CHAPTER 1
Magnetocaloric effect in perovskite manganites
A.Author, B.Author2
1
Affiliations
email
Abstract
Room temperature magnetic cooling based on magnetocaloric effect (MCE) has attracted
considerable attention in recent years as a promising environmentally friendly alternative to
conventional gas-compression cooling. In this purpose, meny research has been done on
various magnetic materials and today a number of materials are considered as promising
candidates in magnetic cooling technology. Among various magnetocaloric materials,
perovskite manganites (R1-xAxMnO3, where R is rare-earth ions and A is alkali earth ions)
have become prominent due to their important properties such as large magnetic entropy
change, large adiabatic temperature change, much smaller thermal or magnetic hysteresis,
low cost and higher chemical stability for long term use. In addition, the Curie temperature
and saturation magnetization of manganites are strongly doping dependent which makes
these materials more convenient to use as a magnetic refrigerant at various temperatures.
Since the last decade, no detailed review has been conducted on the magnetocaloric
properties of manganites. Therefore, in this review, magnetocaloric properties of many
manganites and their potentials for use in magnetic refrigeration were given in detail in order
to give hints for future magnetocaloric studies in manganites.
Keywords
Please list up to 5 keywords describing your chapter.
Section Headings
1.
Introduction
2.
Magnetocaloric Effect
2.1.
Historical
Development
Refrigeration...................................................
of
Magnetic
2.2. Basic Thermodynamic of Magnetocaloric Effect.....................................................
2.3.
Measurement of Magnetocaloric Effect...................................................................
2.3.1.
2.3.2.
2.3.3.
2.4.
3.
Direct Measurements....................................................................................
2.3.1.1.
Measurements Under Variable Magnetic Field.........................
2.3.1.2.
Measurements Under Static Magnetic Field..............................
Indirect Measurements.................................................................................
2.3.2.1.
Magnetisation Measurements......................................................
2.3.2.2.
Specific Heat Measurements........................................................
Semi Theoretical Determination Methods..................................................
2.3.3.1.
Determination From Resistivity Measurements........................
2.3.3.2.
Determination From Landau Theory.........................................
2.3.3.3.
Determination From Mean-Field Method.................................
2.3.3.4.
Determination From A Phenomenological Model.....................
Magnetic Cooling.......................................................................................................
Perovskite Manganites.........................................................................................................
3.1.
Structural and Magnetic Properties of Manganites...............................................
3.2.
Magnetocaloric Properties of Perovskite Manganites............................................
3.2.1.
A-site Substitution In Manganites...............................................................
3.2.1.1.
(La-A)MnO3 (A= Ca, Sr, Ba, Cd, Pb, N, K, Ag, Bi)..................
3.2.1.2.
La(Ca-A’)MnO3 (A’= Sr, Ba, Pb, K, Na, Ag, Mg ) ...................
3.2.1.3.
La(Sr-A’)MnO3 (A’= Ba, K, Ag, Mg)..........................................
3.2.1.4.
(La-A)CaMnO3 (A=Nd,Tb, Dy, Gd, Ce, Y, Sm, Bi, Eu, Ho).....
3.2.1.5.
(La-A)SrMnO3 (A=Er, Eu, Gd, Ce, Pr, Nd, Bi).........................
-.
3.3.
4.
(A1-xA’x)MnO3 (A=Nd, Pr, Sm, Gd, Na, Eu A’=Ca,Sr, Pb, Bi)
Mn-Site Substitution In Manganites...........................................................
3.2.2.1 Mn-site substitution with Al………………………………….....
3.2.2.2
Mn-site substitution with Co………………………………........
3.2.2.3
Mn-site substitution with Cr…………………………………....
3.2.2.4
Mn-site substitution with Fe………………………………….....
3.2.2.5
Mn-site substitution with Cu……………………………………
3.2.2.6
Mn-site substitution with Ni………………………………….....
3.2.2.7
Mn-site substitution with Ga……………………………………
3.2.2.8
Mn-site substitution with Ti…………………………………….
3.2.2.8
Mn-site substitution with V……………………………………..
3.2.2.8
Mn-site substitution with Sn………………………………….....
3.2.2.9
Mn-site substitution with B, Bi, Gd, In, Ru, Sb, Si, Zn, Li……
Comparison of Magnetocaloric Materials..............................................................
Final Remarks....................................................................................................................
References.................................................................................................................................
1.
INTRODUCTION
In today's modern society, cooling at room temperature has become an indispensable
technology at every point of daily life, such as houses, public buildings, and air conditioning
in vehicles, to store food in homes and markets. So far, refrigerators based on gas
compression and expansion logic have been widely used for cooling applications. The use of
chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) gases as a refrigerant in
conventional cooling technology has brought with it serious environmental concerns due to
global warming, especially damaging the ozone layer [1]. According to Montreal Protocol,
the prohibition of the use of CFC and HCFC gases, replacing them with hydrofluorocarbons
(HFC) that do not contain chlorine and therefore do not harm the ozone layer, does not solve
the problem. Because HFC is greenhouse gas with global warming potential higher than CO2
[2]. According to the Montreal Protocol, the use of HFC gases will be banned in the
following years [3]. Therefore, due to severe environmental concerns, alternative
technologies should be introduced that can offer more appealing solutions to environmental
problems rather than existing gas cooling technology.
In recent years, the development of new magnetic refrigeration (MR) technology based on the
magnetocaloric effect (MCE) has brought a new and promising alternative to conventional
gas cooling technology [4]. In a magnetic refrigerator based on the GD element, although the
cooling efficiency reaches 60% of the theoretical limit, this ratio remains only 40% in gascompressed refrigerators [5]. At the same time, magnetic refrigeration is an environmentally
friendly and cost-effective technology that saves up to 30% energy compared to conventional
gas refrigeration technology. The use of magnetic refrigeration technology with high energy
efficiency will reduce CO2 emissions by reducing fossil fuel consumption. It also prevents
the use of gases that damage the ozone layer (CFC), greenhouse gases and hazardous
chemicals. Magnetic refrigeration has significant advantages such as small volume, chemical
stability, low cost, non-toxic and not causing sound pollution. Magnetic refrigeration has
been used in scientific applications for a long time for refrigeration under 1 K [6]. However,
since most ferromagnetic materials do not exhibit adequate magnetocaloric properties around
room temperature, there are no commercial practices of magnetic refrigeration at room
temperature. For this reason, a large portion of the studies in the field of magnetic
refrigeration is still related to the discovery of materials at various temperatures (especially at
room temperature) and with sizeable magnetocaloric effect.So far, many promising materials
have been reported to be candidates for magnetic refrigeration, such as Ni-Mn-Ga alloys [7],
Mn-As-Sb alloys [8], La-Fe-Co-Si alloys [9-12], Mn-Fe-P-As alloys [13], and La-Ca-Sr-MnO manganites [14-36]. Among these magnetocaloric materials mentioned, perovskite
manganites have been prominent because of its significant properties, such as extremely large
magnetic entropy and adiabatic temperature variations, much smaller thermal or magnetic
hysteresis, higher chemical stability for low cost and long-term use. Also, changing
manganites depending on the Curie temperature and doping concentration of the saturation
magnetization makes it possible for these materials to be used as magnetic refrigeration at
different temperatures.
2.
MAGNETOCALORİC EFFECT
In 1881, the magnetocaloric effect (MCE), first observed by German scientist Emil Warburg
[37] on a piece of iron, refers to the change occurring at its temperature by applying a
magnetic field to a material. This substantial truth is directly related to the entropy of the
material. When a magnetic field is implemented to a material that is insulated from its
environment, the magnetic moments of the material that are randomly oriented are directed in
the same direction, and in this case, the defined entropy of the system is reduced. As a result,
the system increases its temperature by a few degrees to restore the decreasing entropy
balance. Therefore, the material refrigerates the environment by making heat absorption. This
situation underlies magnetic refrigeration technology. When the magnetic field is removed,
the magnetic moments gain a random oriented direction again, the entropy of the system
increases and the metal gets absorbed (Figure 2.1).
Figure 2.1. Schematic representation of the magnetocaloric effect [38].
2.1. HİSTORİCAL DEVELOPMENT OF MAGNETİC REFRİGERATİON
W.F. Giauque [6] initiated the first studies on the origin of the magnetocaloric effect in 1927.
It has been shown that extremely low temperatures (0.25 K) can be reached using
paramagnetic salts. With this study, experimental studies aimed at reducing low temperatures
(mK) to ultra-low temperatures (µK) have gained speed. Today, this technology is used to
obtain ultra-low temperatures.
The magnetocaloric effect and its technological applications, which have been going on over
the past decades and today, are proceeding intensively. In particular, studies that encourage
the idea of developing magnetocaloric refrigeration that can operate at room temperature and
be an alternative to contemporary refrigerations have been provided with the studies made
with gadolinium and Gd-based alloys. Studies on the magnetocaloric properties of Gd and
Gd-based alloys, which were introduced by G.V Brown [39] in 1976, have accelerated the
development of modern magnetic refrigerants that can operate around room temperature.
First magnetocaloric effect studies in LaMnO-based, doped perovskite-type film alloys, were
conducted by D.T. Morelli [40] in 1996. In studies with LaAMnO (A=Ca, Ba, Sr) film alloys,
positive results were obtained regarding the magnetocaloric effect. In parallel to this study, in
1996, the higher magnetocaloric effect was observed by X.X. Zhang [41] in LaCaMnO
ceramic bulk materials. Thus, studies carried out with LaMnO-based, doped perovskite-type
alloys have gained speed. The high magnetic entropy exchange, called GMCE (Giant
Magnetocaloric Effect) was first observed in 1997 by V.K. Pecharsky and K.A.Jr.
Gschneidner [42],in the alloy of Gd5(Si,Ge)4 ,several times larger than pure Gd. In addition,
we can summarize some of the different alloys that are still being studied today that Gd-based
and exhibit GMCE, metallic alloys such as; GdDy, GdTy, Gd(Si-Ge), La(Fe-Si)H, MnFe(PAs), and especially in the last few years, FeSiB-based amorphous and doped amorphous
alloys.
In Figure 2.2, the number of publications published in international journals, which have been
received from the Web of Science since 1994 and entered into ISCI (International Science
Citation Indexes), are distributed by year. As a result, scientific studies for the development
of materials showing magnetocaloric and superior magnetocaloric effects, as can be seen
from the graph, show a rapid increase to the present day.
Figure 2.2. According to Web of Science and ISCI (International Science Citation Indexes)
data, distribution of the number of publications associated with magnetocaloric effect by
years.
2.2. BASIC THERMODYNAMICS OF MAGNETOCALORİC EFFECT
If the magnetic field is applied adiabatically in a ferromagnetic material around Curie
temperature (TC), unpaired spins are directed towards the direction of the area. As a result,
the magnetic entropy of the solid matter decreases and the lattice entropy of the sample
increases. Due to the increase in lattice entropy, the sample increases its heat to allow the
decrease in magnetic entropy to rise again. As a result, when the field is eliminated, the spin
turns back randomly, the magnetic entropy increases, and the heat of the sample decreases
with the lattice entropy.
Under constant pressure, the entropy of a magnetic solid, S(T, H), can be written as three
different entropy sums [5-22,43].
S(T,H) = SM (T,H) + SLat (T) + SEl (T)
(2.1)
Here, SM , magnetic , SLat , lattice ve SEl , shows electronic entropy. Change of magnetic
entropy of a magnetic material
(2.2)
it is given in the form. Similarly, the adiabatic temperature variation of a magnetic material;
(2.3)
is given in the form.
The term adiabatic temperature change (ΔTad) and isothermal magnetic entropy change (ΔSM)
relate to the terms magnetization, magnetic field strength and heat capacity under constant
pressure and constant temperature. According to Maxwell's Equations [43];
(2.4)
a correlation can be established. With the integration of this equation;
(2.5)
and
(2.6)
it is obtained in the form. It depends on both ΔSM(T)ΔH and ΔTad(T)ΔH temperature and ΔH.
In most studies, a given ΔH field change is calculated as a function of the temperature, or a
function of the ΔH field change for a given temperature. The characteristic behavior of both
magnetocaloric effects depends on the properties of the material. Therefore, without
experimental measurements, knowledge about these behaviors is complicated to predict.
As is known, the magnetocaloric effect is the physical basis of magnetic refrigeration
systems. Hence, one of the technologically essential parameters such as maximum magnetic
entropy change and operating temperature is relativistic absorption power (RCP), which
represents the magnetic cooling efficiency. In short, the maximum value of the |∆SM| or
∆Tad curves is found by multiplying the half-height temperature width (δTFWHM) of the
entropy change curves with the maximum value (Figure 2.3).
RCP(S) = ⏐ΔSMmax⏐ × δTFWHM
2.7
RCP(T) = ΔTad × δTFWHM
2.8
Figure 2.3. Calculating the RCP value of the thermal curve of entropy change for the
La0.97Bi0.06MnO3 alloy [30].
2.3. MEASUREMENT OF MAGNETOCALORİC EFFECT
We can assemble the measurement methods of magnetocaloric effect in two main groups.
First, the magnetocaloric effect can be measured using direct methods [43]. Secondly, it can
be calculated using indirect methods like using magnetization or heat capacity measurements
[18,19,43]. Whether direct or indirect methods are used, measurements or calculations are a
function of the temperature and magnetic field. In comparison, both techniques have the
advantages and disadvantages of each other.
Direct measurement methods only give adiabatic heat change (ΔTad). Temperature values are
found without any processing of data, and the magnetocaloric effect is easily obtained by
taking the difference between the two temperature values. However, direct measurement
usually has time delays, and it is difficult to measure it for small changes in temperature. In a
direct measurement, massive experimental mistakes are unavoidable if the measuring devices
are not calibrated well or the material is not properly isolated.
Indirect MCE measurements allow the calculation of both ΔTad(T)ΔH and ΔSM(T)
ΔH
using
experimental heat capacity data or will enable the calculation of ΔSM(T)ΔH alone using
experimental magnetization measurements. Indirect measurement gives practical results in
any temperature range. However, experimental data must be processed to calculate MCE.
2.3.1. DİRECT MEASUREMENTS
2.3.1.1. MEASUREMENTS UNDER VARIABLE MAGNETİC FİELD
Thermal isolation of the sample is of great significance in the direct measurement method. In
this method, the initial temperature of a thermally isolated sample is measured in an initial
area ( Ti(Hi) ). Then, the area is subtracted from the starting value ( Hi ) to the final value ( Hf
), and the final temperature of the sample is measured ( Tf(Hf) ). The difference between the
temperature values obtained by using these two field values is taken. Thus, the adiabatic
temperature change is found.
ΔTad(Ti)ΔH = Tf - Ti
(2.9)
Adiabatic temperature change is here, to change the ΔH a given field, is a function of the
initial temperature (Ti). The form of the magnetic field applied to the sample is pulsed when
implementing the field or removing the field. On the other hand, it can be in the form of steps
with a magnetic field exchange ratio of ∼ 10kOe/s [43]. Weiss and Forrer first proposed
the direct measurement method in 1926 by using an electromagnet to create and eliminate the
field using the switch-on technique [44]. Later, in 1969, Clark and Callen made the first
measurements using the yttrium iron core under a powerful magnetic field (above 110 kOe)
[45]. In both studies, a thermocouple was used to measure the temperature. In 1988, Green
used the same method but used a superconducting selenoid instead of an electromagnet to
reach higher fields [43].
Another method of direct measurement is the differential thermocouple method proposed by
Kuhrt in 1985, which provides more accurate results [43].
2.3.1.2. MEASUREMENTS UNDER STATİC MAGNETİC FİELD
The field generated by an electromagnet is approximately 20 kOe. However, if a
superconducted selenoid is used, this field value can be increased to 100 kOe or above. A
heat distribution occurs due to the magnetocaloric behavior of the sample to attain the desired
field value of electromagnets in the elapsed time. To eliminate this undesirable situation,
observations by Tishin [46] in 1988 revealed that for temperatures above 30 K, the time to
reach the desired value of the field should not be higher than 10 seconds. In order to
overcome these difficulties, which are related to the duration of the applied field to reach the
desired value, the static magnetic field method based on the logic of fast positioning of the
sample into the static magnetic field of a superconducting selenoid was first developed by
Nikitin [47] in 1985, Gopal [48] in 1987 and Tishin [46] in 1988. According to this method,
the sample is initially outside the selenoid, and when selenoid reaches the desired field, the
sample is quickly placed at the center of the selenoid (∼ 1s.), and the temperature is
measured.
2.3.2. INDIRECT MEASUREMENTS
2.3.2.1. MAGNETIZATION MEASUREMENTS
Magnetic entropy change (ΔSM) using experimental isothermal magnetization data ( M(H) )
can be calculated using equation 2.5. The numerical integration and derivative (∂M/∂T)
derivative of equation 2.5 can be calculated in the desired temperature and magnetic field
range. In 1993, McMichael [49] introduced the following simple formula for numerical
computation of ΔSM .
(2.10)
Figure 2.4 displays the calculation of magnetic entropy change (ΔSM) using typical M-H
curves for LaCaMnO structure. According to this, the field between the M-H curves (
T+ΔT/2 ) in two different temperature ranges ( T ve T+ΔT ) is calculated, and ΔSM change
corresponding to the temperature range can be obtained according to the following equation
2.11.
A
(2.11)
M( H
M( H ,
Artan
Figure 2.4. Calculation of magnetic entropy change(ΔSM) from the field between M-H
curves.
2.3.2.2. SPECİFİC HEAT MEASUREMENTS
MCE and magnetic entropy change can be determined from the measurements of temperature
dependence of heat capacity in different magnetic fields. This method was developed by
Brown [39] in 1976 and Gschneidner [50] in 1996. Entropy change of material ΔH=H2-H1
under magnetic field change can be calculated as follows,
(2.12)
2.3.3. SEMI THEORETICAL DETERMINATION METHODS
2.3.3.1. DETERMINATION FROM RESISTIVITY MEASUREMENTS
Many researchers have shown that there is a strong correlation between electrical and
magnetic properties. [51,52]. In manganites, both the magnetocaloric effect and
magnetoresistance properties usually occur around the temperature of the magnetic transition
phase. This shows that there is a correlation between magnetic entropy change and resistivity
(ρ). In this context, Xiong [53], ∆SM between ρ
(2.13)
He proposed that there was a correlation. Here α, is the adjustment parameter and reflects the
magnetic properties of the material. A constant of α is calculated using different functions (
[51],
[52],
[53] ) that
correlate M and ρ. For La0.67Ca0.33MnO3 alloy, α=21.74 emu/g has been found [53]. Figure
2.5 shows the magnetic entropy change obtained from conventional magnetization curves and
resistance measurements for La0.67Ca0.33MnO3 alloy.
Figure 2.5 Comparison of magnetic entropy change calculated from resistance measurements
and magnetization curves [53].
2.3.3.2 DETERMINATION FROM LANDAU THEORY
Landau expansion of magnetic free energy, F(M,T), in terms of magnetism,
(2.14)
shown above is given in the form of the formula. Here c1(T), c3(T) ve c5(T) are called
Landau coefficients, and depends on the temperature. Magnetic entropy change using
condition
in equilibrium condition
(2.15)
as written in the form of. c1(T), c3(T) ve c5(T) coefficients are found as the function of
temperature using experimental magnetization curves (Figure 2.6). Magnetic entropy change
is detected using the obtained coefficients and Equality 2.15.
Figure 2.6 For La0.67Ca0.31Mg0.02MnO3 alloy c1(T.g/emu), c3(T.g3/emu3) and c5(T.g5/emu5)
coefficients and calculated magnetic entropy change [33].
2.3.3.3 DETERMINATION FROM MEAN-FIELD METHOD
As it is known, the theory of Landau relies on the logic of the expansion of magnetism in
terms of forces; it can not take into account the magnetic behavior in higher fields. Since this
limitation is particularly significant in low temperatures and high fields, a generalized meanfield analysis method has been developed, which is also profoundly successful in determining
the magnetocaloric properties of materials [54]. The general mean-field principle is used as
defined in the form
(2.16)
to obtain the mean-field parameters from experimental magnetization data [55]. Here Hexc=
λM is the exchange field and f is called the state function. Inverse f-1(M) function is defined
(2.17)
in the form. From experimental M-H curves, H and T values for specific magnetization
values are determined, (Figure 2.7a), and graphs of H/T against 1/T are drawn (Figure 2.7b).
For each M value, the slope of the drawn curves is equal to Hexc (=λ1M+ λ3M3+….).
Figure 2.7. a) Obtaining mean-field data for specific magnetization values (M=10,20 ve 30
emu/g) in M-H curves. b) H/T-1/T curves [56].
After the exchange field is determined, the graph is drawn in the second step, corresponding
to the magnetization values of the exchange field values (Figure 2.8a). After the exchange
field is obtained, the second step of this method is to create a scale graph (f function), M
corresponding (H + Hexch)/T, where the data defining the system is collected on a curve
(Figure 2.8b). f scale function, with the function given below, directly proportional with
magnetic entropy
M -(H+Hexc)/T eğrileri çizilir (Şekil…).
Figure 2.8 Exchange field transformation in response to magnetism b) Transformation of M
corresponding (H + Hexch)/T [56].
(
), Thus, the magnetic entropy change between the H1 and H2 fields
(2.18)
is given in the form. Magnetic entropy change is obtained by using the f function given in
Equation 2.18 and figure (2.8b). Figure (2.9) shows the magnetic entropy value obtained
from La0.665Er0.035Sr0.3MnO3 alloy [56].
Figure 2.9. Comparison of magnetic entropy change in La0.665Er0.035Sr0.3MnO3 alloy with the
Mean-field method and experimentally obtained [56].
2.3.3.4 DETERMINATION FROM A PHENOMENOLOGICAL MODEL
In addition to experimental studies, magnetic and magnetocaloric properties of magnetic
materials are studied using theoretical models [57]. From Haman's theoretical model and
magnetization measurements (M(T)), it is possible to determine the magnetocaloric
properties of magnetic materials. In the studies carried out, it is claimed that this model is
more accessible and gives more precise results than previously used theoretical models [58].
In this model, magnetization changes according to temperature
(2.19)
are presented in the form. Here, Mi and Mf show the initial and end values of the
ferromagnetic-paramagnetic
transition
and
region,
as
shown
in
figure
is given in the form of.
2.10.
Here,
Figure 2.10 Change of magnetization for La0.94Bi0.6MnO3 with temperature.
Here B gives the magnetization precision
in a ferromagnetic state before the
transition. The Sc provides the precision of magnetization in Curie temperature
.
Magnetic entropy change using Equation 2.19 and Equation 2.5
(2.20)
is obtained in the form. Similarly, adiabatic temperature change
(2.21)
is given in the form. Here Cp is the heat capacity per mol, in the constant magnetic field. In
Figure 2.11, ΔSM and ΔTad changes calculated in the 0.1 T magnetic field for La0.94Bi0.06Mn1xCrxO3
(x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) alloys are observed using the Equations 2.20 and
2.21 and experimentally measured magnetization curves.
Figure 2.11. For La0.94Bi0.06Mn1-xCrxO3 alloy, magnetic entropy change calculated in 0.1 T
magnetic field and adiabatic temperature change.
2.4
MAGNETIC COOLING
Since the discovery of the magnetocaloric effect, studies have been in progress on the
development of magnetic cooling systems. The idea of magnetic cooling is used for more
than 50 years to cool ultra-cold environments more, which are not yet fully achieved as
commercially cooling systems [6, 37]. Since magnetic cooling technology is capable of
competing with today's commercial cooling systems, it is one of the greatest of the efforts to
produce the cooling technology of the future. Also, since solid materials are used as
refrigerant material, they have significant advantages compared to gas-cycled commercial
systems. The fact that magnetic refrigerations do not carry any adverse features such as noise,
excessive vibration, oil or gas leakage, abrasion, and dependence on gravitation, is the main
reason for to be seen as the future refrigerations. Simply, the cycle of the cooling process is
shown in figure 2.12 [23]. A magnetic cooler that can be operated at room temperature, first
developed by Brown [39] in 1976 and Steyert [59] in 1997, has increased interest in this field
in recent years.
Figure 2.12. Schematic representation of magnetic cooling system and gas cycled commercial
cooling system comparatively [38].
In Table 2.1, the development of magnetic cooling systems with room temperature operating
characteristics are given by years. In recent years, the use of permanent magnets in prototype
magnetic cooling systems is an exciting feature. Besides, systems developed in recent years
have smaller volumes, and this is seen as an essential improvement for commercial use.
When the historical development of magnetic cooling systems is examined, it is predicted
that in a not-so-distant future, magnetic cooling systems will replace commercial refrigerants.
The studies carried out today are continued under two central headings: the development of
materials with magnetocaloric effect with superior efficiency, enduring and economic system
design.
Table 2.1. Some magnetic cooler prototypes that can operate at room temperature [60].
Research and Development Group &
System
Year
Type
Brown [39]
Reciprocat
The first device to use Gd (1976)
ing
Ames Lab./Astronautics
Reciprocat
USA (1997)
ing
Mater. Science Institute
Spain (2000)
Rotating
Chuba Electric/Toshiba
Reciprocat
Japan (2000)
ing
University of Victoria
Reciprocat
England (2001)
ing
Astronautics
USA (2001)
Rotating
Sichuan Inst. Tech./Nanjing Uni.
Reciprocat
China (2002)
ing
Tspan (K)
Magnetic Field
(kOe)
47
70(S)
10
50(S)
5
9.5(P)
21
40(S)
14
20(S)
20
15(P)
23
14(P)
Chuba Electric/Toshiba
Reciprocat
Japan (2002)
ing
Chuba Electric/Toshiba
Japan (2003)
Rotating
27
6(P)
10
7.6(P)
4
8(P)
Lab. d’Electrontechnique Grenoble
Reciprocat
France (2003)
ing
The Universities of Victoria and
Reciprocat
47/51
20(P)
Quebec, Canada (2004)
ing
14
20(S)
Washington State University
Reciprocat
USA (2004)
ing
5
20(P)
Graduate School of Engineering of the
Hokkaido University
Japan(2005)
Reciprocat
10
20(P)
18/10
15(P)
5
15(P)
Rotating
12
15(P)
Rotating
13
14(P)
Rotating
11.5
15(P)
9
12(P)
6-7
14(E)
2
23(P)
16
11(P)
11
23(E)
7.8
8(P)
16
15.8(P)
Chinese Academy of Science
China (2006)
Baotou Research Institute of Rare Earth
China(2006)
Astronautics Corporation of America
USA(2007)
University of Victoria
Canada(2007)
Sichuan University
China(2007)
Riso National Laboratory
Denmark(2007)
Bahl et al. [40?]
(2008)
Hirano et al. [41?]
(2009)
Cooltech Applications
France(2009)
Campinas State University
Brasil(2009)
Dupuis et al.
(2009)
Korea Advanced Institute of Science
and Technology
ing
Reciprocat
ing
Reciprocat
ing
Reciprocat
ing
Reciprocat
ing
Reciprocat
ing
Reciprocat
ing
Rotating
Reciprocat
ing
Reciprocat
Korea(2009)
University of Genoa
Italy(2009)
Trevizoli et al.
(2010)
Korea Advanced Institute of Science
and Technology
Korea(2011)
Balli et al.
(2011)
ing
Reciprocat
Park et al.
(2012)
University of Victoria
Canada(2013)
Korean Advanced Institute of Science
and Technology
Korea(2013)
Chinese Academy of Science
China(2013)
University of Salerno
Italy(2014)
Technical University of Denmark
Denmark(2015)
Institute of Non Ferrous Metals
Poland(2016)
15,5(P)
4.4
16.5(P)
14
15(P)
20
14.5(P)
29
14.7(P)
5-10
10.3(P)
26.8
14(P)
33
16(P)
20
14.1(P)
Rotating
7.9/14.9
15(P)
Rotating
13.5
12.5(P)
Rotating
10.2
12(P)
2.5
8(P)
Reciprocat
ing
Reciprocat
ing
Reciprocat
ing
Tura et al.
(2011)
Technical University of Denmark
Denmark(2011)
5
ing
Rotating
Reciprocat
ing
Reciprocat
ing
Rotating
Reciprocat
ing
Reciprocat
ing
Federal University of Santa Catarina
Rotating
7.1
15(P)
Brasil(2016)
Benedict et al.
Rotating
21
15(P)
(2016)
* P=Permanent Magnet, S=Superconducting magnet, E=Electromagnet
[40?] Bahl CRH, Petersen TF, Pryds N, Smith A. A versatile magnetic refrigeration test
device. Rev Sci Instrum 2008;79:093906
[41?] Hirano S, Kawanami T, Nakamura K, Fumoto K, Ikegawa M, Hirasawa S. A
development of spherical-shaped magnetocaloric materials using power coating
method. In: Proceedings of the third international conference on magnetic refrigeration
at room temperature 2009.
3. PEROVSKITE MANGANİTES
Mixed-valence perovskite manganites structures have been synthesized and examined since
the second half of the twentieth century due to their superior structural, magnetic, electrical
[21,22,41,43] and magnetocaloric properties [16-36]. Magnetic and structural properties of
polycrystalline mixed-valence manganites (R1-xAxMnO3, here R rare-earth ions A alkaline
earth ions) initially produced by Jonker and Santen [61] in 1950, were examined, and the
dependence of these properties to dope concentration (x) was determined. The basis of
magnetic properties in these alloys is defined by the double-exchange (DE) mechanism by
Zener [62]. The advantages of Perovskite manganites compared to Gd and GdSiGe alloys
are primarily have low cost and longevity chemical stability. Also, Curie temperature in
manganese can be regulated to the desired temperature range with doping of different
elements has made this kind of material a sought-after candidate for magnetic cooling at
room temperature. First, the magnetocaloric properties of these alloys were examined by
Morelli [40], Zhang [41] and Guo [63] and determined to be proper for technological
applications, leading to further studies.
3.1. STRUCTURAL AND MAGNETİC PROPERTIES OF MANGANITES
In general, perovskite manganites has a chemical structure that can be formulated with
RMnO3 (R rare-earth cations). With the doping of atoms with different properties and ionic
radiations, instead of R and Mn, perovskite manganites can exhibit a very diverse electrical
and magnetic properties.
In MCE field studies, usually, as an R cation, a Lanthanide group element is used. Then,
another alkaline-earth element with a value of +2 is doped to this lanthanum-based structure.
The general formula of this doped perovskite manganite structure is R1-xAxMnO3. Here, A, +
2 valenced divalent metal (Ca, Ba, Sr, Li, Na, K, Y…) and R represents (La, Pr, Nd, Gd, Dy,
Er…) rare-earth elements with a + 3 valenced lanthanides. As shown in figure 3.1, a
manganite perovskite structure is in the form of a cubic lattice, where six oxygen is placed
around the manganite atoms in octahedral order, and a lanthanide group of rare-earth
elements is located in the core of the cubic lattice. When the element A is not doped to the
structure, in the case of x=0, the perovskite manganite structure is R3+Mn3+O32- - and the
crystal structure is as shown in figure 3.1.
For x=0 (RMnO3) all of the manganese in the structure are in the form of (3d4) Mn3+. For x=1
(AMnO3) all of the manganese are in the form of (3d3) Mn4+. Since electrons are localized on
manganese, in the structure corresponding to both x=0 and x=1, the electrons exhibit
antiferromagnetic properties, and the structure is nonconducting.
R
A
=
=
M
O
Figure 3.1. Schematic representation of LaCaMnO3 perovskite-manganite structure.
In Perovskite manganites, due to the ionic radius of ions doped to the structure, structural
dispersion and the magnitude of these dispersions are determined by a parameter named the
Goldschmidt tolerance factor given in equation 3.1.
( 3.1)
Here, r represents ionic radiations. While t=1 is the ideal cubic structure that is not dispersed,
t<1 is the ideal structure that is dispersed. The ionic radius of some atoms used in the
perovskite structure is given in Table 3.1. To understand the changes in the magnetic and
electrical properties of manganites by doping atoms with different ionic radius, the magnitude
of the tolerance factor is a significant parameter.
Figure 3.2 displays the phase diagram of the magnetic phases in the structure for La1xCaxMnO3
alloy, depending on Ca (x) concentration. [64]. At high temperatures, at all x
concentration values, the composition is in the paramagnetic insulator (PM-I) phase. At lower
temperatures, depending on x, various ferromagnetic-metallic (FM-M), ferromagneticinsulator (FM-I) and antiferromagnetic-insulator (AFM-I) phases occur. As MCE is generally
observed around Curie temperature, the Curie temperature of the material is desired to be
around the room temperature. As shown in Figure 3.2, the maximum temperature value at
which PM-FM transition is observed corresponds to the concentration ratio (260 K) x=0.33.
Table 3.1. The ionic radius of some oxides in the perovskite structure (ds=low spin).
Radius
Ion
(A)
Ion
Radius
(A)
Radius
Ion
(A)
Al3+
0.535
K+
1.64
Rb+
1.72
B3+
0.23
La3+
1.032
Sm3+
1.24
Ba2+
1.61
Mn2+
0.83
Sn2+
1.30
Bi3+
1.03
Mn3+
0.645
Sr2+
1.44
Ca2+
1.00
Mn4+
0.53
Ti4+
0.605
Cd2+
1.31
Na+
1.39
V3+
0.74
Co3+
0.61
Nd3+
1.27
V4+
0.63
Fe3+
0.645
Ni3+
0.69
Y3+
0.90
Ga3+
0.62
Pb2+
1.49
Gd3+
1.107
Pr3+
1.29
O2-
1.40
S
ı
c
a
k
FA
A
C
G
Figure 3.2. Magnetic phase diagram of the structure La1-xCaxMnO3 according to the amount
of x concentration [64].
Magnetic and electrical properties of perovskite manganite structures, depending on the
changes in concentration ratio, can be explained by these changes in phase diagrams. Figure
3.3 shows the phase diagram for the R1-xAxMnO3 structure, reflecting magnetic and electrical
properties as a function of bandwidth and concentration ratio [65]. Here R represents rareearth elements of lanthanide group and A represents alkaline-earth elements. As shown in the
figure, depending on the concentration ratio and the bandwidth, A-type AFM-I, F-type FMM, A-type AFM-M, C-type AFM-I and G-type AFM-I like many that have different
magnetic properties and conductivity phase is seen.
B
a
n
t
g
e
n
yalıtk
yalıtk
Deşik
konsantrasyonu
Figure 3.3. Phase diagram reflecting magnetic and electrical properties as a function of
bandwidth and concentration ratio for R1-xAxMnO3 structure [65].
3.2. MAGNETOCALORİC PROPERTİES OF PEROVSKİTE MANGANITES
3.2.1. A-SITE SUBSTITUTION IN MANGANİTES
3.2.1.1. (La-A)MnO3 (A= Ca, Sr, Ba, Cd, Pb, N, K, Ag, Bi)
Magnetic properties of manganites in the structure of (La1-xCax) MnO3 have been studied
inclusively. The most comprehensive researched group of manganite can be called. The
magnetocaloric properties of (La1-xAx)MnO3 (A = Ca, Sr, Ba) manganite films were first
reported by Morelli [40]. Although the obtained |∆SM| value is not very low, (under 1T
magnetic field change 0.5 J/kg.K) it has been shown that the peak temperature of |∆SM| can
be adjusted in the range of 250-350 K depending on the dope concentration. Guo [63],
revealed that the (La1-xCax)MnO3 polycrystalline sample has a much higher |∆SM| value (For
1.5 T magnetic field x=0.2, 0.25 and 0.33 respectively; in 230 K 5.5 J/kg.K, in 224 K 4.7
J/kg.K and in 260 K 4.3 J/kg.K) in the concentration range of 0.20 ≤ x ≤
0.33. These values are much higher than the |∆SM| value (4.2 J/kg.K) observed under the
same magnetic field change for pure Gd [43]. Zhang [41], has shown that La0.67Ca0.33MnO3
(x = 0.33) alloy has a smaller |∆SM| value (0.6 J/kg.K) for 1 T magnetic field change. For the
same sample, Ulyanov [68] found higher |∆SM| values (5.04 J/kg.K ve 6.25 J/kg.K) for 0.5 T
and 1 T magnetic field changes. This discrepancy among the equivalent samples can be due
to different preparation methods or different chemical compositions of the samples. It has
been observed that La0.67Ca0.33MnO3 alloy has the highest |∆SM| value among the various x
concentrations examined [15, 21, 69]. Sun [15] and Mira [21], in their study, proposed that
the greater magnetic entropy change was caused by sudden decreases in magnetism occurring
in the first-degree magnetic phase transition. It should also keep in mind that, with the switch
like the magnetic phase transition from the first degree to the second degree, the |∆SM| curve
more broad, even though the |∆SM| value decreases dramatically. This condition is one of the
features sought for magnetic cooling [14,21]. Hueso [70] explained that in nano-sized
La0.67Ca0.33MnO3_δ alloy, which is synthesized by the sol-gel method, peak temperature could
be adjusted without repressing the magnetic entropy change. In the same study, it was noted
that magnetic entropy change is proportional to grain size. Guo [71] examined the effect of
grain size on magnetic entropy change in La0.75Ca0.25MnO3 alloy produced by the sol-gel
method. For samples with 120 nm ve 300 nm grain size, Curie temperatures were measured
177 ve 224 K. It has also been observed that the magnetic entropy change has decreased with
the shrinking of the grain size, but the |∆SM| change has become broader. Biswas [72]
observed a reverse magnetocaloric effect in the polycrystalline sample of La0.125Ca0.875MnO3
alloy. This situation is attributed to the presence of a non-homogenous magnetic structure
formed by the antiferromagnetic phases of the composite. Under 7 T magnetic field change,
negentropy change has been reported as -6.4 J/kg.K. In recent years, to produce perovskite
manganites, a new method named mechanical alloying or high-energy ball milling method
has come into use [31, 73, 74]. Studies have revealed that the ball milling method has a
significant number of advantages, such as low cost, high efficiency, low-temperature
synthesis and the ability to adjust grains in the desired size from micrometer to nanometer. In
many studies, magnetic and magnetocaloric properties of manganites produced using highenergy ball milling method were examined.
In 2014, in a study conducted by Gencer and his colleagues, the La067Ca033MnO3 alloy was
produced using a high-energy ball milling method [31]. It was observed that the perovskite
structure was formed for the milling period above 4 hours. When the milling period was over
40 hours, it was seen that the perovskite structure was dispersed entirely and the amorphous
structure has been shown to have emerged. In the 24-hour milled sample, it has been reported
that particle size varies from nm to a few µm. Magnetic entropy change for 12 hours milled
sample was measured as 0.3 J/kgK in a 6 T magnetic field. Although this value is relatively
small compared to the entropy values of samples produced by other methods, magnetic
entropy change has been observed to have a considerably wide temperature range. Although
the |∆SM| value in manganites produced by ball milling method is quite low, the fact that the
magnetic entropy change has an extensive temperature range makes these samples appealing
magnetic coolers under room temperature. Bourouina [73] examined the structural, magnetic
and magnetocaloric properties of the nano-particle Pr0.5Sr0.5MnO3 alloy produced using highenergy ball milling method. The samples obtained during various grinding times were
subjected to thermal treatment of 1400 C0 for 20 hours. Structural analyses have shown that
all samples have tetragonal symmetry. When the grinding time was over 16 hours, it was
observed that the average particle size was reduced to nano-size. For 4, 12 and 16-hour
milled samples, Curie temperature was reported as 250, 240 and 235 K, respectively. 5 T for
magnetic field change, 4, 12 and 16 hours for samples produced by grinding, respectively;
|∆SM| = 2.27, 2.57, 2.58 J/kg.K and RCP= 216.33, 214,92, 204.31 J/kg were reported. Again
in a different study, using solid-state, sol-gel and ball milling method, La0.78Dy0.02Ca0.2MnO3
alloy is produced [74]. Structural analyses have shown that all the samples have an
orthorhombic structure. Also, in samples produced by a solid-state and sol-gel method, grain
sizes were in the order of micrometers, and from samples produced by a ball milling method,
smaller particle sizes were obtained. Interestingly, the samples produced by a sol-gel method
exhibit the first phase transition, while the samples produced by the other two methods show
the second phase transition. The maximum magnetic entropy values of samples produced by
solid-state, ball milling and sol-gel methods under a 2 T magnetic field change, respectively;
1.78, 1.83 ve 4.24 J/kg.K has been reported. In the sample produced by the sol-gel method,
the magnetic entropy change is much more significant than the others, which is attributed to
the second-order phase transition. In spite of this, the greatest RCP value (RCP values of
samples produced by solid-state, ball milling and sol-gel methods under 2 T magnetic field
change,respectively; 106, 112 ve 76 J/K) was reported in the samples produced by ball
milling method. The results showed that the samples produced by the high-energy ball
milling method might be promising candidates in the field of magnetic cooling.
Phan [75] studied magnetic and magnetocaloric properties in (La1-x)
0.8Ca0.2MnO3
(x= 0.05,
0.1, 0.2 and 0.3) alloy which contains voids on the La side. Interestingly, La-side voids have
not only improved the magnetocaloric properties, but also Curie temperature. Likewise, Hou
[76] examined the effect of La-side voids on La0.67- xCa0.33MnO3 (x = 0, 0.02, 0.06, and 0.1)
alloy. The maximum |∆SM| value was measured as 2.78 J/kg.K for x=0.02 sample in 277 K
and 1 T magnetic field. Chen [77] examined the effect of La-side voids on Tc and
magnetocaloric properties in (La0.8-yCa0.2)MnO3 alloy. The Curie temperature, for y= 0 value
of 182 K increased its value to 260 K for y=0.05, was seen. Besides, it was observed that Tc
remained almost constant for higher y values. It is noted that the nature of the magnetic phase
transition is transformed into second order for y=0 and y=0.01, and the first order for y=0.03
and 0.1. It is observed that the |∆SM| value in 1 T magnetic field increases from
7.7 mJ/cm3K for y=0 to 22.3 mJ/cm3K for y=0.03. In 2007, Adıguzel and his colleagues
examined the effect of sintering temperature on structural, magnetic and magnetocaloric
properties of La067Ca033MnO3 alloy produced using the polymeric precursor route method
[26]. The lattice parameters of the samples showed that the sintering temperature increased
up to 1150 C0. Magnetization measurements showed that Curie temperature increased from
241.3 K for the 600 C0 sintered sample to 268.8 K for 1000 C0 sintered sample. Similarly,
magnetic entropy change has been found to increase due to sintering temperature. At 1150 C0
sintered sample, the highest |∆SM| value is measured as 4.8 J/kg.K in 1 T magnetic field.
A large number of researches have been carried out on (La1-xSrx)MnO3 manganites, to make
the magnetocaloric effect available at room temperature. First, Szewczyk [78] examined the
magnetocaloric properties of La1-xSrxMnO3 (x = 0.120, 0.135, 0.155, 0.185 and 0.200)
manganites. In the research, the |∆SM| value was also increased depending on the increased
SR ratio. The highest adiabatic change in 7 T magnetic field, for (∆Tad) x=0.200, was
reported as 4.15 K. Mira and colleagues [21], were found that the |∆SM| value under 1 T
magnetic field change, for La0.67Sr0.33MnO3 polycrystalline alloy, is 370 K at 1.5 J/kg.K. This
value is similar to the research results obtained by Xu and his colleagues [17]. For 1 T
magnetic field change, the maximum |∆SM| value for the La0.65Sr0.35MnO3 sample has been
reported by Phan [79] as 2.12 J/kg.K at 305 K. For the La0.8Sr0.2MnO3 alloy produced
by the carbonate precursor method, under 2 T magnetic field
change, |∆SM| value has been reported by Pekala [80] as 1.7 Jkg.K at 275 K. Immediately
after this, magnetocaloric properties of the same La0.8Sr0.2MnO3 alloy, produced by the solgel method, were examined for the polycrystalline and nanosize forms [81]. Under 2 T
magnetic field change, |∆SM| value has been reported, respectively, as 2.2 J/kg.K at 301 K
and 0.5 J/kg.K at 295 K. Given the temperatures where |∆SM| maximums
are observed, it is seen that these samples are promising for magnetic cooling at room
temperature.
First studies on magnetocaloric properties of La1-xBaxMnO3 manganites have been reported
by Phan [82], Zhong [83] and Xu [17]. The magnetocaloric properties of La0.7Ba0.3MnO3
polycrystalline manganites were first published by Phan [82]. For 1T magnetic field change
and at 336 K, |∆SM| value has been reported as 1.6 J/kg.K. Zhong [83] examined the effect
of oxygen ratio in La2/3Ba1/3MnO3-δ (δ = 0, 0.02, 0.05, 0.08 and 0.1) manganites on magnetic
and magnetocaloric properties. With the increase of δ value, it has been observed that the
value of |∆SM| has decreased considerably. For δ=0 in 1 T magnetic field and at 350 K, the
|∆SM| value has been reported as 2.7 J/kg.K [83]. This value is comparatively different from
the |∆SM| value obtained by Xu [17] for La0.67Ba0.33MnO3 alloy. It is stated that this
discrepancy can be caused by differences in sample preparation and chemical compositions.
Magnetic and magnetocaloric properties of La1-xBaxMnO3 (x = 0.1, 0.2 and 0.3) manganites
were examined by Tonozlis [84]. It was reported that Curie temperature increased from 181
K to 319 K, depending on Ba concentration. For 2 T magnetic field change, it has been
reported that the |∆SM| value, for x=0.1 at 1.51 J/kg.K has increased from the value, for
x=0.3 to 2.61 J/kg.K. In recent years, Hussain [85] examined the magnetic and
magnetocaloric properties of La1-xBaxMnO3 (x = 0.30, 0.35 and 0.40) manganites. For
La0.7Ba0.3MnO3, under 2.5 T magnetic field, they reported the |∆SM| value as 2.06 J/kg.K at
342 K. The results revealed that these samples could be used as magnetic coolers around
room temperature. Das [86] examined magnetocaloric properties of La0.7Ba0.3−zNazMnO3 (0
< z < 0.15) alloy. For z= 0.05, 0.1 and 0.15 Curie temperatures were reported as 317, 313 and
312 K respectively. The largest magnetic entropy change, under 0.8 T magnetic field change
was observed as 1.18J/kg.K for z= 0.05. Regaieg [87] examined the magnetic and
magnetocaloric properties of La0.8Na0.2−xKxMnO3 (0≤x≤0.2) alloy. In K-doped samples,
it was reported that the Curie temperature remained constant at around 330 K. Under 1 T
magnetic field change, for x=0 at 325 K, the |∆SM| value was measured as 1.32 J/kg.K. The
magnetic entropy value for x=0.2 is down to 0.91 J/kg.K at 335 K. Luong [88] examined the
magnetocaloric properties of La1-xCdxMnO3 (x = 0.1, 0.2, and 0.3) manganites. Under
H=1.35 T magnetic field, |∆SM| value for x=0.3 as 2.88 J/kg.K at 140 K and for x=0.2, 1.01
J/kg.K at 150 K. Besides, it was reported that there were significant differences between the
temperature value and Tc, where the |∆SM| peak values were observed. Moreover, the
resistance of the samples increased with the Cd ratio. These abnormalities have been
attributed to the uniform distribution of the grains in the sample. Hamad [89] examined the
magnetocaloric properties of the
La1−x Cd
x
MnO3 alloy. The distribution of magnetic
entropy change of the La1−x Cd x MnO3 alloy is much more uniform than gadolinium. This is
the desired characteristic for an Ericsson cycled magnetic refrigerator. Also, in La1−x Cd
x
MnO3 alloy, it was seen that temperature ranges from 150 K to room temperature could be
obtained with different Cd concentrations. For this reason, La1−x Cd x MnO3 alloys make it
possible to be used as a magnetic cooler in various temperature ranges.
A significant number of studies have been carried out on the understanding of magnetocaloric
properties of La1-xPbxMnO3 manganites. Chau [90] examined magnetocaloric properties of
La1-xPbxMnO3 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) alloys and reported that the |∆SM| value with
increasing Pb ratio increased to x=0.3 and decreased again for higher Pb values. The
maximum |∆SM| value for the x=0.3 sample was measured as 1.35 T at 358 K and for the
magnetic field 1.53 J/kg.K. In another study [91] he measured |∆SM| and
∆Tad values in La1-xPbxMnO3 (x = 0.1, 0.2, 0.3) alloys and observed the maximum |∆SM|
value in x=0.2 sample. Under 1.5 T magnetic field, ∆Tad values were reported as 0.68 K at
292 K for x=0.2 and 1 K at 349 K for x=0.3. Tozri [92], said the |∆SM| value, for
La0.8Pb0.1MnO3 sample, under 1 T magnetic field as; 0.43 J/kg.K at 201 K.
For La1-xNaxMnO3 alloy, Zhong [20, 93] showed that the |∆SM| peak temperature could be
adjusted within the range of 195-334 K. On the other hand, for La1-xKxMnO3 alloy, he has
shown that it can be adjusted in the range of 230-334 K. For La1-xNaxMnO3 alloy, under 1 T
magnetic field change, it has been reported that the |∆SM| value for x=0.075, 0.1, 0.165 and
0.2 is respectively; 1.32, 1.53, 2.11 and 1.96 J/kg.K [20]. Das [94] later examined the
magnetocaloric properties of La1-xKxMnO3 (x = 0.05, 0.1, 0.15) alloy produced by the
pyrophoric method. In this study, it was explained that adding potassium increases Curie
temperature in the structure to 264 K for x=0.05 and 310 K for x=0.15. At the same time the
highest |∆SM| value, under 1 T magnetic field change for La0.85K0.15MnO3 has been
reported as 3 J/kg.K at 310 K. Again, the same author [95] examined the magnetic and
magnetocaloric properties of the La1−xKxMnO3 alloy produced in nano-size by using the
pyrophoric method. It has been detected that the Curie temperature varies between 260 and
309 K, depending strictly on the K ratio. Magnetic entropy changes and adiabatic changes
have been detected to increase proportionally with the K ratio. It has been reported that
maximum |∆SM| ve ∆Tad values for La0.85K0.15MnO3 alloy, under 1 T magnetic field
change, are respectively; 3 J/kg.K and 2.1 K. In 2011, Juan [96] examined the relationship
between magnetocaloric properties and degree of calcination in the nanoparticle La1alloy. For samples calcined at 600 C0, 800 C0 and 1000 C0 , in the
xKxMnO3
La0.85K0.15MnO3 alloy, under 2 T magnetic field change, the |∆SM| values at 274 K have
been reported as; 2.02, 3.06 and 3.56 J/kg.K respectively. Tang [97] found that La1xAgxMnO3
(0 ≤ x ≤ 0.3) alloys have a huge magnetocaloric effect. In La0.8Ag0.2MnO3
alloy, for 1 T magnetic field exchange, the |∆SM| value has been reported as; 3.4 J/kg.K.
This value is higher than the value of the Gd element. Coşkun [98] examined the magnetic
and magnetocaloric properties of La1−x Ag
x
MnO
3
(0.05 ≤ x ≤ 0.25) alloy. The
Curie temperature increased from 200 K for x=0.05 to 290 K for
x=0.25, depending on the concentration of Ag. Under 3 T magnetic field
change, relative cooling power for x=0.1, 0.15 and 0.25, were reported as; 82.49, 82.61 and
127.37 J/kg respectively. Gamzatov [99] studied the magnetocaloric properties of
La0.9Ag0.1MnO3, La0.8Ag0.2MnO3, and La0.8Ag0.15MnO3 alloys. When the La0.8Ag0.15MnO3
alloy is sintered at 1373 K, under 2.6 T magnetic field change, the |∆SM| value has been
reported as 2.8 J/kg.K at 270 K. Aliev [100] examined the magnetocaloric properties of La1xKxMnO3
(x = 0.05, 0.1, 0.11, 0.13, 0.15, 0.175) alloys. The highest adiabatic change was
reported as 2.05 K and 1.66 K for La0.87K0.13MnO3 and La0.85K0.15MnO3 alloys under 1 T
magnetic field change.
In 2014, İzgi and his colleagues [30] examined the magnetic and magnetocaloric properties of
La0.94Bi0.06MnO3 alloy in detail. The structural dispersions observed in La0.94Bi0.06MnO3 alloy
from the ideal cubic lattice to orthorhombic lattice are related to the effect of Jahn-Teller and
the polarized 6s2 lone-pair characteristic of Bi3+ ions. Magnetization measurements revealed
that tiny amounts (x=0.06) of Bi doping in LaMnO3 alloy caused ferromagnetic regulation.
For the sample, under 1 T magnetic field change, at 209 K, considerably high magnetic
entropy change ( |ΔSm|=1.58 J/kg.K) has been reported. In 2015, Kolat and colleagues [34]
systematically examined the magnetocaloric properties of La1-xBixMnO3 (x=0.01, 0.03, 0.06,
0.1 and 0.2) alloys. For x=0.01, 0.03, 0.06, 0.1, 0.2 Curie temperatures reported as; 234, 224,
209, 198, 149 K, respectively. Saturation magnetization has been reported as; 89, 88, 85, 84,
80 emu/g, respectively. Ferromagnetism observed in La1-xBixMnO3 alloys is explained by
ferromagnetic super-exchange interaction between Mn3+-O-Mn3+ ions. It has been reported
that the |ΔSm| value, under 1 T magnetic field change, has decreased from the value of 2.42
J/kg.K for x=0.01 to 0.79 J/kg.K for x = 0.2. These |ΔSm| values obtained are comparable to
the magnetic entropy change observed for many manganites. For example, 1.58 J/kg.K value
observed for X=0.06, is comparable to the value [101] of 1.55 J/kg.K, which is measured at
La0.67Sr0.33MnO3 alloy in the same magnetic field, and the value [102] of 1.6 J/kg measured
in the La0.67Ba0.33MnO3 alloy.
3.2.1.2. La(Ca-A’)MnO3 (A’= Sr, Ba, Pb, K, Na, Ag, Mg)
As discussed in detail in section 3.2.1.1, La1-xCaxMnO3 alloys exhibit the highest
magnetocaloric effect among existing manganites [15,21,26,31,40-43, 56,59, 60-64].
However, since Curie temperatures are below room temperature, where the change in
magnetic entropy is seen in general, it limits the use of La1-xCaxMnO3 alloys as a magnetic
cooler at room temperature. For this reason, new dopings are being investigated that raise
Curie temperature around room temperature without altering the high magnetic entropy
change in La1-xCaxMnO3 alloys. In this context, a large number of studies have been carried
out on the displacement of Ca element with elements such as Sr, Ba, Pb, K, Na, Ag. Phan
[14, 103] reported a relatively high magnetocaloric effect in a monocrystalline
xSrxMnO3
La0.7Ca0.3-
(x = 0.05, 0.10, 0.20 and 0.25) alloy, around room temperature. For x=0.05, under
5 T magnetic field change and at 275 K, the |∆SM| value measured as 10.5 J/kg.K. This
value is more significant than the magnetic entropy change of the Gd element. Therefore,
monocrystalline manganite is one of the most intriguing candidates for magnetic cooling at
room temperature. Again, a lot of studies have been done on the
polycrystalline samples of the same example. In these studies, it
was observed that there was a decrease in magnetic entropy
change as a result of the increase in Sr ratio. Sun [104] reported
the |∆SM| value at 315 K for 2 T magnetic field change in the alloy of
La0.7Ca0.2Sr0.1MnO3 as 2.85 J/kg.K. Again, Li [105] reported the |∆SM| value, at
317 K for 2 T magnetic field change in the La0.5Ca0.3Sr0.2MnO3 alloy, as 1.52 J/kg.K. In a
study conducted by Gou [22], it was reported that there was a structural phase transition in
La0.75Ca0.25-xSrxMnO3 alloy depending on the Sr ratio.
For x≤0.125, the
composition has an orthorhombic phase, while for X≥ 0.125 it has a
rhombohedral phase [22]. Kim [106] examined the magnetic and magnetocaloric
properties of La0.7Ca0.3-xSrxMnO3 (x = 0.120, 0.135 and 0.150) alloy to elucidate the effect
of lattice structure on Tc and magnetocaloric properties. For X=0.135, the magnetization
curve observed a two-phase reduction in the values 309 and 320 K corresponding to the
Curie temperatures of orthorhombic and rhombohedral phases, while the samples x= 0.12
and 0.15 exhibit the typical magnetization behavior at Curie temperatures of 300 and 323 K.
Also for X=0.12, 0.13 and 0.15, the |∆SM| value measured as; 1.87, 1.72 and 1.7 J/kg.K ,
respectively. Studies have revealed that the increase in Sr ratio has a positive effect on Curie
temperature, especially when shifting from orthorhombic to rhombohedral phase, although it
has a sharp increase in Curie temperature, it has an adverse impact on |∆SM| [14,22,101106]. Mira [21] conducted a comprehensive study of the magnetic and magnetocaloric
properties of La2/3(Ca1-xSrx)1/3MnO3 (x = 0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1) alloys.
Depending on the Sr ratio, the decrease in |∆SM| value is attributed to the change like the
magnetic phase transition. In the study, the samples produced for the x•0.15 ratio showed
the first-degree magnetic phase transition. At higher Sr rates, it was observed that the nature
of phase transition changed from first to second degree. Phan [82] examined magnetocaloric
properties of La0.7Ca0.3-xBaxMnO3 (x= 0.12, 0.24 and 0.3) alloys and reported that the
|∆SM| value decreased by increasing Ba ratio. Under 1 T magnetic field change, the |∆SM|
value is 1.85 J/kg at 298 K for X=0.12.K, 1.72 J/kg.K at 320 K for x=0.24 and 1.6 J/kg.K at
336 K for x=0.3 is given. Sun [107] examined the magnetocaloric properties of
La2/3(Ca,Pb)1/ 3MnO3 alloy and |∆SM| value for 7 T magnetic field change is reported as;
7.5 J/kg.K at 290 K and ΔTad value is reported as 5.6 K. The value of
|∆SM| for
La2/3(Ca,Pb)1/ 3MnO3 was lower than the value of |∆SM| for La2/3Ca1/3MnO3 alloy under the
same field change [102]. Phan [108] studied the magnetocaloric properties of
La0.6Ca0.3Pb0.1MnO3, La0.7Ca0.2Pb0.1MnO3, and La0.7Ca0.1Pb0.2MnO3 alloys. The maximum
|∆SM| value under 1.35 T magnetic field change for La0.7Ca0.1Pb0.2MnO3 has been reported
as; 3.72 J/kg.K. at 337 K. Hanh [109] examined magnetocaloric properties of La0.7Ca0.3xPbxMnO3
(x = 0.05, 0.01, 0.15 and 0.2) alloys. Under the magnetic field change of 1.35 T,
for x=0.05 and 0.2 samples, respectively; at 270 and 337 K, the same |∆SM| value (3.72
J/kg.K) reported. The magnetocaloric properties of La0.7Ca0.3-xKxMnO3 (x = 0.05, 0.075 and
0.1) polycrystalline perovskites were examined by Bejar [110]. Under 2 T magnetic field
change, |∆SM| values has been reported as; for x=0.05 3.95 K/kg.K at 270 K, for x=- J/kg.K at 281 K and for x=0.1 3.49 J/kg.K at 272 K, respectively. Koubaa [111]
examined the magnetic and magnetocaloric properties of La0.65Ca0.35-xNaxMnO3 alloy, and it
has been reported that the Curie temperature increased from 248 K for x=0 to 315 K for
x=0.2, with the increase of Na ratio. Under 5 T magnetic field change, the maximum |∆SM|
value has been reported as; 3 J/kg.K for x=0.05 and 5.8 J/kg.K for x=0.2. The magnetic and
magnetocaloric properties of La0.5Ca0.5- xNaxMnO3 alloy were studied by Mehri [112], and
he reported that with the increasing Na ratio, Curie temperature decreased and magnetic
entropy change increased.
The same author [113] examined the magnetic and
magnetocaloric properties of the La0.5Ca0.5-xAgxMnO3 alloy and observed characteristics
similar to those found in Na-doped perovskites. In recent years, Kolat [33] studied the
magnetic and magnetocaloric properties of La0.67Ca0.33-xMgxMnO3 (x = 0, 0.02, 0.05, 0.1,
0.2, 0.33) alloys. It has been reported that the Curie temperature decreases from 267 K for
x=0, to 96 K for x=0.33, along with the amount of Mg. The reason for the decrease in Curie
temperature and similarly in saturation magnetism was attributed to the weakening of
ferromagnetism in the Mg-doped samples. Under 1 T magnetic field change, |∆SM| value
is decreased from 4.07 J/kg.K for x = 0 to 0.41 J/kg.K for x=0.33. This reduction in
magnetic entropy change is attributed to the decrease in the saturation magnetism and the
transformation of the nature of the magnetic phase transition from first degree to second
degree. As can be seen from the results, combining A’= Sr, Ba, Pb, K, Na, Ag, Mg
compounds instead of Ca in La(Ca-A’)MnO3 alloy causes an increase in Curie temperature
and decrease in magnetic entropy change in general. With the determination of appropriate
doping elements, manganites can be produced which provides ample magnetic entropy
change at suitable temperatures and can be used as a magnetic cooler at room temperature.
3.2.1.3. La(Sr-A’)MnO3 (A’= Ba, K, Ag, Mg)
Phan
[79],
reported
considerably
sizeable
magnetic
entropy
change, |∆SM|=2.26 J/kg.K, under 1 T magnetic field change, at 354 K, in the
La0.6Sr0.2Ba0.2MnO3 alloy. Koubaa [114] examined magnetocaloric properties of La0.7Sr0.3xAgxMnO3
(x = 0.05, 0.1, 0.15 and 0.2) composites and observed that the Curie temperature
decreased from 365 K to 286 K with the increase of Ag element from x=0 to x=0.2. The
most considerable magnetic entropy change, for La0.7Sr0.2Ag0.1MnO3 alloy, under 1 and 7 T
magnetic field changes, has been reported as respectively; 0.9 and 4.5 J/kg.K. Again, by the
same scientist, the magnetocaloric properties of La0.7Sr0.3-xKxMnO3 (x = 0.05, 0.1, 0.15 and
0.2) alloys have been examined [115]. In this sample, it was observed that the Curie
temperature decreases from 365 K for x=0, to 328 K for x=0.2, depending on the K value.
Under 1 T magnetic field change, |∆SM| value for x=0.05 and 0.15 samples has been
reported as; 1.37 and 1.2 J/kg.K. Wang [116] studied the magnetic and magnetocaloric
properties of La 0.67 Sr 0.33-x Mg x MnO 3 (x = 0, 0.05, 0.15 and 0.2) alloy. With the increase
of Mg ratio, both Curie temperature and saturation magnetization has been reported to
decrease. Under 5 T magnetic field change, the |∆SM| value for x= 0, 0.05, 0.15, 0.2,
measured as; 2.49, 1.28, 1.37 and 1.3 J/kg.K respectively. Mg-doped manganites have
shown almost a constant magnetocaloric effect, in a considerably wide temperature range
(50 K to 300 K), which is essential for magnetic cooling.
3.2.1.4. (La-A)CaMnO3 (A=Nd,Tb, Dy, Gd, Ce, Y, Sm, Bi, Eu, Ho)
Many studies have been carried out on the (La-A)CaMnO3 group of manganites, and it has
been shown that magnetocaloric properties can be improved by substitution of the La
element with elements such as Nd, Bi, Tb, Dy, Gd, Ce, Y, Sm, Eu, Pr, Ho. Wang [117]
examined the influence of the substitution of La element with Nd element on
magnetocaloric properties in La0.7-xNdxCa0.3MnO3 (x = 0, 0.05, 0.1, 0.15 and 0.20) alloy.
Highest |∆SM| value, under 1 T magnetic field change, has been reported for x=0.2 as 2.31
J/kg.K
at
213
K.
Chen
[16]
examined
the
magnetocaloric
properties
of
(La1−xRx)2/3Ca1/3MnO3 (R = Gd, Dy, Tb, Ce x = 0–0.2) alloys, and she was observed that
Curie temperature decreased due to partial displacement of the La element with Gd, Dy, and
Tb elements. Interestingly, the maximum |∆SM| conversion for all doping elements was
observed at the ratio of x = 0.1. For (La0.9Dy0.1)2/3Ca1/3MnO3 component, under 1.5 T
magnetic field change and at 175 K, the highest |∆SM| value has been reported as 6.06
J/kg.K. Zhang [41] pointed that the partial displacement of La element with Y element, as in
La0.60Y0.07Ca0.33MnO3 alloy, reduces both magnetocaloric properties and the Curie
temperature. The |∆SM| value measured as 1.46 J/kg.K, under 3 T magnetic field change at
230 K. Anwar [118] examined the effect of Sm doping at different ratios instead of La, on
magnetic and magnetocaloric properties in La0.7-xSmxCa0.30MnO3 (0≤x≤0.3) alloy. As a
result of magnetization and Arrott analysis, it was concluded that the sample exhibited a
first-degree ferromagnetic phase transition for x=0 and that the other samples exhibited a
second-degree ferromagnetic phase transition. Curie temperature has decreased from the
value of 182 K for x=0.05 to 109 K for x=0.3. Although the change in magnetic entropy
(1.75 J/kg.K under 1 T magnetic field) is quite significant for x=0, the |∆SM| value has
decreased considerably due to irregularities caused by Sm in Sm-containing samples.
Another reason for the reduction in magnetic entropy change is that the nature of the phase
transition shifts from first to second degree. Zhang [119] examined the magnetocaloric
properties of La0.65−xEuxCa0.35MnO3 (x = 0, 0.05, 0.10 and 0.15) alloy prepared by sol-gel
method. With the increasing of the Eu ratio, Curie temperature has decreased. The most
substantial magnetic entropy exchange, under 1.5 T magnetic field change, has been
reported as 5.778 J/kg.K for La0.6Eu0.05Ca0.35MnO3 alloy. Ning [120] examined the
magnetic and magnetocaloric properties of nanoparticle (La0.8Ho0.2)2/3Ca1/3MnO3 , ranging
from 50 to 200 nm in size, and (La0.5Ho0.5)2/3Ca1/3MnO3 alloys, produced by the sol-gel
method. Under 5 T magnetic field change, for (La0.8Ho0.2)2/3Ca1/3MnO3 at 100 K, |∆SM| has
been reported as 1.19 J/kg.K, and for (La0.5Ho0.5)2/3Ca1/3MnO3 at 152 K, |∆SM|= 2.03
J/kg.K. Anwar [121] examined the magnetic and magnetocaloric properties of
x)CexCa0.25MnO3
La(0.75-
(x = 0.0, 0.2, 0.3 and 0.5) alloy and reported that Curie temperature
decreased due to Ce concentration (for x= 0, 0.2 ve 0.3 respectively; 255, 213 and 150K).
Under the magnetic field changes of 1.5 and 4 T, the most considerable magnetic entropy
change has been reported as 3.31 and 6.4 J/kg.K for La0.55Ce0.2Ca0.25MnO3. Gencer [23]
examined the effect of Bi doping on sintering temperature and magnetic and magnetocaloric
properties of La0.62Bi0.05Ca0.33MnO3. Interestingly, the result of Bi dopings on magnetic and
magnetocaloric properties has been determined to have a positive effect on sintering
temperature as well. Even a small rate of Bi (x=0.05) has been observed to reduce the
sintering temperature to 200 C0. The Bi-doped sample also exhibited a remarkably high
magnetic entropy value. Under 1 T magnetic field and at 248 K, this value has been reported
as 3.5 J/kg.K. This value is superior the magnetic entropy change value of the initial sample
of the La0.67Ca0.33MnO3 alloy. Although magnetic entropy change is high enough, the
decrease in Curie temperature restricts the use of these samples as magnetic coolers at room
temperature. Atalay [24] systematically studied the effect of Bi dopings on the magnetic and
magnetocaloric properties of
La0.67_xBixCa0.33MnO3 (x= 0, 0.05, 0.1, 0.2) alloy. The
temperature of Curie was reported as 267, 248, 244 and 229 K for samples of x=0, 0.05, 0.1
and 0.2, respectively. Magnetic entropy change, |∆SM|, has reached the maximum value of
6.08 J/kg.K for the x=0.05 sample, under 3 T magnetic field change. The most significant
decrease in magnetic entropy change was observed in the x=0.2 sample. Gutierrez [122]
studied the magnetocaloric properties of (La0.55Bi0.15)Ca0.3MnO3, alloy. Curie temperature
has been reported to be 230 K. Neutron diffraction experiments have revealed that under this
temperature there are localized antiferromagnetic phases within the ferromagnetic phase of
the sample. Under 9 T magnetic field change, |∆SM|=1.1 J/kg.K and ∆Tad= 2.3 K were
reported as. In La0.7Ca0.3MnO3 alloy, as a result of the substitution of La elements with
different elements, magnetic entropy change in general increases or remains constant, while
Curie temperature, where entropy change is observed, is generally observed to change into
temperatures beneath room temperature. This situation restricts the use of manganites as a
magnetic cooler at room temperature and shows that it may be a beneficial magnetic cooler
applicant in the range of about 210-270 K.
3.2.1.5. (La-A)SrMnO3 (A=Er, Eu, Gd, Ce, Pr, Nd, Bi)
As a result of the substitution of La ions with Er and Eu ions in La0.7-xErxSr0.3MnO3 (x =
0.014, 0.035, 0.14 and 0.21) and La0.7-xEuxSr0.3MnO3 (x = 0.035, 0.14 and 0.21) alloys,
Amaral [123] revealed that the Curie temperature decreases around room temperature, and
the magnetic entropy change remains constant. Bouderbala [124] observed that
La0.7−xEuxSr0.3MnO3 (x=0, 0.1, 0.2 and 0.3) polycrystalline manganites
exhibited
structural
phase
transitions
from
rhombohedral
symmetry to orthorhombic symmetry for x≥0.1. It is seen that the Curie
temperature decreases from the value of 343 K for x=0.1 to 272 K for x=0.3, depending on
Eu concentration. All samples have been reported to exhibit high magnetic entropy change at
a level that can be used as a magnetic cooler around room temperature. Similarly, Sudharshan
[125] studied the magnetic and magnetocaloric properties of La0.7-xEuxSr0.3MnO3 (x=0.0, 0.1,
0.2, 0.3) alloy. Similar to previous studies, the crystalline structure
has been transformed from the rhombohedral phase for x=0; to
orthorhombic phase for x≥0.1. Under 6 T magnetic field change, the magnetic
entropy change value is increased from 3.88 J/kg.K for x=0 to the value of 5.03 J/kg.K for
x=0.03. The magnetocaloric properties of La0.7−xPrxSr0.3MnO3 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5,
0.6, 0.7) alloy were measured by Gamzatov [126], using direct and indirect methods.
Magnetic entropy change under 1.8 T magnetic field change was measured in the range of;
|∆SM|=1.84 and 4.21 J/kg.K. The adiabatic change was measured between ∆Tad=1.09 and
1.75 K. Phromchuai [127] examined the magnetocaloric properties of the La0.75xGdxSr0.25MnO3
(x=0-0.3) alloy produced by the sol-gel method. Although Curie temperature
decreases with raising Gr ratio, the magnetic entropy change, under 0.7 T magnetic field
change was increased from the value of 0.93 J/kg.K for x=0 to 1.14 J/kg.K for x=0.3. A
significant magnetic entropy change was first reported by Kallel [128] in the Ce-doped
La0.7Sr0.3MnO3 alloy. For (La0.56Ce0.14)Sr0.3MnO3 alloy, the magnetic field change has been
reported as; 1.55 and 4.78 J/kg.K, under 1 and 5 T magnetic field changes at 357 K. Anwar
[129] observed that the Curie temperature in La0.7−xCexSr0.3MnO3 (0≤x≥0.3) alloy,
decreased from the value of 370 K for x=0 to 310 K for x=0.3, depending on the Ce
concentration. It has been observed that the magnetic entropy change increases up to x=0.15,
with the increasing Ce ratio. Under 2 T magnetic field change, for La0.55Ce0.15Sr0.3MnO3
alloy, the maximum magnetic entropy change has been reported as; 2.12 J/kg.K at 356 K.
Çetin [130] noted that the Curie temperature in (La1−xSmx)0.67Pb0.33MnO3 (x = 0, 0.1, 0.2,
0.3) alloy decreased from the value of 358 K for x=0 to 286 K for x=0.3, depending on Sm
concentration. Under 3 T magnetic field change, the adiabatic change is measured as
ΔTad=1.3 K for x = 0.3. In the study of Dhahri [131], magnetic entropy change in La0.7xEuxBa0.3MnO3
(x ¼=0.05, 0.1 and 0.15) alloy, for x=0.15, has been reported as 2.3 J/kg.K,
under 1 T magnetic field change, at 298 K. Barik [132] examined the impact of Bi dopings on
the magnetic and magnetocaloric properties of La0.7−xBixSr0.3MnO3 (x=0.0–0.4) alloy. It is
observed that the Curie temperature decreases from the value of 365 K for x=0 to 191 K for
x=0.3. Under 5 T magnetic field change, it has been observed that the magnetic entropy
change rises from the value of 4.56 J/kg.K for x=0 to 5.02 J/kg.K for x=0.05. With an
increasing Bi ratio, |∆SM| has decreased again (3.1 J/kg.K for x=0.3).
3.2.1.6. (A1-xA’x)MnO3 (A=Nd, Pr, Sm, Gd, Na, Eu A’=Ca,Sr, Pb, Bi)
In recent years, studies on magnetocaloric properties of manganites with charge-order (CO)
configuration have begun to increase. The most characteristic features of CO manganites are
that they exhibit two consecutive phase transitions. The first of these is the first phase
transition from the antiferromagnetic phase to the ferromagnetic phase at lower temperatures.
The other is the second-degree transition from ferromagnetic metallic phase to paramagnetic
insulator phase at higher temperatures [133]. CO is a configuration based on the localization
of charge carriers and naturally competing with ferromagnetic double-exchange activation.
CO in manganites is a form of configuration in which transition metals (Mn3+ and Mn4+) with
different oxidation state form similar to the arrangement of checkers. This configuration
usually causes charges to be localized and prevents electrons from hopping from one ion to
another. This provides the structure of a semiconductor or insulator character.
CO
configuration in manganites usually occurs in the case of mixed-valence and x= 1/8, 1/2 and
3/4 ratios.
Si and colleagues [134] examined magnetocaloric properties in Nd2/3Sr1/3MnO3 alloy, under 1
T magnetic field change and at 257.5 K the |∆SM| value has been reported as 3.25 J/kg.K.
In Nd1−xSrxMnO3 (x=0.3, 0.5) monocrystalline alloy, for x=0.3, the Nd0.7Sr0.3MnO3 alloy
under 1.4 T magnetic field change and at 203 K, in an extensive temperature range; the
adiabatic change has been reported as, ∆Tad=20 K and |∆SM|= 3.82 J/kg.K [135].
For x=0.5, an ample magnetic entropy change was observed in the antiferromagnetic CO
alloy given with Nd0.5Sr0.5MnO3 , under quite small magnetic field change (1.4 T) and around
the Neel temperature of 150 K [135]. Phan [136] informed that the (Nd1−xYx)0.7Sr0.3MnO3
(x = 0 and 0.07) alloy exhibited first-order phase transition for x=0 and second-order phase
transition for X=0.07. The Curie temperature was reported as 240 K for x=0 and 170 K for
x=0.07. For the studied samples; under 5 T magnetic field change, magnetic entropy change
was measured as 8 J/kg.K, and the relative cooling power RCP was measured in the range of
200-246 J/kg. Beiranvand [137] examined the magnetic and magnetocaloric properties of
Gd1−xCa xMnO3 and Nd1−x Ca
x
MnO3 (
) alloys. The highest magnetocaloric
properties were observed in both samples at temperatures below 140 K and low x values.
Low magnetic hysteresis and high entropy change have made these materials magnetic
refrigerants sought at low temperatures. In his study for R0.15Ca0.85MnO3 (R = Y, Gd and Dy)
alloy, Dhal [139] demonstrated that these alloys exhibit an antiferromagnetic phase transition
at Neel temperatures of 111, 119, and 112 K, respectively, for R=Y, Gd, and Dy in the
magnetic field measurements under 0.5 T magnetic field. The magnetocaloric effect was
calculated from isothermal magnetization curves, and the inverse magnetocaloric effect was
seen in the antiferromagnetic transition region. Magnetocaloric properties of the
Nd0.5Sr0.5MnO3 alloy were first reported by Sande [139]. Under 1 T magnetic field change
and TCO= 155 K a substantial magnetic entropy change, such as 2.8 J/kg.K, has been
observed. Besides, it has been found that the measured |∆SM| value around the first phase
transition is approximately three times greater than the observed value around the second
phase transition. This is related to the suppression of CO-configuration as a result of the
heightened state of electron mobility under the applied magnetic field. In her study for the
same sample, Chen [1123] reported a much larger |∆SM| =7.5 J/kg.K value, under 1 T
magnetic field change and at 183 K. In 2010, Fan [140] reported a considerable decrease in
the Curie temperature and magnetic entropy value of Nd0.5Ca0.25Sr0.25MnO3, alloy. The
|∆SM| value was measured as 0.77 J/kg.K, under 1 T magnetic field change, and at 175 K.
The same author [141] reported the value of |∆SM| =3.14 J/kg.K,
under 1.5 T magnetic field for Nd0.6La0.1Sr0.3MnO3 alloy. Nanto [142] stated that
Nd0.5Sr0.5MnO3 monocrystalline sample was exhibited antiferromagnetic CO phase transition
at the temperature of TCO= 152 K. On the other hand, he reported that the sample showed a
ferromagnetic, paramagnetic phase transition at Tc= 272 K. Maximum magnetic entropy
change, |∆SM|, has been reported as 1.65 J/kg.K in first-order phase transition and -1.13
J/kg.K in the second order gas transition. Cao [143] examined the magnetocaloric properties
of Eu1−xSrxMnO3 (x = 0.5, 0.6, 0.7 and 0.8) alloy systematically. Under 5 T magnetic field
change, the most considerable magnetic entropy change measured as 0.31 J/kg.K for sample
Eu0.5Sr0.5MnO3. It was observed that the magnetic entropy change of the Eu0.5Sr0.5MnO3
sample, which was subjected to thermal treatment at 800 C0 again for 12 hours, increased to
the value of 3.03 J/kg.K.
Ayaş [144] studied the impact of Pr doping on the magnetic and magnetocaloric properties in
(La1−xPrx)0.85Ag0.15MnO3 (0.0 ≤ x ≤ 0.5) alloy. Rietveld analysis revealed
that
the
alloy
has
a
rhombohedral
phase
for
x≤0.2
and
orthorhombic phase for x≥0.3. It has also been observed that the
average
particle
size
is
reduced
depending
on
the
Pr
concentration. All samples exhibit a second-order magnetic phase transition. The Curie
temperature decreases from 262 K to 138 K, depending on the increase in Pr value. Under 5
T magnetic field change, magnetic entropy change and relative cooling capacity were
measured in the range of, respectively; |∆SM| = 7.9-2.88 J/kg.K and RCP=-
J/kg.
In 2000, Chen [145] examined magnetocaloric properties of Pr1-xSrxMnO3 (x = 0.3, 0.4, and
0.5) polycrystalline manganites, and they measured the maximum |∆SM| value for x=0.5
as 7.1 J/kg.K, under 1 T magnetic field change, at 160 K. Among the samples prepared for
x=0.3, 0.4 and 0.5, the Pr0.5Sr0.5MnO3 example prepared for x=0.5 exhibits CO configuration
at the CO transition temperature given as TCO=161 K. Chen [146] explained that the
temperature of Curie increased from 205 K to 267 K, while the CO transition temperature
increased from 161 K to 183 K, as a result of the displacement of Pr with Nd in (Pr1yNdy)0.5Sr0.5MnO3
(y = 0, 0.3, 0.5, 0.7 and 1.0). For all samples, it has been observed that the
|∆SM| value (6.5 J/kg.K for x=0, 8J/kg.K for x=1) remains almost constant under 1 T
magnetic field change. It is designated that the Curie temperature decreases from the value of
310 K for x=0 to 252 K for x=0.4, depending on the Bi doping in the (Pr1-xBix)0.6Sr0.4MnO3
(0 ≤ x ≤ 0.4) alloy [147]. For 0=0.06, the magnetic entropy change has been reported as
1.11 J/kg.K and 4.78 J/kg.K, under 1 and 7 T magnetic field changes. In another study,
Gomes [148] examined the magnetocaloric properties of Pr1-xCaxMnO3 (0.3 ≤ x ≤ 0.45)
manganites and reported positive and negative magnetic entropy changes in a relatively high
value. For Pr0.68Ca0.32MnO3 sample, the |∆SM| value was measured as positive 24 J/kg.K at
21.7 K and negative 27 J/kg.K at 31 K, under 5 T magnetic field exchange. As can be
comprehended from the results, these samples are particularly suitable for magnetic cooling
at low temperatures. The same authors [149] investigated the contribution of COconfiguration to magnetic entropy change in Pr1-xCaxMnO3 (0.2 ≤ x ≤ 0.95) alloy in
another study. The value of ∆SM observed in Co manganites was shown to be due to the
overlapping of negative entropy change resulting from the spin configuration (∆Sspin) and the
positive
entropy
change
resulting
from
charge-order
configuration
((∆SCO).
Immediately after that, Phan [150] has reported a reasonably high
|∆SM| value (8.52 J/kg.K) in monocrystalline Pr0.63Sr0.37MnO3, alloy, under 5
T magnetic field exchange at 300 K. In a relatively low magnetic field change, obtaining a
substantial magnetic entropy change is one of the characteristics sought to be applicable to
active magnetic refrigerants at home. For commercial applications of magnetic cooling at
room temperature, a monocrystalline sample of Pr0.63Sr0.37MnO3 can be a candidate. To
completely understand the properties of Pr-based samples, the same author [151] examined
the magnetocaloric properties of Pr1-xPbxMnO3
(0.1 ≤ x ≤ 0.5) alloy in detail. Under
1.35 T magnetic field change, |∆SM| value for x=0.1, 0.4 and 0.5 has been reported as;
3.91, 3.68 and 3.34 J/kg.K respectively. These values are greater than the magnetic entropy
change of the Gd element. More importantly, these entropy changes are obtained under a
relatively low magnetic field, which can be produced by permanent magnets. Bingham [152]
examined the magnetocaloric properties of the charge ordered alloy Pr0.5Sr0.5MnO3. It has
been observed that the system has exhibited paramagnetic-ferromagnetic phase transitions at
Tc=255 K and following this, at TCO=165 K , it has exhibited that phase transitions from CO
ferromagnetic phase to CO antiferromagnetic phase. The value of |∆SM|= 7.5 J/kg.K
obtained at TCO=165 K, is two times larger than the value of |∆SM|= 3.2 J/kg.K,
observed at Tc=255 K, under 5 T magnetic field change. In Pr0.5M0.1Sr0.4MnO3 (M = Eu,
Gd and Dy) alloy, the temperature of Curie for Eu, Gd and Dy doped samples were measured
as 270, 258 and 248 K respectively [153]. The Arrott plots designate that all samples exhibit
a second-order magnetic phase transition. Under 1 T magnetic field change, the magnetic
entropy change was reported as 1.37, 1.23 and 1.18 J/kg.K for M= Eu, Gd and Dy,
respectively.
Sarkar [154] studied the magnetocaloric properties of monocrystalline Sm0.52Sr0.48MnO3
alloy. At 125 K, |∆SM| value was reported as 5.9 J/kg.K, under 1 T magnetic field change.
In another study, it was observed that Sm1−xSrxMnO3 (x = 0.42, 0.44, 0.46)alloy was
exhibited CO configuration, for x=0.44 [155]. Magnetization measurements have revealed
that all samples show a first-order phase transition. For x=0.42, 0.44, 0.46, the Curie
temperature has been reported as Tc= 130, 143, 133 K. The highest magnetic entropy change
was measured as; |∆SM| =4.61 J/kg.K for x=0.44 under 5 T magnetic field change. It was
found to be RCP=151.42, 140.15, 135.91 J/kg for x=0.42, 0.44 and 0.46 under the same field
change. Zashchirinskii [156] examined the ceramic structure of Sm0.55Sr0.45MnO3 alloy and
the magnetocaloric properties of three different samples of the monocrystal structure
subjected to thermal treatment in oxygen and air environment. The temperatures at which the
maximum adiabatic change (ΔTad) observed were reported as 143.3 K for the ceramic
structure, 244 K for the monocrystal sample obtained in the oxygen environment, and 143 K
for the monocrystal sample obtained in the air environment. At these temperature values,
ΔTad changes were measured as 0.8, 0.41 and 0.4 K respectively.
The magnetic field-welded metamagnetic phase transition and magnetocaloric properties
were investigated in the Charge-Ordered Pr0.68Ca0.32-xSrxMnO3 (x=0, 0.1,0.18,0.26 and 0.32)
alloy, by Kolat [29]. In low Sr concentrations, (x=0 and 0.1), magnetization curves have
shown CO transition of around 185 K. In subsequent Sr concentrations, peaks representing
CO transition were observed to disappear. For x=0 and 0.1 samples, sharp step-like
metamagnetic transitions were seen in magnetization curves (M-H). At low Sr
concentrations, two different abnormal magnetic entropy changes were observed just below
and above the Curie temperature. The positive ∆SM value (0.45 J/kg.K for x=0.1, under 3 T
magnetic field change) found on Tc was attributed to the CO transition. The relatively high
magnetic entropy change (-26.2 J/kg.K for x=0 at 38 K, -6.5 J/kg.K for x=0.1 at 83 K, under
5 T magnetic field change) observed at lower temperatures is attributed to very sharp steplike metamagnetic transitions. For following Sr concentrations, negative magnetic entropy
change related to FM-PM phase transition was observed around Curie temperature. The
peak temperature at which ∆SM is seen has increased from the value of 203 K for
x=0.18 to 267 K for x=0.32 with the increasing Sr ratio. It is observed that the magnetic
entropy change, under 1 T magnetic field change, decreases from the value of -4.1 J/kg.K
for x=0.18 to -4.4 J/kg.K for x=0.32. Again, from magnetization measurements for
Pr0.67Ca0.33MnO3 alloy, it has been determined that this alloy has shown a CO-phase
transition of around 200 K [32]. The FM phase, which is more pronounced, has emerged
below 56 K. The sharp step-like metamagnetic transitions observed at 5 K are correlated
with phase separation. At 5K, after the sample was exposed to 7 T magnetic fields, it was
found that its magnetic properties show completely FM behavior. More interesting is that
even when the sample is heated at the temperature above the CO transition temperature, the
sample remains in the FM phase without turning to its former magnetic properties. The
change under 5 T magnetic field and the change in the highly negative magnetic entropy of
26.18 J/kg.K, observed at 38 K, are attributed to the metamagnetic transition. In recent
years, Gencer [35] has examined the magnetic and magnetocaloric properties of
Pr0.68Ca0.32−xBixMnO3 (x = 0, 0.1, 0.18, 0.26 and 0.32) alloy. Only for sample x=0, CO
phase transition was observed at Tco=200 K. For subsequent Bi concentrations, the chargeorder phase transition has been disappeared. With the increase in Bi concentration; the Curie
temperature, saturation magnetism, and magnetic entropy change have been observed to
decrease. The decrease in Curie temperature and saturation magnetization was attributed to
the non-homogenous magnetic structure and the attenuation of double-exchange interaction
in Bi-doped samples. The reduction in the magnetic entropy change (1.094 J/kg.K for x=0.1,
0.475 J/kg.K for x=0.32, under1 T magnetic field change) is due to the decrease in the
saturation magnetism and the difference in the nature of the phase transition. Several A-site
doped manganite's magnetocaloric properties are abstracted in Table 3.1.
3.2.2. Mn-SITE SUBSTITUTION IN MANGANİTES
As discussed above, the magnetic, conductance and magnetocaloric properties of manganites
in the A-site doping state, where the different ionic radius and oxidation state elements are
used, are obliquely changed depending on carrier density (Mn3+/Mn4+) or structural
parameters (Mn–O bond distance, Mn–O–Mn bond angle). In the case of Mn-site doping, in
addition to structural parameters, since various transition metals (TM) substitute Mn ions,
new transition metal ions supplemented to the structure, and Mn ions produce new exchange
interactions (Mn-Mn, Mn-TM, TM-TM) between them. This situation signifies that the
magnetic and conductance and therefore the magnetocaloric properties are directly affected.
For this reason, the doping of Mn-site in manganite is quite attractive. Until today, many
different transition metals (Fe, Cr, Cu, Al, Ni, Co, Sn, Si, Ru, Ti, Ga, V, Sb, Gd, In, Zn, Li) in
FM
metallic
manganites
(La0.67Ca0.33Mn1-xTMxO3)
and
CO-insulator
manganites
(La0.5Ca0.5Mn1-xTMxO3) have been replaced with Mn, and magnetic and magnetocaloric
properties of these manganites have been examined [157-213].
3.2.2.1 Mn-site substitution with Al
TKA [157] studied the effect of Al doping on the magnetic and magnetocaloric properties of
La0.57Nd0.1Sr0.33Mn1−xAlxO3
(0.0≤x≤0.3) alloy. The study explained that Curie
temperature varies between 238 and 342 K and is strictly related to Al concentration. It is
reported that under 1 T magnetic field change, magnetic entropy change increases from 2.31
J/kg.K for x=0 to 3.58 J/kg.K for x=0.3, depending on the quantity of Al. The high |∆SM|
change, occurring in a low area such as 1 T and a reasonably wide temperature range, has
brought these alloys to an essential point in the magnetic cooling field. Again, in Al-doped
La0.7Sr0.3Mn1-xAlxO3 (0 ≤ x ≤ 0.2) alloy, for x≥0.15, the samples showed similar
behavior to the Griffiths phase above the Curie temperature [158]. This abnormal
paramagnetic behavior is linked to the presence of ferromagnetic clusters within
paramagnetic domains. The Curie temperature has decreased from the value of 366.74 K for
x=0 to 226.44 K for x=0.2. It is reported that the magnetic entropy change calculated using
the phenomenological theoretical model under 0.01 T magnetic field decreased from |∆SM|
=162.34 erg/g.K for x=0 to |∆SM| =9.022 erg/g.K for x=0.2. Dhahri[159] studied the effect
of Al and Sn ions doping simultaneously on magnetic and magnetocaloric properties in the
La0.7Ca0.1Pb0.2Mn1_x-yAlx SnyO3 (0 ≤ x,y ≤ 0.075) alloy. It has been observed that the
Curie temperature is decreased from the value of 310 K for x=0 to 290 K for x=0.075.
Magnetic entropy changes were reported as; |∆SM| = 3.7, 2.7, 2.3 and 2 J/kg.K for x,y= 0,
0.025, 0.05 and 0.075, respectively.
3.2.2.2 Mn-site substitution with Co
In the study of Bau [160] for the La0.7Sr0.3Mn0.05Co0.95O3 alloy, which is rich in Co, Curie
temperature was measured at 190 K. Magnetic entropy change under 4.5 T magnetic field
change was reported as; |∆SM|= 1.41 J/kg.K. Although the |∆SM| value is meager, the
entropy change overspreading an extensive temperature range is a sought-after feature for the
technological applications of magnetic cooling. In La0.67Pb0.33Mn1−xCoxO3 alloy, Curie
temperature was measured as 297, 285, 272 and 260 K for x=0.15, 0.2, 0.25 and 0.3,
respectively, depending on the Co-doping [161]. Although Curie temperature decreases by
Co ratio, magnetic entropy change is increasing. Under 1 T magnetic field change, for
x=0.15, 0.2, 0.25 and 0.3, |∆SM| value was reported as; 2.73, 2.92, 3 and 3.22 J/kg.K
respectively. In another study [162], Curie temperature for x=0, 0.03, 0.06 and 0.08 was
reported as 360, 345, 324 and 316 K, respectively, depending on the Co concentration for
similar La0.67Pb0.33Mn1−xCoxO3 (x=0, 0.03, 0.06, 0.08) alloy. In addition, magnetic entropy
change was reported as 4.32, 0.58, 0.26, 0.19 J/kg.K. In the previous example [161], although
magnetic entropy change increased depending on the Co-ratio, |∆SM| decreased significantly
in Co-doped samples into this study. The discrepancies in these studies may be because of the
sample preparation conditions or the Co ratios that are determined differently. Zhang [163]
examined the magnetic and magnetocaloric properties of the La0.7Ca0.3Mn1-xCoxO3 (x=0–
0.05) alloy. Magnetization measurements showed that Curie temperature decreased from 270
K for x=0 to 215 K for x=0.05, depending on Co concentration. Magnetic entropy change,
under 1.5 T magnetic field change for x=0 has been reported as 5.9 J/kg.K and for x=0.05 as
4.8 J/kg.K. As can be seen from the results, the temperature range of semi-maximums
(δTFWHM) increases from 12 to 16 K, even though the |∆SM| change with increasing Co ratio
drops a small amount. Despite the increase in δTFWHM width, it has been determined that all
samples still exhibit a phase transition from the first order. In La0.7Sr0.3Mn1-xCoxO3 (x=0,
0.05,0.1) alloy, both Curie temperature and magnetic entropy change are strictly dependent
on Co quantity [164]. For x=0, 0.05 and 0.1, the Curie temperatures were measured,
respectively; 338, 260 and 300 K. Magnetic entropy change under 1.5 T magnetic field
change was reported as 1.36, 1.17 and 0.92 J/kg.K for x=0, 0.05 and 0.1 respectively. This
decrease in |∆SM| value was attributed to the weakening of the DE interaction between Mn3+
and Mn4+ due to the ions being added. In general, the sudden change in the phase transition
zone of magnetism and a high saturation magnetism causes a change in magnetic entropy. In
La0.7Sr0.3Mn1-xCoxO3 alloy, the number of Mn3+-Mn4+ pairs that interact with FM with the
doping of Co3+ and Co4+ ions instead of Mn leads to decrease in magnetization and |∆SM|
value. Magnetization measurements for La0.8Ba0.1Ca0.1Mn1-xCoxO3 (x = 0, 0.05 and 0.10)
alloy informed a large deviation between magnetization measured in zero field cooling (ZFC)
conditions and magnetization curves measured in magnetic field cooling (FC) conditions
[165]. his divergence between the ZFC and FC magnetization curves has been connected to
the canted ferromagnetic structure resulting from the conflict between the FM interactions
existing in the structure and AFM interactions. The Curie temperature has decreased from the
value of 282 K for x=0 to 214 K for x=0.1. In the study, it was found that all the samples
exhibited a second phase transition. Magnetic entropy change under 5 T magnetic field
change has been reported as 3.2, 2.5, and 0.8 J/kg.K, for x=0, 0.05, and 0.1 respectively. In
another study, the effect of Co-doping (Pr0.7Ca0.3Mn1-xCoxO3, 0≤x≤0.1) on magnetic
and magnetocaloric properties of Pr0.7Ca0.3MnO3 alloy with CO configuration
was examined [166]. While the Pr0.7Ca0.3MnO3 alloy exhibits CO configuration, it was
observed that CO configuration in Co-doped alloy was absent and all samples exhibited a
phase transition from a paramagnetic configuration to a ferromagnetic configuration. The
Curie temperature has increased from the value of 105 K for x=0 to 116 K for x=0.1.
Magnetic entropy change under 5 T magnetic field change was reported as |∆SM|= 0.8, 2.2,
3.1 and 3.2 J/kg.K , for x=0, 0.02, 0.05 and 0.1, respectively. As we can see from the results,
Co doping in perovskite manganites with FM configuration have an adverse effect on Curie
temperature and magnetic entropy change, whereas in CO ordered Tc and
|∆SM| are
observed to increase. Relative cooling power (RCP) was found to be 378.2 J/kg for
Pr0.7Ca0.3Mn0.95Co0.05O3 alloy, under 5 T magnetic field change. The reported RCP value is
92% of the RCP value measured under the same 5 T magnetic field of Gd element considered
as one of the most significant materials in the field of magnetic cooling.
3.2.2.3 Mn-site substitution with Cr
In La0.7Sr0.3Mn1–xCrxO3 (x=0, 0.2, 0.5 and 0.5) alloy, the temperature of Curie is measured as
369, 286, 242, 226 K for x=0, 0.2, 0.4 and 0.5, depending on the Cr ratio [167]. A
comparable decrease in Curie temperature was also observed in magnetic entropy change.
For x=0, 0.2, 0.4 and 0.5, |∆SM| value was reported as 1.27, 1.203,
0.473 and 0.279 J/kg.K. The first point that draws attention here is that until x=0.2,
|∆SM| value has almost never changed. Also, the temperature at which the |∆SM| change is
seen is adjacent to room temperature. In higher Cr ratios, the |∆SM| value has greatly
reduced. The effect of Cr doping in Pr0.6A0.4Mn1-xCrxO3 (A =Ca and Sr, x=0, 0.04) perovskite
structure on samples containing Ca and Sr was examined [168]. In the sample of
Pr0.6Ca0.4Mn1-xCrxO3 , it was observed that the CO antiferromagnetic configuration observed
for x=0 was converted into a ferromagnetic metallic phase with the doping of Cr. In the
sample of Pr0.6Sr0.4Mn1-xCrxO3 the structure for x=0 is already in the FM metallic phase, and
the Curie temperature and saturation magnetization have been observed to decrease with the
doping of Cr. Magnetic entropy change for x=0 in Pr0.6Ca0.4Mn1-xCrxO3 alloy was observed at
a negative value of 325 K and considerably small. As we move towards low temperatures,
∆SM increased and reached a maximum value of 255 K. At TCO =240 K , the sign of ∆SM has
changed. Around 230 K, the positive peak has given (∆SM= 0.656 J/kg.K for ∆H=5 T) .
Under 200K, ∆SM has been negative again. As a result of the
decreasing temperature, the value increases; ∆SM=-4.7 J/kg.K has reached
its value, at around 10 K for ∆H=5 T. Unlike the sample of Pr0.6Ca0.4MnO3 , in the sample of
Pr0.6Ca0.4Mn0.96Cr0.04O3, ∆SM has a negative value in a wide temperature
range. ∆SM measured as - 5.99 J/kg.K, under ∆H=5 T and at Tc=155 K. Unlike the
sample that contains Ca, magnetic entropy change in the sample that contains Sr is smaller.
When Tc=290 K; ∆SM falls from the value of - 2.51 J/kg.K for x=0 to ∆SM= - 2.08 J/kg.K
for x=0.04, with the increasing of Cr ratio. The results reveal that one of the ways to increase
the magnetocaloric effect in CO manganites is Cr doping to the structure. Abdelkhalek [169]
reported that the alloy of La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3, , which has been doped with Cr and Fe
at the same ratio, exhibited a phase transition to second order. When the transition
temperature is around at Tc=212 K and under 1 and 5 T magnetic changes the |∆SM|
measured as 0.43 and 1.75 J/kg.K , respectively. Oumezzine [170] studied the magnetic and
magnetocaloric properties of La0.67Ba0.33Mn0.9Cr0,1O3, alloy. As a result of the calculations, it
was understood that the sample exhibited a second-order magnetic phase transition. Under 5
T magnetic field change and at Tc=324 K , magnetic entropy change |∆SM| = 4.2 J/kg.K and
relative cooling power RCP=238 J/kg.K values have made this alloy one of the samples
sought in the magnetic cooling field at room temperature. Again, he studied the effect of Cr
doping on magnetic and magnetocaloric properties in a La0.75Sr0.25Mn1–xCrxO3 (x = 0.15, 0.20
and 0.25) alloy, systematically [171]. Curie temperatures were measured as Tc= 317, 278,
and 253 K, for x=0.15, 0.2 and 0.25, respectively. Magnetic entropy change under 5 T
magnetic field change for x=0.15, 0.2 and 0.25, were reported as respectively; |∆SM|= 3.5,
3.85 and 4.2 J/kg.K and relative cooling power RCP = 289, 323 and 386 J/kg. As Tc
decreased due to Cr ratio, |∆SM| and RCP values were increased. Bellouz [172] studied the
effect of Cr doping on the magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3Mn1xCrxO3
(x= 0.05, 0.1 and 0.15) alloy. Magnetization measurements explained that double-
exchange interaction in the structure was weakened with increased Cr ratio. It has been
observed that the Curie temperature decreases from the value of 338 K for x= 0.05 to 278 K
for x=0.15, due to the weakening of the DE interaction. It has been determined that the
structure exhibits a second-order magnetic phase transition. It was observed that the magnetic
entropy change decreased from the value of 4.04 J/kg.K for x=0.05 to 0.78 J/kg.K for x=0.15,
under 5 T magnetic field change. The RCP value reported for La0.65Eu0.05Sr0.3Mn1-xCrxO3
alloy is about 54% of the RCP value of pure Gd element, making these materials one of the
most striking examples of magnetic cooling. The Rietveld analysis applied in
La0.7Sr0.1Ca0.2Mn1-xCrxO3 (x = 0, 0.05 and 0.1) alloy has revealed that Cr doping changes
structural parameters such as Mn-O bond length, Mn-O-Mn bond angle [173]. It is observed
that the displacement of Mn ions with Cr ions reduces the 2p-3d hybridization between O and
Mn and therefore the bandwidth of Mn ions. As a result of the increasing Cr ratio, Curie
temperature decreases from 294 K to 255 K, confirming these results. Under 5 T magnetic
field change, the magnetic entropy change decreased from 6.2 J/kg.K for x=0 to 3.8 J/kg.K
for x=0.1. At the same time, the relative cooling power, RCP, has increased from 234.5 J/kg
for x=0 to 240 J/kg for x=0.1. These results show that Cr doping in some perovskites instead
of Mn restrains the improvement of magnetocaloric properties. This situation is explained by
the weakening of the ferromagnetic interaction between Mn3+-Mn4+ in Cr-doped manganites.
In La0.5Sr0.5Mn1-xCrxO3 (x=0.05, 0.1, 0.15 and 0.2) alloy, depending on the Cr doping, the
Curie temperature was observed to decrease from 319 K for x=0.05 to 251 K for x=0.2 [174].
From Arrott drawings, it was observed that the magnetic phase transition is second degree.
Magnetic entropy change has been reported as 2.77, 1.91, 1.59 ve 1.35 J/kg.K, for x = 0.05,
0.1, 0.15 and 0.2, respectively, under 5 T magnetic field change. As with magnetic entropy
change, RCP values have been shown to decrease. Under 5 T magnetic field change, cooling
power for x=0.05, 0.1, 0.15 and 0.2 was reported as RCP= 288, 213, 152 and 122 J/kg.
Gencer [175] examined the effect of Cr doping on the magnetic and magnetocaloric
properties of La0.94Bi0.06Mn1-xCrxO3 (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) alloy. It has been
observed that all samples have orthorhombic symmetry. In this study, we have determined
that the Curie temperature decreases from 209 K for x=0 to 127 K for x=0.25, with a Cr
concentration, and the magnetization measurements reveal that the saturation magnetization
(Ms) decreases from 86.13 emu/g for x= 0 to 35.69 emu/g for x=0.25. The decrease in Curie
temperature and saturation magnetization in Cr was attributed to the weakening of
ferromagnetic interactions in Cr-doped samples. Under 5 T magnetic field change magnetic
entropy change value and relative cooling power value, for x=0.0, 0.05, 0.1, 0.15, 0.2 and
0.25, were reported as, respectively; |ΔSm| = 5.51, 2.42, 2.47, 1.99, 1.61, 1.26 J/kg.K and
RCP= 264.53, 253.15, 202.21, 162.46, 129.86, 100.41 J/kg. The decrease in the
magnetocaloric effect was attributed to the reduction in the saturation magnetism by the
doping of Cr.
Studies have revealed that doping Cr instead of Mn in manganites often reduces the
improvement of magnetocaloric properties [167-175]. The doping of Cr3+ ions in manganites
instead of Mn3+ results in a decrease in Curie temperature [167-175]. In most studies, the
reduction of Tc was explained by the antiferromagnetic superexchange interaction between
Cr3+-O-Mn3+ and Cr3+-O-Cr3+. Magnetization measurements have proven strong conflict
between FM double-exchange interaction and AFM super-exchange interaction in Cr-doped
alloys. Also, as compared to the ionic radius of Mn3+ (= 0.645 Å) ion, Cr3+ (= 0.615 Å) ion
has a smaller ionic radius because Cr is expected to have a structural effect on the alloy. In
this case, structural disruptions may have an indirect effect on the ferromagnetic DE
interaction in structure.
3.2.2.4 Mn-site substitution with Fe
The effect of Fe doping on magnetic and conductivity properties of manganites rather than
Mn has been studied for many years, but the impact on magnetocaloric properties has been
started to be examined in recent years, and exciting results have been reported [176-183,51].
Nisha [176] investigated the structural magnetic and magnetocaloric properties of nanocrystal
La0.67Ca0.33Mn1-xFexO3 (x = 0.05, 0.2) alloys. The average particle size was reported as 15 nm
and 42 nm respectively for x = 0.05 and 0.2. Although the ionic radius (0.645 A0) of the Fe3+
ion is equal with the Mn3+ ion, the increase of the lattice parameters and unit cell volumes
due to the Fe ratio can be caused by the lattice distortions resulting from the random
dispersion of Fe and Mn ions. The other possibility is that there are Fe3+ ions in the structure
as well as Fe4+ ions. Because Fe4+ (=0.585 A0) ions have a higher ionic radius than Mn4+
(=0.53 A0) ions. Magnetization measurements have revealed that La0.67Ca0.33Mn0.95Fe0.05O3
alloy has a super paramagnetic configuration and La0.67Ca0.33Mn0.8Fe0.2O3 alloy has spin-glass
type configuration. In many studies, although it is possible that La0.67Ca0.33MnO3 alloy
showed a phase transition of the first degree, the Arrott drawings show a phase shift of the
second degree magnetic transition for both Fe-doped samples. The Curie temperature has
dropped from the value of 162 K for x=0.05 to 92 K for x=0.2. Under 5 T magnetic field
change, magnetic entropy change for x= 0.05 was decreased from the value of |ΔSm| =2.3
J/kg.K to 0.3 J/kg.K for x=0.2. Magnetization measurements have explained that both
samples still do not reach saturation under the 5T magnetic field. Magnetic entropy change is
so small that it is attributed to the second-degree magnetic phase transition and the samples
still do not reach saturation even under 5 T magnetic field. In contrast, the effect of Fe doping
on magnetocaloric properties of LaMnO3 alloy with antiferromagnetic insulator phase has
been studied [177]. It has been reported that LaMn0.9Fe0.1O3 alloy shows a second-order
magnetic phase transition. Magnetization measurements have shown that the structure has a
short-range ferromagnetic configuration. It has been reported that the magnetic entropy
change at 137 K as |ΔSm| =3.8 J/kg.K , under 5 T magnetic field change. X-ray studies for
Nd0.67Ba0.33Mn1−xFexO3 (0≤x≤0.1)
alloy have shown that unlike nano-sized produced
manganites [176], Fe3+ and Mn3+ ions in the polycrystalline structure have no significant
influence on the structural parameters of the composite because they have the same ionic
radius [178]. Magnetization measurements have shown that the alloy exhibits ferromagnetic
behavior for x=0 and 0.02, and the samples exhibit spin-glass-like behavior for x ≥0.05.
The temperature of Curie was measured as 150, 131, 61, 50 and 40 K for x=0, 0.02, 0.05,
0.007 and 0.1. Magnetic entropy change has been reported as |ΔSm| = 3.91 and 2.97 J/kg.K
for x=0 and 0.02, under 5 T magnetic field change. The relative cooling power was reported
as RCP= 265 and 242 J/kg. Magnetization measurements for La0.67Sr0.22Ba0.11Mn1-xFexO3 (0
≤x ≤ 0.3) alloy have given that x=0 and 0.1 samples exhibit an FM-PM phase transition
[179]. A large deviation was observed between ZFC and FC curves for
0.2 ≤ x ≤ 0.3 samples. This condition shows that there are FM and AFM interactions
competing with each other in the structure. Curie temperature dropped from the value of 360
K for x=0 to 94 K for x=0.2. Under 5 T magnetic field change for x=0, 0.1 and 0.2, magnetic
entropy change were reported as, respectively; |ΔSm| =2.46, 2.43 and 0.91 J/kg.K and relative
cooling power were reported as; RCP= 169, 241 and 70 J/kg, respectively. In La0.8Ca0.2Mn1-xFexO3 (x = 0, 0.01, 0.15, 0.2) alloy, depending on the Fe ratio, Curie temperature decreased
from 223 K for x=0 to 70 K for x=0.2 [180]. Magnetic entropy change under 5 T magnetic
field change for x= 0, 0.01, 0.15 and 0.2, were reported as, respectively; 4.42, 4.32, 1.6 and
0.54 J/kg.K. The results confirm that magnetic properties are strongly related to Fe
concentration. In 2016, the effect of Fe doping on the magnetic and magnetocaloric
properties of La0.67Sr0.33Mn1-xFexO3 (x=0, 0.05,0.1 and 0.2) [181] and La2/3Ba1/3Mn1-xFexO3
(x=0.0–0.10) [182] alloys, which exhibit similar magnetic properties, was examined. In
La0.67Sr0.33Mn1-xFexO3 alloy, Curie temperature decreased from 355 K to 100 K, depending
on Fe concentration. Similar behavior at Curie temperature has been observed in
La2/3Ba1/3Mn1-xFexO3 alloy. In La0.67Sr0.33Mn1-xFexO3 alloy, magnetic entropy change under 3
T magnetic field exchange, for x=0, 0.05, 0.1 and 0.2, measured as 1.66, 0.59, 0.79 and 0.42
J/kg.K, respectively. In La2/3Ba1/3Mn1-xFexO3 alloy, magnetic entropy change under 2.5 T
magnetic field change, first increased from the value of 1.06 J/kg.K for x=0 to 1.46 J/kg.K for
x=0.025. In later Fe concentrations, x ≥0.05 was less than 1.14 J/kg.K.
In La0.7Te0.3Mn1−xFexO3 (x=0.1 and 0.3) alloy, it was seen that the alloy was a rhombohedral
structure for x=0.1 and orthorhombic structure for x=0.3, depending on the Fe concentration
[183]. The Curie temperature has decreased from the value of 171 K for x=0.1 to 78 K for
x=0.3, depending on the Fe concentration. The magnetic entropy change for x=0.1 was
reported as |ΔSm| = 1.17 K/kg.K and RCP= 80 J/kg, and for x=0.3; |ΔSm| = 0.44 J/kg.K and
RCP= 49 J/kg, under 2 T magnetic field change. Gencer [51] examined the magnetic and
magnetocaloric properties of La0.94Bi0.06Mn1-xFexO3 (x = 0, 0.05, 0.075 and 0.1) alloy. With
increased Fe ration, it has been observed that saturation magnetism, Curie temperature, and
magnetic entropy change have decreased. The temperature of Curie was measured as 209,
185, 160, 156 K for x=0, 0.05, 0.075, 0.1, respectively. Magnetic entropy change were
reported as, for x=0; |ΔSm| = 5.51 J/kg.K and RC= 259 J/kg, for x=0.075; |ΔSm| = 2.89 J/kg.K
and RC=225 J/kg respectively, under 5 T magnetic field change. The decrease in the
magnetic entropy change is attributed to the decrease in the saturation magnetism and the
change in the nature of the phase transition from first to second order.
In general, substitution of Mn3+ ions with Fe3+ ions in Fe-doped manganites resulted in a
decrease in Curie temperature [176-183,51]. Since the ionic radius (0.645 A0) of the Fe3+ ion
is equal to the Mn3+ ion, it is expected not to affect the structural parameters. In spite of this,
significant changes have been observed in some manganite's, in lattice parameters and
volume of unit cells due to Fe concentration [176]. This situation is attributed to structural
irregularities due to the random dispersion of Fe and Mn ions in the structure. Another
possibility of these structural dispersions is that Fe3+ ions are present in the composition as
well as Fe4+ ions. Because Fe4+ (=0.585 A0) ions have a bigger ionic radius than Mn4+ (=0.53
A0) ions. However, the impact of structural parameters on magnetic properties is considerably
small. In Fe-manganites, the decrease in Tc temperature, in general, has been attributed to the
emergence of new antiferromagnetic interactions in the structure by the addition of Fe3+ ions.
In many studies, Fe3+-O-Fe3+ and Fe3+-O-Mn3+ antiferromagnetic superexchange
interactions, which occur with Fe doping, weaken ferromagnetic Mn3+-O-Mn4+ double
exchange interactions. As a result of this, the Curie temperature has been concluded to
decrease.
3.2.2.5 Mn-site substitution with Cu
In La0:77Sr0.23Mn1-xCuxO3 (0.1 ≤x ≤0.3) alloy, it was observed that Curie temperature
dropped from 325 K for
Interestingly,
x=0.1 to 242 K for x=0.3, depending on Cu ratio [184].
magnetic
entropy
change
has
decreased
with
increasing Cu substance in samples with a rhombohedral phase for
x ≤ 0.2. However, for x≥0.3, the structure has converted into the
orthorhombic
phase,
and
the
magnetic
entropy
change
with
increasing Cu ratio has begun to increase again. Magnetic entropy change
for x=0.1, 0.2 and 0.3 were reported as |ΔSm| =4.41, 2.68, 3.36 J/kg.K, and RCP= 570, 396,
330 J/K, under 1 T magnetic field change. In the study, RCP values obtained for
La0:77Sr0.23Mn1-xCuxO3 alloys were proclaimed to be around 60% of RCP values reported for
pure Gd. Moreover, under 1 T magnetic field, |ΔSm| =4.41 J/kg.K, observed for x=0.1, it is
claimed that the |ΔSm| value is 26% greater than the |δm| value reported for pure Gd. For this
reason, it has been stated that Cu-doped manganites may have potential coolers for magnetic
cooling around room temperature due to their high |ΔSm| and RCP values. In addition to the
study of Cu doping in LaSrMnO3 manganites, the effect of Cu doping on the magnetic and
magnetocaloric properties of La0.7Sr0.25Na0.05MnO3 [185] and La0.65Sr0.3Ce0.05MnO3 [186]
alloys were investigated, which are doped Na and Ce in low ratios (x=0.05) instead of Sr.
La0.7Sr0.25Na0.05Mn1−xCuxO3 (x = 0, 0.05, 0.10, 0.15, 0.20)alloy [185] is rhombohedral in all
Cu proportions. X-ray photoelectron spectroscopy (XPS) revealed that Cu2+ and Cu3+ ions
coexist in x=0.15 and 0.2 samples. Magnetization measurements showed that Curie
temperature and magnetic entropy change decreased with the increasing Cu ratio. Curie
temperatures were reported as 362, 359, 320, 274, 167 K for x=0, 0.05, 0.1, 0.15 and 0.2,
respectively. Magnetic entropy changes under 2 T magnetic field change were reported as
2.2, 2.02, 1.75, 1.39, 0.67 J/kg.K for x= 0, 0.05, 0.1, 0.15 and 0.2. Although Tc and |ΔSm|
values decrease with Cu ratio, relative cooling power has an average value of around 87 J/kg.
Similarly, in La0.65Sr0.3Ce0.05Mn1-xCuxO3 (0 ≤ x ≤0.15) [186] alloy, all Cu ratios were
found to be in the rhombohedral structure. From the analysis of the crystallographic data, it
was observed that there was a strong correlation between structural characteristics and
magnetic properties. For example, the decrease in Curie temperature is thought to be related
to the degradation of MnO6 octahedrons in Cu-doped samples. Experimental measurements
have confirmed that the Cu doping in the structure instead of Mn disrupts the formation of the
Mn3+-O-Mn4+ bond and weakens the ferromagnetic DE interaction between Mn3+ and Mn4+
ions. The Curie temperature is measured as 360, 330, 305, 275 K for x=0, 0.05, 0.1, 0.15,
respectively. Magnetic entropy change under 1 T magnetic field change has been reported as
1.49, 1.34, 1.5, 1.08 J/kg.K for x=0, 0.05, 0.1, 0.15, respectively. The change in the |ΔSm|
value with the Cu ratio is similar to the results obtained in a study conducted by Hagary[184].
In La0:77Sr0.23Mn1-xCuxO3 (0.1 ≤x ≤0.3) alloys, when |ΔSm| decreased for x ≤ 0.2,
it was observed that the |ΔSm| value was reallocated due to the
structural phase
transition
for
x≥0.3. Interestingly, in
the
La0.65Sr0.3Ce0.05Mn1-xCuxO3 (0 ≤ x ≤0.15) alloy, the |ΔSm| value fell to x=0.05 and
increased again for x=0.15. Nanto [187] studied the magnetic and magnetocaloric properties
of La0.7Ca0.3Mn1–xCuxO3 (0.0 ≤ x ≤ 0.03) alloy. The Arrott drawings show that all Cudoped samples exhibit a first-degree phase transition.Depending on the amount of Cu, it has
been observed to decrease. For x=0, 0.01, 0.02 and 0.03, Tc= 260, 248, 230, 217 K was
measured. 1 T for magnetic field change, magnetic entropy change has been reported as;
|ΔSm|= 4.32, 3.46, 2.98, 2.74 J/kg.K and RCP= 45, 42, 39, 47 J/kg.
3.2.2.6 Mn-site substitution with Ni
In 2012, Zhang [188] examined the effect of Ni doping on the magnetic and magnetocaloric
properties in La0.7Sr0.3Mn1-xNixO3 (x = 0, 0.01, 0.02, 0.03) alloys. In the examination, Ni ions
were shown to be in the Ni2+ state of the composite. In this case, increasing the amount of
Ni2+ in the structure causes the concentration of Mn4+ ions to increase. In this case, the
increase in the number of Mn4+ ions causes a decrease in the number of Mn3+-O-Mn4+ pairs
interacting ferromagnetic. In this case, it causes ferromagnetism to decrease and therefore
Curie temperature to drop in Ni-doped alloys. For x=0, 0.01, 0.02 and 0.03, TC= 362, 356,
350 and 347 K were reported. Magnetic entropy change has been reported as |ΔSm| = 2.33,
2.27, 2.26 and 2.21 J/kg.K, for x=0, 0.01, 0.02 and 0.03 respectively, under 1.5 T magnetic
field change. As can be seen from the results, although the effect of small Ni doping on
magnetic entropy change in La0.7Sr0.3MnO3 alloy is quite insignificant, the temperature in
which entropy change observed is shifting towards room temperature. Semli [189] examined
the magnetic and magnetocaloric properties in Pr0.7Ca0.3Mn1−yNiyO3 (0 ≤ y ≤0.1) alloy.
For y=0, the magnetization curve points to the maximum CO configuration observed at a
value of 215 K. In the magnetization curves obtained for later Ni concentrations, it was
observed that the corresponding peak was lost. As a result, it has seen that the contribution of
Ni has disrupted the CO configuration observed for y=0. Also, depending on the Ni
concentration, Curie temperature decreased from the value of Tc= 106 K for y=0.02 to
Tc=118.4 K for y=0.1. Magnetic entropy change for y= 0.02, 0.05 and 0.1 has been reported
reported as; |ΔSm|= 2, 2.96 and 2.94 J/kg.K and RCP= 239.5, 352.2 and 308.7 J/kg, under 5 T
magnetic field change. Oumezzine [190] studied the magnetic and magnetocaloric properties
in La0.6Pr0.1Ba0.3Mn1-xNixO3 (0 ≤ x ≤ 0.3) nanocrystal alloy. Increasing the concentration
of Ni in the structure results in an increase in the rate of Mn4+ ions compared to Mn3+ ions. In
other words, it increases the hole concentration and therefore decreases the density of the Eg
electrons. This circumstance caused magnetization and Curie temperature in the system to
decrease from 215 K for x=0 to 131 K for x=0.3. The magnetic entropy change has decreased
from the value of 1.97 J/kg.K for x=0 to 0.65 J/kg.K for x=0.3, under 5 T magnetic field
change. Similarly, the relative cooling power decreased from RCP=230 J/kg for x=0 to 62
J/kg for x=0.3, depending on Ni concentration. In La0.7Ca0.3Mn1−xNixO3 (x=0, 0.02, 0.07 and
0.1) nanocrystal alloy, similar to previous Ni-doped manganites, it has been observed that the
Curie temperature decreased from 264 K for x=0 to 174 K for x=0.1 [191]. The reduction in
Curie temperature and Ni concentration is explained by the weakening of the FM doubleexchange interaction between Mn3+-O-Mn4+ as a result of the substitution of Mn3+ ions with
Ni2+ ions. The Arrott drawings determine that all samples exhibit a second-order magnetic
phase transition. Interestingly, the second-order phase transition of the La0.7Ca0.3MnO3 alloy
is attributed to the nanosize of the samples. The magnetic entropy change for x=0, 0.02, 0.07
and 0.1 under 1.5 T magnetic field change was reported as; |ΔSm|= 0.85, 0.77, 0.63 and 0.59
J/kg.K respectively.
As can be seen in studies related to the doping of Ni in manganites, relocation of Mn3+ ions
with Ni2+ ions represses the effect of FM double exchange interaction and consequently
results in the decrease in magnetization, Curie temperature and magnetic entropy in Ni-doped
manganites. The increase in the amount of Ni2+ in the structure causes a decrease in the
number of Mn3+ ions (eg electron density) and therefore the increase in the number of Mn4+
ions (hole concentration). This situation results in a decrease in the number of Mn3+-O-Mn4+
pairs interacting ferromagnetic in the structure, and an increase in the number of Ni2+–O–
Ni2+ and Mn4+–O–Mn4+ pairs interacting antiferromagnetic. In this case, the interaction of
FM double exchange is repressed and consequently results in a decrease in the magnetization
and Curie temperature of Ni-doped manganites.
3.2.2.7 Mn-site substitution with Ga
In La0.7Ca0.15Sr0.15Mn1-xGaxO3 (x =0, 0.025, 0.05, 0.075 and 0.1) alloy, it was reported that
the Curie temperature dropped from 336.5 K for x=0 to 244.5 K for x=0.1, depending on the
Ga ratio [192]. The Arrott drawings revealed that the samples up to x=0.05 ratio exhibited
first-degree phase transition, and in later Ga concentrations, phase transition transformed its
nature from first degree to second degree. Magnetocaloric calculations showed that the
magnetic entropy change decreased from the value of 5.15 J/kg.K for x=0 to 1.86 J/kg.K for
x=0.1 with increased concentration of Ga, under 5 T magnetic field change. Again,
magnetocaloric properties have been examined in similar La0.75Ca0.08Sr0.17Mn1-xGaxO3 (0 ≤
x ≤0.2) alloy where Sr and Ca ratios are different [193]. Magnetization measurements
showed that the Curie temperature decreases with the increasing Ga ratio. Curie temperatures
were reported as 336, 285, 241 and 135 K, respectively, for x=0, 0.05, 0.1 and 0.2. Magnetic
entropy change for x= 0, 0.05, 0.1, 0.2 were reported as, respectively; |ΔSm|= 2.87, 1.92, 1.57,
1.17 J/kg.K and RCP= 97.5, 83, 101, 89 J/kg, under 2 T magnetic field change. The effect of
non-magnetic Ga-ion on magnetic and magnetocaloric properties of La0.7(Ba, Sr)0.3Mn1xGaxO3
(x=0, 0,1, 0,2) alloy was examined by Tlili [194]. Magnetization measurements show
that Curie temperature decreased from 316 K for x=0 to 300 K for x=0.2. The Arrott
drawings reveal that the samples exhibit a second-order phase transition. Magnetic entropy
change for x=0, 0.1 and 0.3 were reported as; 1.27, 1.16, 1.02 J/kg.K and RCP= 75.74, 72.35,
71.29 J/kg, under 2 T magnetic field change. As explained in the results, the change in
magnetic entropy reduces with the increase in the ratio of Ga.
With the increasing Ga ratio, the rate of Mn3+/Mn4+ , in other words, the FM doubleexchange interaction between Mn3+ and Mn4+ ions causes the decrease. When Mn ions are
replaced with non-magnetic Ga3+ ions, the number of Mn3+ ions decreases and some Mn3+-OMn4+ bonds existing in the structure transform into the form of Ga3+-O- Mn4+ and Ga3+-OGa4+ . Also, it causes a decrease in the mobility of eg electrons. Thus, the doping of Ga3+ ions
in manganites causes FM double-exchange interaction to be repressed and consequently
magnetization and Curie temperature to decrease.
3.2.2.8 Mn-site substitution with Ti
Kallel
[195]
calculated
magnetic
entropy
change
according
to
Ti
ratio
in
La0.70Sr0.30Mn0.90Ti0.10O3 alloy in two different ways, magnetization measurements and
Landau theory [195]. For non-Ti-doped La0.70Sr0.30MnO3 alloy, Curie temperature was 369 K,
and Curie temperature was 210 K for Ti-doped La0.70Sr0.30Mn0.90Ti0.10O3 alloy.Magnetic
entropy change was increased from the value of |ΔSm|= 2.31 J/kg.K and RCP= 69 J/kg for
La0.70Sr0.30MnO3 to |ΔSm|= 2.94 J/kg.K ve RCP= 288 J/kg for La0.70Sr0.30Mn0.90Ti0.10O3, under
5 T magnetic field change. The RCP value obtained for La0.70Sr0.30Mn0.90Ti0.10O3 is greater
than the La0.70Sr0.30MnO3 alloy where various TM elements are doped, up to 70% of the RCP
value measured in the equivalent field of Gd element. The compatibility between
experimental and theoretical calculations shows the effect of magnetoelastic interaction and
electron interaction on magnetocaloric properties. Exchange energy interaction by varying
the magnetic field applied around phase transition contributes additionally to magnetic
entropy change. Phong [196] systematically examined the effect of Ti doping on magnetic
and magnetocaloric properties in La0.7Sr0.3Mn1-xTixO3 (0 ≤ x ≤0.3) alloy. Magnetization
studies have revealed that doping of Ti weakens the FM double-exchange interaction, and
causing Curie temperature to decrease (364, 307, 236, 132ve 55 K, for x= 0, 0.05, 0.1, 0.2
and 0.3). Besides, by increasing the Ti ratio, the temperature of the
metal-insulator transition decreases and transforms into the
insulator phase for x ≥ 0.2 samples. Magnetic entropy change under 0.01 T
magnetic field change has been reported as |ΔSm|= 0.0162, 0.0042 and 0.0071 J/kg.K, for
x=0, 0.05 and 0.1, using the phenomenological model of Hamadin [75]. The behavior of the
results is consistent with the results given in reference [40]. Again, it was observed that Curie
temperature dropped from 363 K for x=0 to 125 K for x=0.2, in Ti-doped
La0.7Sr0.25Na0.05Mn1-xTixO3 (0≤ x ≤0.2) alloy [197]. Magnetic entropy change was
decreased from the value of |ΔSm|= 4.34 K/kg.K and RCP= 298 K/kg for x=0.2 to |ΔSm|= 2.03
K/kg.K and RCP= 273 K/kg for x=0.2, under 5 T magnetic field change. In the same year, the
effect of Ti doping on magnetocaloric properties in La0.67Ba0.22Sr0.11Mn1-xTixO3 (x= 0, 0.1,
0.2 and 0.3) alloy was examined [198]. Similarly, the Curie temperature has dropped from the
value of 344 K for x=0 to 267 K for x=0.1. It has been reported that |ΔSm|=2.75 J/kg.K and
RCP= 290 J/kg for x=0; |ΔSm|=1.33J/kg.K and RCP= 255 J/kg for x=0.1, under 5 T magnetic
field.
Smiy
[199]
examined
the
magnetic
and
magnetocaloric
properties
of
La0.5Pr0.2Sr0.3Mn1−x TixO3 (x =0.0 and 0.1) alloys. The Curie temperature has dropped from
280 K for x=0 to 123 K for x=0.1. |ΔSm|=1.969 J/kg.K and RCP= 285 J/kg for x=0 decreased
to |ΔSm|=1.309 J/kg.K and RCP= 162 J/kg for x=0.1, under 5 T magnetic field change.
In general, if the results are taken into consideration, it is perceived that the doping of Ti in
manganites instead of Mn has an adverse effect on the Curie temperature and magnetic
entropy change. One of the highlights is that even small amounts of Ti doping have a
substantial effect on magnetic and magnetocaloric properties compared to other Mn site
dopings. As can be seen from the results, the Ti doping in minimal ratios has caused a
significant reduction in Curie temperature. It has been concluded that doping Ti significantly
reduces the temperature of the ferromagnetic configuration in the structure. Since the Ti4+
ions do not have any 3d electrons, they do not exhibit any magnetic properties. When Mn
ions are substituted with Ti ions, the ferromagnetic Mn3+-O-Mn4+ interactions cause a
reduction. Since Ti4+ ions do not exhibit magnetic properties and there are no further
interactions to suffice this decline, the decrease in Tc is faster than in other transition metals.
3.2.2.8 Mn-site substitution with V
Kolat [25] examined the magnetic, electrical and magnetocaloric properties of
La0.67Ca0.33Mn09V1O3 alloy. In the study, V atoms in the structure are shown to be in the state
of V4+. For La0.67Ca0.33MnO3, alloy, only one magnetic phase transition was observed at
TC=267 K, two different phase transitions were observed for La0.67Ca0.33Mn0.9V0.1O3 alloy at
Tc1=223 K and Tc2=190 K. Two different magnetic transitions observed in the magnetization
curve of V-doped alloy are explained by the presence of two ferromagnetic phases in the
structure. As a result of EDX analysis, it was concluded that two different transition
temperatures belong to the La0.67Ca0.33Mn0.9V0.1O3 and La0.4Ca0.6Mn0.21V0.79O3 phases. The
magnetization curves, under 3 T magnetic field and at 5 K, showed that the saturation
magnetization decreased from 92 emu/g for x=0 to 77 emu/g for x=0.1. The reduction of the
saturation magnetism in V-doped alloy means that V in the structure represses the
ferromagnetic interactions. In V-doped alloy, the decrease of the Curie temperature is one of
the proofs of the weakening of ferromagnetism. Two peaks were observed around Tc1 and Tc2
temperatures in the magnetic entropy change curve obtained for La0.67Ca0.33Mn0.9V0.1O3
alloy. Under 1 T magnetic field change, the maximum magnetic entropy change obtained for
La0.67Ca0.33MnO3
has
a
value
of
4
J/kg.K,
whereas
the
value
for
V-doped
La0.67Ca0.33Mn0.9V0.1O3 alloy has been reported as |ΔSm|=2.4 J/kg.K. As it can be seen, in Vdoped alloy, |ΔSm| value has decreased significantly. This decrease in magnetic entropy
change is explained by the weakening of ferromagnetism and therefore the decrease in
saturation magnetism, in V-doped alloy. In another V-doped La0.6Nd0.1(CaSr)0.3Mn0.9V0.1O3
alloy, magnetization measurements have revealed that the alloy exhibits a second-order
magnetic phase transition [200]. Curie temperature measured as 298 K. Magnetic entropy
change and RCP values were reported as 4.266 J/kg.K and 205.35 J/kg respectively under 5 T
magnetic field change. The obtained |ΔSm| value is almost two times more significant than the
same V-doped La0.67Ca0.33Mn0.9V0.1O3 sample. Also, the temperature (298 K) at which |ΔSm|
change is observed in this example corresponds to room temperature. The RCP value
obtained for this sample is about 49.72% of the value obtained for pure Gd in the equivalent
field. Therefore, this alloy is one of the examples that are sought for cooling at room
temperature. Mansouri [201] studied the magnetic and magnetocaloric properties of
La0.7Sr0.2Ca0.05Li0.05Mn1−xVxO3 (x = 0 and x = 0.05) alloy. The Curie temperature has
dropped from the value of 271 K for x=0 to 266 K for x=0.05. It has been determined that all
samples exhibit a second-order phase transition. Under 5 T magnetic field change, it has been
reported that; |ΔSm|= 5.4 J/kg.K and RCP=211.5 J/kg, and |ΔSm|= 4.8 J/kg.K and RCP=195.5
J/kg for x=0.05. Similarly, it has been reported that Curie temperature drops from 262 K for
x=0 to 208 K for x=0.5, depending on V ratio in the La0.65Ca0.35Mn1-xVxO3 (0 ≤ x ≤ 0.5)
alloy [202]. The magnetic entropy changes under 5 T magnetic field change were reported as;
5.5 J/kg.K for x= 0, 3.36 J/kg.K for x=0.1, and 5.25 J/kg.K for x=0.5. As pointed in the
results, the |ΔSm| value has descended to the value of x=0.1 first, and then increased again
with the increasing V ratio. The relative cooling power increased from the value of RCP= 125
J/kg for x=0.1 to 207 J/kg for x=0.5, with the increasing V ratio.
3.2.2.8 Mn-site substitution with Sn
Dhahri [203] examined the effect of Sn doping on the magnetic and magnetocaloric
properties of the La0.67Ba0.33Mn1-xSnxO3 (x = 0.05, 0.1 and 0.15) alloy. Curie temperature was
found to be 340, 325 and 288 K, respectively, for x=0.05, 0.1 and 0.15. As can be seen
prominently, the Curie temperatures of these Sn-doped manganites comprise room
temperature. Under 2 T magnetic field change, values have been reported as; |ΔSm|= 1.9
J7kg.K and RCP= 101 J/kg for x=0.05, |ΔSm|= 2.27 J7kg.K and RCP= 120 J/kg for x=0.1,
and |ΔSm|= 2.49 J7kg.K and RCP= 123 J/kg for x=0.15. As can be seen from the results,
while Curie temperature decreases by Sn ratio, |ΔSm| and RCP values increase. Sn, which
doped to the alloy, is a non-magnetic cation with 4d105s05p0 electron configuration. In this
case, the doping of Sn cannot be expected to have a direct magnetic contribution to the
system. As can be seen from the experimental results, the doping of Sn4+ in La0.67Ba0.33Mn1xSnxO3
alloy has revealed that magnetic properties have changed considerably. Given the
load neutrality, the doping of Sn4+ to the structure causes the average valance state of Mn
atoms to shift to Mn3+ state. In this case, the decrease in the number of Mn4+ ions causes the
density of eg electrons to decrease. Also, the substitution of Mn4+ (0.53 A0) ions with Sn4+
(0.69 A0) ions whose ionic radius is bigger causes the variation in structural parameters such
as Mn-O bond length and Mn-O-Mn bond angle. This causes the FM double-exchange
interaction to be weakened by doping Sn to the structure. As a result of the increase in the
ratio of Sn in La0.57Nd0.1Sr0.33Mn1-xSnxO3 (0.05 ≤ x ≤ 0.30) alloy, the Curie temperature
(Tc = 282, 224, 187 and 158 K for x=0.05, 0.1, 0.15 and 0.2 respectively) decreased [204]. In
the examinations, it was found that the samples showed a second-order magnetic phase
transition. Magnetic entropy change has been reported as |ΔSm|=2.8 J/kg.K for x=0.0.5 and
3.22 J/kg.K for x=0.1, under 5 T magnetic field change. A very small amount of Sn doping in
La0.7Ca0.3Mn1−xSnxO3 (x = 0.0, 0.02 and 0.04) alloy has been shown to cause a decrease of
about 80 K at Curie temperature [205]. The Curie temperature was found as Tc=260, 176,
180 K for x=0, 0.02 and 0.04 respectively. |ΔSm| has been reported as 4.32, 1.61, 1.24 J/kg.K
for x= 0, 0.02 and 0.04 respectively, under 1 T magnetic field change. Similarly, in
La0.7Ba0.2Ca0.1Mn1-xSnxO3 (x = 0 and 0.1) alloy [206], Curie temperature decreased from 310
K for x=0 to 290 K for x=0.1. Under 5 T magnetic field , x=0; |ΔSm|= 6.7 J/kg.K abd RCP=
248 J/kg, x=0.1; |ΔSm| 3.21 J/kg.K ve RCP= 237 J/kg.K were reported. Considering the
obtained |ΔSm|, RCP values and temperatures which values are observed, La0.7Ba0.2Ca0.1Mn1xSnxO3
alloy has made one of the samples sought for cooling at room temperature.
3.2.2.9 Mn-site substitution with B, Bi, Gd, In, Ru, Sb, Si, Zn, Li
Kolat [27] studied the effect of B doping on the magnetic and magnetocaloric properties in
La0.67Ca0.33Mn1−xBxO3 (x = 0, 0.1, 0.2 and 0.3) alloy. Curie temperature was measured at
260 and 269 K for x=0 and 0.1. Interestingly, the magnetization curves for x=0 and 0.1 have
only one magnetic transition been observed, while the magnetization curves for x=0.3 have
two magnetic transitions, one of which is Tc1=246.6 K and the other is Tc2= 210.4 K. This is
explained by the presence of two different magnetic phases, confirmed by EDX analysis, in
the structure for x=0.3. The magnetic entropy change has decreased from the value of 6.1
J/kg.K for x=0 to 4.5 J/kg.K for x=0.3, under 3 T magnetic field change. The reduction of
the saturation magnetism from 93 emu/g for x=0 to 67 emu/g for x=0.3 means that
ferromagnetism is weakened in B-doped alloys. In the structure, the substitution of nonmagnetic Bi3+ ions with Mn3+ ions causes some Mn3+-O-Mn4+ bonds to become in the form
of Bi3+-O-Mn4+ and thus weaken ferromagnetism. Gd doping in La0.7Ca0.15Sr0.15Mn1-xGdxO3
(x = 0.0.02 and 0.06) alloy has been shown to reduce Curie temperature (Tc=338, 211 and
203 K for x=0, 0.02 and 0.06) significantly [207]. All of the samples exhibited a secondorder phase transition. Under 2 T magnetic field change, values were reported as;
|ΔSm|=0.925 J/kg.K and RCP=40.5 J/kg for x=0, |ΔSm|=1.2 J/kg.K and RCP=90,7 J/kg for
x=0.03 and |ΔSm|=1.004 J/kg.K and RCP=111.14 J/kg for x=0.06. Laouyenne [208]
examined the magnetic and magnetocaloric properties of La0.8Na0.2Mn1-xBixO3 (0 ≤x
≤0.06) alloy. Curie temperature was recorded as 330, 320, 310 K for x=0, 0.03 and 0.06.
As can be seen from the results, the temperature of Curie is reduced by doping Bi.
Magnetization measurements prove that alloys exhibit a second-order phase transition.
Under 5 T magnetic field change, values were reported as; |ΔSm|=4.73 J/kg.K and RCP= 241
J/kg for x=0, ΔSm|=4.77 J/kg.K and RCP= 218 J/kg for x=0.03, and ΔSm|=5.2 J/kg.K and
RCP= 229 J/kg for x=0.06. As can be seen from the results, doping of Bi to the structure
increases the magnetocaloric properties. The increase in magnetic entropy change, resulting
in an expansion in lattice volume and additional contribution to magnetic entropy change, is
explained by the exchange of structural parameters. Decreasing Curie temperature with
increasing Bi ratio determines that ferromagnetism is weakened in structure. In the structure,
the substitution of Bi3+ ions with Mn3+ ions causes Mn3+ density to decrease and therefore
the number of Mn3+-O-Mn4+ bonds that interact with FM decreases. This argument explains
the decrease in the Curie temperature with the doping of Bi in the structure. Also, when the
ionic radius of the Bi3+ (1.03 A) ion is compared with the ionic radius of the Mn3+ (0.645
A) ion, it is evident that the substitution of the Mn3+ ions with the B3+ ions will cause the
transition in structural parameters such as the Mn-O bond length and the Mn-O-Mn bond
angle. In this case, the decrease in Curie temperature can be attributed to the shrinking of
bandwidth caused by structural deteriorations. The Curie temperature in Sb-doped
La0.67Ba0.33Mn1-xSbxO3 (x=0.01, 0.03 and 0.07) alloy was reported as 326, 316 and 296 K,
respectively, for X=0.01, 0.03 and 0.007 [209]. Magnetocaloric properties were calculated
using the phenomenological model of Hamadın [57]. Under 1.5 T magnetic field change,
values were reported as; |ΔSm|=1.37 J/kg.K and RCP=69.12 J/kg for x=0.01, |ΔSm|=2.26
J/kg.K and RCP=87.86 J/kg for x=0.03 and |ΔSm|=2.74 J/kg.K and RCP=122.26 J/kg for
x=0.07. The decrease of Curie temperature in Sb-doped alloy is attributed to the decrease in
Sb5+ ion density due to the increase in the concentration of Mn4+ ions in the structure.
According to the
equation, the substitution of Mn
atoms with Sb5+ ions will change the ratio of Mn3+/Mn4+ considerably. Since the number of
M3+-O-Mn4+ bonds that interact with FM will decrease, this situation is expected to affect
DE mechanism. The presence of 10 electrons in the 4d shell of Sb5+ ions indicates that these
ions can not directly contribute to magnetic interactions. As a result, with the increase in the
number of Sb5+ ions, ferromagnetism in the structure is weakened, and Curie temperature
drops. Likewise, La0.7Ca0.3Mn1−xZnxO3 (x=0.0, 0.06, 0.08 and 0.1) alloy may be an example
of the doping of non-magnetic elements [210]. As with other non-magnetic dopings, Zn
doping in this alloy has caused a decrease in Curie temperature (Tc=245, 160, 100 and 70 K
respectively for x=0.0.06, 0.08 and 0.1). From the Arrott drawings, it was determined that
the samples exhibited the first-order magnetic phase transition for x<0.06, and the secondorder magnetic phase transition for x≥0.06. Under 5 T magnetic field change, values were
reported as; |ΔSm|=10.3 J/kg.K and RCP=294 J/kg for x=0,
|ΔSm|=5.33 J/kg.K and
RCP=364 J/kg for x=0.06, |ΔSm|=3.52 J/kg.K and RCP=404 J/kg for x=0.08. As in similar
alloys, the drop at Curie temperature is attributed to the weakening of ferromagnetism due to
the magnetic irregularities resulting from the doping of non-magnetic ions. Depending on
the saturation magnetization and the nature of the phase transition, the increase in Zn ratio
has dropped |ΔSm| value. However, due to the increase in Zn ratio and the shift in phase
transition from first degree to second degree, the increase in RCP value has made this alloy
interesting. The magnetocaloric properties obtained are salutary than the many alloys that
have been observed before. With the increase in Si ratio in the La2/3Ca1/3Mn1−xSixO3 (x =
0.05, 0.10, 0.15 and 0.20) alloy, Curie temperature dropped, and magnetic entropy change
remained at a high value almost constant [211]. In the examinations, magnetic entropy
change varies in the range of 4.88-5.48 J/kg.K, under 2 T magnetic field change. On the
other hand, it varies in the range of- J/kg.K, under 7 T magnetic field change. The
inclusion of Si4+ ions in the structure does not directly contribute to magnetic interactions.
They only weaken the long-reach ferromagnetic configuration along the M3+-O-Mn4+ bond
and consequently decrease the saturation magnetization and Curie temperature. Another
example of a non-magnetic cation is the La0.5Sm0.1Sr0.4Mn1-xInxO3 (0 ≤ x ≤0.1) alloy
[212]. In this study, it was observed that the temperature of Curie decreases from 310 K for
x=0 to 251 K for x=0.1, depending on the concentration of In. It has been determined that
the alloy exhibits a second-order phase transition at all In concentrations. Magnetic entropy
change has been reported as |ΔSm|= 5.88, 4.5 and 3.5 J/kg.K for x=0, 0.05 and 0.1, under 5
T magnetic field change. Under the same magnetic field change, the relative cooling power
was calculated as RCP= 181.66, 193.48 and 205.91 J/kg respectively. Since In3+ is a nonmagnetic cation, there is no direct magnetic contribution to the system. The effect of In
doping on magnetic and magnetocaloric properties is indirectly obtained. In3+ doping in
alloy causes Mn3+ ion density to decrease and therefore ferromagnetism to decrease.
Besides, the ionic radius of the In3+ (0.8A) ion causes the change in the structural parameters
by increasing In concentration compared to thE Mn3+ (0.65A) ion. One of the most
prominent Mn-site doping in recent years is the Ru ion. Ru is one of the exceptional
examples that can directly influence the magnetic and conductivity properties of manganites
with local spin interactions compared to other transition metal ions. The alloy of
Pr0.5Ca0.5Mn1-xRuxO3 is one of the examples in which the effect of Ru doping on magnetic
and magnetocaloric properties is studied [213]. In the study conducted by Kumar and his
colleagues, [213] showed that 3% Ru doping to Pr0.5Ca0.5MnO3 alloy disrupts the
antiferromagnetic CO configuration and stabilizes the ferromagnetic configuration to a
steady state. Curie temperature is observed to increase from the value of Tc=213 K for
x=0.03 to Tc=239 K for x=0.1, depending on the Ru concentration. Magnetic entropy
change has been observed to decrease with increasing Ru concentration (|ΔSm|= 4.2, 3.8 and
3.4 J/kg.K respectively for x=0.03, 0.05 and 0.1, under 5 T magnetic field change). Under
the same field change, the relative cooling power increased from RP=284.9 J/kg for x= 0.03
to RCP=303.6 J/kg for x= 0.1, due to Ru concentration. Various Mn-site manganite's
magnetocaloric properties are abstracted in Table 3.1.
3.3. COMPARISON OF MAGNETOCALORIC MATERIALS
The results revealed that the magnetocaloric properties of manganites gave promising
magnetocaloric materials from very low temperatures in an extensive temperature range
above room temperature. When it comes to magnetic cooling at room temperature, materials
that exhibit a high MC effect are one step ahead under very low areas around room
temperature. In figure 3.4, magnetic entropy changes compared to the Gd element of some Asite and Mn-site manganites were observed in a relatively small magnetic field change of 1 T
and Curie temperatures were given. As can be understood from the figure, MCE properties of
many manganites are comparable to those of Gd. Also, with the determination of appropriate
manganites and the appropriate A-site and Mn-site dopings, it is observed that the peak
temperatures where |ΔSm| change is observed can be adjusted under and above room
temperature, i.e., in a reasonably wide temperature range.
Figure 3.4. Comparison of Curie temperatures with maximum magnetic entropy change
values of some perovskite manganite structures with Gd. (Tc=294 K, |∆SM| =2.8 J/kg.K).
Black symbols, A-site doped manganites (1-La0.7Ca0.3MnO3 2-La0.67Ca0.33MnO3-δ 3La0.65Sr0.35MnO3
4-La0.67Sr0.33MnO3
5-La0.67Ba0.33MnO3
6-La0.67Ba0.33MnO2.95
7La0.8Na0.2MnO3 8- La0.835Na0.165MnO3 9- La0.8Ag0.2MnO3 10- La0.78Ag0.22MnO3 11La0.9K0.1MnO3 12- La0.85K0.15MnO3 13-Pr0.55Sr0.45MnO3 14- Pr0.6Sr0.4MnO3 15La0.67(Ca0.85Sr0.06)0.33MnO3 16- La0.67(Ca0.5Sr0.5)0.33MnO3 17-La0.6Ca0.2Sr0.2MnO3 18La0.67Ca0.18Ba0.12MnO3 19- La0.7Ca0.06Ba0.24MnO3 20- La0.62Bi0.05Ca0.33MnO3 21La0.55Nd0.1Ba0.35MnO3 22- La0.6Nd0.1Ba0.3MnO3 23- La0.8Ag0.1K0.1MnO3), Red symbols, Mn-
site doped manganites (1-La0.77Sr0.23Mn0.9Cu0.1O3 2-La0.57Nd0.1Sr0.33Mn0.95Al0.05O3 3La0.57Nd0.1Sr0.33Mn0.9Al0.1O3 4- La0.57Nd0.1Sr0.33Mn0.85Alu0.15O3 5- La0.7Sr0.3Mn0.93Fe0.07O3 6La0.7Sr0.3Mn0.9Fe0.1O3 7- La0.67Ba0.33Mn0.95Fe0.05O3 8- La0.67Ba0.33Mn0.9Fe0.1O3 9La0.67Pb0.33Mn0.75Co0.25O3 10- La0.67Pb0.33Mn0.7Co0.3O3 11- Bi0.4Ca0.6Mn0.8Ru0.2O3 12La0.65Sr0.3Ce0.05Mn0.95Cu0.05O3 13- La0.65Sr0.3Ce0.05Mn0.9Cu0.1O3 14- La0.7Ca0.3Mn0.9Co0.1O3
15La0.7Sr0.3Mn0.9Ti0.1O3
16La0.6Nd0.1Ca0.15Sr0.15Mn0.9V0.1O3
17La0.65Nd0.05Ca0.3Mn0.9Cr0.1O3 18- La0.65Ca0.3Pb0.05Mn0.9Cu0.1O3 19- La0.86Pb0.4Mn0.9Cu0.1O3).
Another essential feature to consider when choosing a magnetic material is the order of phase
transition exhibited by the material. Studies have explained that the magnitude of the
magnetocaloric effect in a material and the temperature range in which it is useful to depend
on the order of phase transition exhibited by the material. Magnetic materials exhibit a phase
transition of first-order or second-order around Curie temperature. Generally, materials
exhibiting the first-order phase transition have a greater MC effect than materials exhibiting
the second-order phase transition. In Figure 3.5, the maximum values of magnetic entropy
changes of some manganites exhibiting the transition from the first and second- order phase,
and the temperatures where this value is observed are seen. As can be seen from the figure,
manganites, which exhibits the first-order phase transition, has a higher |ΔSm| value in
general.
Figure 3.5 Comparison of some manganite's |ΔSm| values exhibiting the first and secondorder phase transition. Black symbols; first-order phase transition (1-La0.7Ca0.3MnO3 2La0.6Sm0.1Ca0.3MnO3 3- La0.94Bi0.06MnO3 4- La0.67Pb0.33Mn0.85Co15O3 5- Pr0.7Sr0.3MnO3 6Pr0.6Sr0.4MnO3 7- Pr0.55Sr0.45MnO3 8- La0.8Ca0.2MnO3 9- La0.5Pr0.2Ca0.3MnO3 10-
La0.7Ca0.275Ba0.025MnO3), red symbols; second-order phase transition (1-La0.5Sm0.2Ca0.3MnO3
2- La0.7Ca0.3Mn0.94Zn0.06O3 3- La0.6Na0.1Ca0.3MnO3 4- La0.75Sr0.1Ca0.15MnO3 5La0.7Ba0.3MnO3 6- La0.7Sr0.3MnO3 7- La0.7Ca0.15Ba0.15MnO3 8- La0.94Bi0.06Mn0.95Fe0.05O3 9La0.8Bi0.2MnO3 10- La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3 11- La0.94Bi0.06Mn0.75Cr0.25O3)
Studies have revealed that the order of phase transition in manganites depends on the
chemical composition of the alloy, the type of element that is doped to the alloy, ionic radius
and magnetic properties. For example, while one of the most studied example,
La0.67Ca0.33MnO3 , exhibits the first-order phase transition, La0.67Sr0.33MnO3 alloy exhibits the
second-degree phase transition. Figure 3.6 displays the magnetic entropy changes of the
Pr0.68Ca0.14Sr0.18MnO3 alloy, which exhibits a first-order phase transition, and the
Pr0.68Sr0.32MnO3 alloy which exhibits the second-order phase transition [29]. As shown in
Figure 3.6, magnetic entropy change occurs in materials exhibiting the first-order phase
transition in a relatively narrow temperature range. In materials exhibiting phase transition of
second-order, |ΔSm| values are relatively smaller. However, the temperature range in which
the maximum entropy change is observed is more extensive.
Figure 3.6. Magnetic entropy changes of a) Pr0.68Ca0.14Sr0.18MnO3 (FOPT) and b)
Pr0.68Sr0.32MnO3 (SOPT) alloys [29].
The RCP value calculated for Pr0.68Ca0.14Sr0.18MnO3 alloy under 1 T magnetic field is 43 J/kg
whereas the RCP value calculated for the La0.67Sr0.33MnO3 alloy under the same field is 142
J/kg. As can be perceived from the results, relative cooling power (RCP), one of the most
important parameters determining the use of magnetic materials as magnetocaloric material,
is greater in the samples exhibiting the transition from the second-order phase. One of the
other important points is the entropy change in the samples exhibiting phase transitions in the
second-order is symmetrical and uniform (Figure 3.6). The ideal magnetic cooler for use in
an Ericsson-type refrigerant must have a constant (or almost constant) magnetic entropy
change in the thermodynamic cycle range. In this case, the manganites exhibiting the
second-order phase transition are considered more relevant in terms of these
characteristics. Also, the thermal and magnetic hysteresis losses present in materials
exhibiting phase transition from the first-order are much lower than in alloys exhibiting
phase transition from the second-order, making it more convenient to use these alloys as a
magnetic cooler at room temperature. Therefore, when choosing a material as a magnetic
cooler, the nature of the phase transition exhibited by the material is one of the crucial
parameters to be considered.
The literature review revealed that particle (grain) sizes in manganites have a significant
effect on magnetic and magnetocaloric properties [214, 215]. A large number of studies have
been carried out for this purpose. For many manganites, the effect of grain size on structural,
magnetic and magnetocaloric properties has been studied in detail [70, 80, 95, 96, 120, 176,
190, 191, 214, 215]. In terms of magnetic properties, in the case of grain size is nano; spinglass, superparamagnetic, high coercivity, saturation magnetism and Curie temperature, such
as changes can be quite attractive and appealing properties. Again, it has been reported that
the order of magnetic phase transition may shift from first to second order with the decrease
in grain size [214, 215]. As a result, since all these properties are closely related to the
magnetocaloric properties of materials, it is presumed that the magnetocaloric properties will
change depending on the grain size. In Figure 3.7, magnetic entropy change for
La0.6Ca0.4MnO3 alloy and Curie temperature change according to grain size are seen [214].
As can be seen in Figure 3.7, both the |ΔSm| value and the Curie temperature increase
significantly with the extension of grain size. Figure 3.8, the expansion of magnetic entropy
change, unlike the |ΔSm| and TC value increases with the shrinkage in grain size. This
behavior enables the development of magnetic cooling systems that can operate in a wide
temperature range by adjusting the particle size appropriately.
The literature review showed promising candidates for magnetic cooling in an extensive
temperature range of materials with magnetic entropy change in substantial quantities.
Figure 3.7 Change in |ΔSm| and TC ‘s grain size in L0.6Ca0.4MnO3 alloy[214]
Figure 3.8 Entropy change for different particle sizes of La0.6Ca0.4MnO3 alloy [214].
When a material is to be used as an active magnetic cooler, the first thing that comes to
mind is that it has substantial |ΔSm| values in low magnetic field changes. However, it is
necessary to remember that the uniform distribution of magnetic entropy change curves
plays a critical role in determining magnetic cooling efficiency [216].
Unfortunately,
nonuniform magnetic entropy change (an undesirable feature for an Ericsson-circulated
magnetic cooler) is a phenomenon observed in magnetocaloric materials such as Gd and
many polycrystal manganites due to its non-homogenous structure [216]. In this context, in
many studies [14, 103, 150, 216], it has been explained that monocrystalline manganite
exhibits superior magnetocaloric properties than polycrystalline manganites. In figure 3.9,
the magnetic field changes of the |ΔSm| and RCP values for the monocrystalline and
polycrystalline forms of the La0.7Ca0.3MnO3 alloy are seen [216]. As shown in Figure 3.9,
the monocrystalline manganite's |ΔSm|
polycrystalline form.
and RCP values are more significant than the
Figure 3.9 Variations of the |ΔSm| and RCP values for mono and polycrystalline forms of the
La0.7Ca0.3MnO3 alloy, under the magnetic field [216].
Besides, asymmetric variations in magnetic entropy curves due to the effects of grain limit in
polycrystalline manganites have become more uniform and symmetrical in monocrystalline
manganites [14, 216]. This condition is due to the absence of grains in monocrystalline
manganites. Another reason for the irregularity observed in entropy curves of polycrystalline
manganites is the presence of different ferromagnetic clusters originating from nonhomogenous structure and stoichiometry. Most importantly, monocrystalline manganites are
exhibit much smaller thermal and field hysteresis than polycrystalline manganites.
Monocrystalline manganites
are
more suitable magnetic
cooler
candidates
than
polycrystalline manganese, considering all of these results.
The literature review has revealed that different preparation methods used to produce
perovskite manganites have significant effects on the structural, magnetic and magnetocaloric
properties of alloys [26, 71].
To date, many methods have been applied to produce
perovskite manganites. Among these, the solid-state reaction method is one of the most
frequently used conventional methods. In this method, keeping the temperature under control
during a solid-state reaction is one of the most critical problems. Also, to achieve a
homogeneous structure in stoichiometry, grain sizes, porosity and purity using solid-state
reaction method, highly high sintering temperatures - K) and reasonably long
annealing times (> 10 hour)) are required. The sol-gel method used as an alternative method
is a chemical method, and it is necessary to overcome the complex chemical processes. In
both methods, the cost is quite high, and efficiency is low. In recent years, a new method
called mechanical alloying or high-energy ball milling method has been introduced to
produce perovskite manganites [31, 73, 74]. Studies have shown that the milling method has
many advantages such as low cost, high efficiency, low-temperature synthesis and the ability
to adjust grains from micrometer to nanometer degree at the desired size. In many studies,
magnetic and magnetocaloric properties of manganites produced using high-energy ball
milling method were investigated. In the study conducted for La067Ca033MnO3 alloy, it was observed
that the perovskite structure was formed for milling time above 4 hours [31].
In the 24-hour milled
sample, it has been reported that grain size varies from nm to a few µm. The Curie
temperature, for 4, 12 and 16-hour milled samples, was reported at 250, 240 and 235 K,
respectively, in Pr0.5Sr0.5MnO3 alloy, which was also produced using a ball milling method.
Under 5 T magnetic field change, values were reported as; |∆SM| = 2.27, 2.57, 2.58 J/kg.K
and RCP= 216.33, 214,92, 204.31 J/kg respectively for 4, 12 and 16-hour milled samples
produced by milling method. Riahi [74] produced La0.78Dy0.02Ca0.2MnO3 alloy using three
different methods, solid-state, sol-gel and ball milling, and compared the magnetic and magnetocaloric
properties of the samples obtained.
Interestingly, the samples produced by the sol-gel method
exhibit the first-order phase transition, while the samples produced by the other two methods
exhibit the second-order phase transition. In the sample produced by the sol-gel method, the
magnetic entropy change is much more significant than the others, which is attributed to the
first-order phase transition. In Figure 3.9, magnetic entropy changes are observed for
La0.78Dy0.02Ca0.2MnO3 alloy produced in three different methods.
Figure 3.10 Magnetic entropy changes of La0.78Dy0.02Ca0.2MnO3 alloy whic is produced by
a) solid-state, b) sol-gel and c) ball milling methods [74].
As can be seen in Figure 3.9, the sample produced by ball milling method has a relatively broad temperature
range, although it has a smaller entropy change. Under 5 T magnetic field change, the most considerable RCP
value was observed as 346.7 J/kg in the sample produced by the ball milling method. The results proved that
manganites produced by ball milling method might be promising candidates in magnetic cooling.
4. FINAL REMARKS
As is known, materials that exhibit a substantial temperature change under changing
magnetic field are termed magnetocaloric materials. A review of studies on magnetocaloric
materials revealed that the temperature change of magnetocaloric materials under a magnetic
field is due to many internal and external factors. Chemical composition, crystal structure and
magnetic state are the most significant internal factors determining the magnetocaloric
properties of a material. The most significant external factors include the temperature, the
pressure of the ambient and the dimension of the field applied. To date, materials that exhibit
magnetocaloric properties far superior to manganites have been reported. However,
considering the features that determine the use of a material as an active magnetic cooler,
such as preparation methods, high resistance, cost, safety in terms of health and chemical
stability, we see that manganites are a prominent step forward. Besides, the Curie temperature
can be easily adjusted in an extensive range to comprise the room temperature where the MC
effect is observed, making manganites one step ahead. Given all these characteristics,
manganites are among the cheapest and most relevant magnetic coolers, currently.