Structural and Magnetic Characterization of Co50Mn30InxSn(20-x) (x= 0, 5, 10, 15, and 20) Samples for Magnetocaloric Effect

Contents

Acknowledgement

Abstract

Chapter One: Magnetocaloric effect and Heusler alloy
1.1 Introduction
1.2 Thermodynamics of magnetocaloric effect
1.3 Measurements of MCE
1.4 Magnetic Refrigeration and Materilas
1.5 Magnetocaloric materials
1.6 Heusler alloy
1.7 Aim of the work

Chapter Two: Sample Preparation and Experimental Techniques
2.1 Preparation of samples
2.1.1 Arc melter
2.1.2 Melt Spinner
2.1.3 Sample Preparation
i) Master alloy
ii) Ribbon Preparation
2.2 Instrumentations
2.2.1 Preparation of system for annealing sample
2.2.2 Differential Sanning Calorimetry
2.2.3 Powder X-ray diffractometer
2.2.4 Physical Property Measurement System(PPMS)
2.2.5 Vibrating Sample Magnetometer(VSM)

Chapter Three: Results and Discussion
3.1 DSC Results
3.2 XRD Results
3.3 Magnetic Measurements
3.3.1 Hysteresis Behavior
3.3.2 Isothermal magnetization measurement
3.3.3 Thermal dependence of the magnetization measurement
3.4 MCE measurements Results

Chapter Four: General Conclusion and Future work

References

Aknowledgement

I would like to express my gratitude to the wonderful people who extended full heartedly their helping hand for accomplishing my Master’s in Nanosicence study program including this thesis.

At first, I would like to thank Prof. Angel Alegría and member’s of the master’s program committee who have accepted me two years ago in the Master’s in Nanoscience program and offered me grant from the Universidad del País Vasco (UPV/EHU) and the Donostia International Physics Centre (DIPC) for the first year of the master’s program.

I would like to thank Prof. Julián González, my research advisor, for accepting me as a member of his research team and having confidence in me. This great opportunity enabled me to immerse myself in a promising research field of Nanoscienc. I appreaciate greatly his continuous supports, guidance and encouragements and for being always ready to listen me during this work.

I would also like to thank Dr. Juan Jose del Val for his helpful assistance, discussions and suggestion for experimental work in the lab. His help and support have given me inspiration and motivated me for working hard in order to accomplish this work.

I’d like to thank all the teachers who contribute to the Master’s in Nanoscience program as well as the students involved in the courses.

I am also thankful to the member of my research group Dr. M. llyn for his supports in the measurements and discussions during this work. I have benefited a lot from his indepth knowledge and experience in this field.

Also, I would like to thank Prof. J.J .Suniol and his group at Universidad de Gerona for allowing me to have access to prepare my samples in their University laboratory.

I greatly thank to my close friends, Yasmin Khairy, Zakaria El-Degway and Afaf El- Syed, who were always willing to give me their hands whenever I needed them. I highly appreciate their friendship and support in those days when I was, sometimes, lost and panicked with my work.

I would also like to express my heartfelt gratitudes for the important role my parents, sisters & brothers have played in my life. Particularly, my elder brother Mr. Harun-or- Rashid’s guidance & supports for my stutdies and research were very important for my academic endeavours. They taught me to work hard, pursue my goals and dreams while respecting others and being grateful for every opportunity given in life. They also taught me the value of education and knowledge.

Finally, I would also like to express my acknowledgement for the financial support from the magnetism group of the University ofBasque country.

Abstract

Nowadays magnetic refrigeration which is based on the Magnetocaloric Effect (MCE) is one of the more relevant topics in the scientific research owing to its very important technological applications, which derive from the attempts made to replace the gas refrigerating technology involving, among other transcendental aspects, a low impact in the environment and an expected higher energetic efficiency. Furthermore, new and improved magnetocaloric materials are one of the cornerstones in the development of room temperature magnetic refrigeration. This master’s work has mainly been concerned in a development of a deep and systematic study of magnetic and magnetocaloric properties of ribbon samples of Heusler alloy family of materials with compositions susceptible of exhibiting MCE, For this purpose, we have focused on ribbons of chemical composition Co5oMn30InxSn(2o-x) (x= 0, 5, 10, 15, and 20).

Co50Mn30InxSn (20-x) Heusler alloys were prepared by arc melting admixtures of the pure elements in the desired quantities under argon atmosphere. The ribbons have been prepared by melt spinning technique. X-ray characterization proved the face centered cubic austenite Heusler alloy phase (211) crystal structural with lattice constant a = 0.5947 nm of as cast and annealed ribbons. Differential Scanning Calorimetry was used to find the temperatures of the recrystallization phase transitions. Magnetization was measured for as-cast and annealed samples. Both Curie temperatures and coercivity were found to be lower in the as-cast samples than in the glass tube annealed ones. Monotonic decreasing of the Curie temperature was observed in the series with the growth of the In content.

MCE has been evaluated for one of the samples (x = 15) and a maximum value of entropy change is 2.37 JKg-1K-1 have been found, which is the same order of magnitude that reported of Gd alloys.

CHAPTER ONE THEORETICAL BACKGROUND OF MAGNETOCALORIC EFFECT AND HEUSLER ALLOY

In this chapter the main term and conceptions about magnetocaloric effect (MCE) are introduced. The models and approaches usually used for description of MCE, such as thermodynamics and statistic approaches are considered. Different measurement methods of MCE have also been included. The main contributions to the MCE as a magnetic refrigeration and magnetocaloric materials are also regarded. Finally attentions are paid to concerning about Heusler alloys and as well as the aim of the present work.

1.1 Introduction

Magnetic refrigeration which is based on the Magnetocaloric effect (MCE) is one of the more relevant topics in the scientific research owing to its very important technological applications, which derive from the attempts made to replace the gas refrigerating technology involving, among other transcendental aspects, a low impact in the environment and an expected higher energetic efficiency. The magnetocaloric effect or adiabatic temperature change is defined as the intrinsic property of magnetic materials which is expressed by its variance under the action of an external magnetic field which means that MCE is the heating or cooling (i.e., the temperature change) of a magnetic material due to the application of a magnetic field [1,2].

In 1881 Warburg was originally first observed the magnetocaloric effect or adiabatic temperature change by the application of a magnetic field in iron [1]. The origin of the MCE was explained independently by Debye and Gaiauque in 1926 and 1927 respectively. They also suggested the first practical use of the MCE: the adiabatic demagnetisation, which means that it is able to reach temperatures, lowers than that of liquid helium, which had been the lowest achievable experimentally temperature. It is the response of a magnetic material to a changing field that is evident as a change in its temperature in the vicinity of the Curie temperature [2-5].

Obviously, the MCE is intrinsic to all magnetic materials. It is represented as the coupling between the total entropy of a magnetic material and the external magnetic field which changes the magnetic part of the total entropy of the material. When a magnetic field is applied for a simple ferromagnetic material near its curie temperature, the spins tend to align parallel to the magnetic field resulting lowers the magnetic entropy. To compensate the loss in the magnetic entropy in an adiabatic (isentropic) process, the temperature of the material increases. On the other hand, when the magnetic field is turned off the spins tend to become random causes increases in the magnetic entropy and the material cools. This phenomenon is called MCE.

Recently, there is a lot of interest in using the MCE as an alternative technology for refrigeration, from room temperature to the temperatures of hydrogen and helium liquefaction ( « 20-4.2 K). The magnetic refrigeration offers the prospect of an energy- efficient and environment friendly alternative to the common vapour- cycle refrigeration technology in use today [4-6].

1.2 Thermodynamics of magnetocaloric effect

In order to explain the origin of the magnetocaloric effect, one can use thermodynamics, which relates the magnetic variables (magnetisation and magnetic field) to entropy and temperature. Tishin et al. [1], Pecharsky et al. [2, 7, 8], and Planes et al [9] have reported about the thermodynamics of magnetocaloric effect very clearly [1, 2, 7, 8, and 9]. All ferromagnetic materials intrinsically show MCE, although the intensity of the effect depends on the properties of each material. The physical origin of the MCE is the coupling of the magnetic sublattice to the applied magnetic field, H, which changes the magnetic contribution to the entropy of the ferromagnetic solid.

Fig 1.1 shows the two basic process of the MCE when a magnetic field is applied or removed in magnetic system which is the equivalence to the thermodynamics of a gas is evident: the isothermal compression of a gas (when a pressure is applied and the entropy decreases) is analogous to the isothermal magnetisation of a paramagnetic or a soft ferromagnetic (by applying H and the magnetic entropy decreases), while the subsequent adiabatic expansion of a gas (when lower pressure at constant entropy and temperature decreases) is equivalent to adiabatic demagnetisation (removing H, the total entropy remains constant and the temperature decreases since magnetic entropy increases).

illustration not visible in this excerpt

Figure 1.1: Schematic picture that shows the two basic processes of the magnetocaloric effect when a magnetic field is applied or removed in a magnetic system the isothermal process, which leads to an entropy change, and the adiabatic process, which yields a variation in temperature.

In Figure 1.2 the thermodynamics of the MCE in a ferromagnet near its magnetic ordering temperature (Curie temperature, T_c ) is schematically illustrated. At constant pressure, the value of the entropy of a ferromagnet (FM) depends on both H and temperature T, whose contributions are the lattice (Slat ) and electronic (SEI ) entropies, as

for any solid, and the magnetic entropy (Sm ).

illustration not visible in this excerpt

From equation 1.1 only the magnetic entropy is of interest, since the change of lattice entropy (Slatl ) and electronic entropy (SEI ) with magnetic field are negligible compared with magnetic entropy. It is shown form figure 1.2 the total entropy for a ferromagnetic material in two constant fields, H0 which is usually taken to be zero in most applications, H1 which is a non-zero magnetic field.. The magnetic part of the entropy is also shown for each one [ S_M (H_i ) ] and [ S_M(H₀) ].

illustration not visible in this excerpt

Two relevant processes are shown in the diagram in order to understand the thermodynamics of the MCE:

i) When the magnetic field is applied adiabatically (i.e., when the total entropy of the system remains constant during the magnetic field change) in a reversible process, the magnetic entropy decreases, but as the total entropy does not change, i.e.

illustration not visible in this excerpt

Then, the temperature increases. This MCE or adiabatic temperature rise can be visualised as the isentropic difference between the corresponding s(t, H) functions as shown in fig 1.2 by the horizontal arrow and it is a measurement of the MCE in the material.

illustration not visible in this excerpt

ii) When the magnetic field is applied isothermally (T remains constant), the total entropy decreases due to the decrease in the magnetic contribution, and therefore the entropy change in the process as shown in fig 1.2 by the vertical arrow is defined as

illustration not visible in this excerpt

Both, the adiabatic temperature change, [illustration not visible in this excerpt], and the isothermal magnetic entropy change, [illustration not visible in this excerpt], are characteristic values of the MCE and it is obvious that both quantities are functions of the initial temperature, T₀, and the magnetic field variation [illustration not visible in this excerpt].

Therefore, it is straightforward to see that if rising the field increases magnetic order (i.e., decreases magnetic entropy, which is the case for simple paramagnetic and ferromagnetic materials), then [illustration not visible in this excerpt] is positive and magnetic solid heats up, while [illustration not visible in this excerpt] is negative, But if the field is reduced, the magnetic order decreases and [illustration not visible in this excerpt] is thus negative, while [illustration not visible in this excerpt] is positive, giving rise to a cooling of the magnetic solid.

The [illustration not visible in this excerpt] and [illustration not visible in this excerpt] are correlated with the magnetization, M, the magnetic field strength, the heat capacity at constant pressure (C), and the absolute temperature by one of the fundamental Maxwell’s equations [2],

illustration not visible in this excerpt

Integrating the above equation for isothermal -isobaric process, we obtain

illustration not visible in this excerpt

This equation indicates that the magnetic entropy change is proportional to the derivative of magnetization with respect to temperature at constant field and to change of the magnetic field.

Considering the total entropy of the systems(t,н)p, its total differential can be written as:

illustration not visible in this excerpt

Where P is the pressure for an adiabatic-isobaric process, the left hand side and the third term of equation (1 .7) are both 0. Combining the Maxwell equation (1.5) and (1.6), and considering [illustration not visible in this excerpt] , where C (T, H) is the heat capacity at constant field, the infinitesimal adiabatic temperature change can be expressed by

illustration not visible in this excerpt

Integrating the above equations we have the value of the MCE or adiabatic temperature change,

illustration not visible in this excerpt

All the above equations are obtained using the general principles of thermodynamics and can be used to describe the MCE on a macroscopic scale.

In paramagnets, the lattice contribution to the heat capacity is negligibly small at temperatures close to absolute zero. At higher temperatures where the lattice heat capacity of the paramagnet is large the small generated MCE heat is absorbed by the lattice degrees of freedom of the solid and practically no temperature change can be observed. In ferromagnets there are two opposite forces, i.e. the ordering force due to exchange interaction of the magnetic moments, and the disordering force of the lattice thermal vibrations, are approximately balanced near the T_c. Hence, the isothermal application of a magnetic field produces a much greater increase in the magnetization (i.e. an increase in magnetic order and consequently, a decrease in magnetic entropy, [illustration not visible in this excerpt] ) near the Curie point than far away from it. The effect of magnetic field above and below T_c is significantly reduced because only the paramagnetic response of the magnetic lattice can be achieved for T » T_c, and for T « T_c the spontaneous magnetization is already close to saturation and can not be increased much more.

In the ferromagnetic state the calculation of magnetic entropy change, [illustration not visible in this excerpt] can be done by using the Maxwell equation (1 .6), to obtain [illustration not visible in this excerpt].

1.3 Measurements of the magnetocaloric effect

To determine the MCE, different methods have been suggested in literature [11, 12, 13, and 14]:

1.3.1 Direct measurement

MCE can be measured directly or it can be calculated indirectly from the measurement of magnetization or field dependence of the heat capacity or by exposing a sample to an increasing magnetic field under adiabatic conditions [2, 15], Ponomarev [16], Clark and Callen [17] and Kuhrt et a/.18] have directly measured the temperature of the sample with a thermocouple during the application or removal of a magnetic field. Direct measurements of the adiabatic temperature change, [illustration not visible in this excerpt] by moving the sample in and out of a magnetic field region and recording the changes in the temperature. Readily this can be achieved when the thermally insulated sample is inserted in to a magnetic field. Also under adiabatic conditions, the reverse effect is obtained when removing of the sample from the magnetic field.

Direct techniques in order to measure the MCE always involve the measurement of the initial (T_o) and final (T_f) temperature of the sample, when external magnetic field is changed from an initial (Ho) to a final value (H_f). Then the measurement of the adiabatic temperature change is the difference between Tf and to for a given To which can be expressed as:

illustration not visible in this excerpt

and

illustration not visible in this excerpt

Direct MCE measurements can be performed using contact which means when the temperature sensor is directly connected with the sample and non-contact techniques in which means when the temperature sensor is not directly connected with the sample [2].

A rapid change of the magnetic field is needed to perform direct measurements of MCE. Therefore, the measurements can be carried out either by changing the field [19] or by moving the samples in and out of a constant magnetic field region [20].Using immobilized samples and pulsed magnetic fields, direct measurements from (1-40 )Tesla (T) have been reported. When electromagnets are used, the magnetic field is usually reduced to less than 2T. When the sample or the magnets are moved, permanent or superconducting magnets are usually employed, with a magnetic field range of 0.1-10T.

By considering the entropy change Casanova et al [21] have reported a direct observation of the entropy change in a first-order phase transition which was obtained by using a differential scanning calorimeter in which the transition is field induced under the application of an external magnetic field. They suggested that this procedure enables direct evaluation of the magnetocaloric effect in materials showing first-order magnetostructural phase transitions. In addition, regarding the entropy change Tocado et al [22] have developed a new method, which is based on using the adiabatic calorimeter designed to measure heat capacity by the heat pulse method, for determining the entropy directly.

The adiabatic temperature change can also be measured directly by changing magnetic field. Balli et al have measured adiabatic temperature change directly under a field of 1.48T, for the sample [illustration not visible in this excerpt] [23] which is a value close to half of that measured for Gd.

The accuracy of the direct experimental techniques depends on the errors in thermometry and in field setting, the quality of thermal insulation of the sample, the possible modification of the reading of temperature sensor due to the applied field, etc. As Pecharsky and Gschneidner [2] have been pointed out, the accuracy is claimed to be in the 5 to 10 % range.

1.3.2 Indirect measurements

The other techniques for determining the MCE are indirect. Indirect methods which are often used include the ones based on magnetization measurements: by determining the magnetization curves at various temperatures, the magnetic contribution to the entropy change asm can be determined and another on Calorimetric method: by measuring specific heat capacity as a function of temperature at various magnetic field strengths.

Usually the direct measurement which only yield the adiabatic temperature change, but the indirect experiments allow to calculation of both ATad (T, ah) and asm (t, Ah) in the case of heat capacity measurements, orjust asm (t, AH) in the case of magnetisation measurements. On the contrary, magnetization must be measured as a function of T and

H. By numerical integration of equation (1.6) this allows to get asm (t, ah) and it is very important as a rapid search for potential magnetic refrigerant materials [24].

The accuracy of [illustration not visible in this excerpt] which are calculated from magnetization data depends on the accuracy of the magnetic moment, temperature, and magnetic field measurements. Also it is affected by the reason that in Eq.1.6 (dM, dH and dT), are replaced by the measured changes [illustration not visible in this excerpt]. Considering all these effects Pecharsky and Gschneidner have been investigated that the error in the value of [illustration not visible in this excerpt] lies within the range of 3-10 % [2, 24].

The measurement of the heat capacity as a function of temperature in constant magnetic fields C(T) H provides the most complete characterization of magnetocaloric effect in magnetic materials. The entropy of a solid can be calculated from the heat capacity as:

illustration not visible in this excerpt

and

illustration not visible in this excerpt

Where S₀ and S_0h are the zero temperature entropies. In a condensed system these are the same (i.e. S₀ = Soh ) [2, 26] and therefore if [illustration not visible in this excerpt] is known, both [illustration not visible in this excerpt] and [illustration not visible in this excerpt] can be obtained [2, 26] for example figure [1.3]

The accuracy in the measurements of MCE using heat capacity data depends critically on the accuracy of С(T) H measurements and data processing, since both ATad (T, AH) and

[illustration not visible in this excerpt] (, AH ) are small differences between two large values (temperature and total entropies). The error in [illustration not visible in this excerpt](T,AH) o[[illustration not visible in this excerpt] (T, AH)], calculated from the heat capacity is given by the expression [2, 27]

illustration not visible in this excerpt

Where [illustration not visible in this excerpt] and [illustration not visible in this excerpt] are the errors in the calculation of the zero field entropy and non-zero field entropy, respectively. The error in the value of the adiabatic temperature change, [illustration not visible in this excerpt] is also proportional to the errors in the entropy, but it is inversely proportional to the derivative to the entropy with respect to temperature [2, 27],

illustration not visible in this excerpt

It should be noted that Equations (1.14 and 1.15) yield the absolute error in MCE measurements and, therefore, the relative errors strongly increase for small MCE values (fig. 1.4) hence, assuming that the accuracy of the heat capacity measurements is not field dependent and the relative error in both [illustration not visible in this excerpt] and [illustration not visible in this excerpt] is reduced for larger [illustration not visible in this excerpt] values.

illustration not visible in this excerpt

1.4 Magnetic Refrigeration and materials

Magnetic refrigeration is based on a fundamental thermodynamic property of magnetic materials which is so called magnetocaloric effect. Magnetic refrigeration has been recognised as being an alternative technology to the conventional vapour compression technology [1]. After the discovery of the MCE in 1881 by Warburg, the effect has been successfully applied for the adiabatic demagnetization refrigeration by which ultra low temperatures are often achieved today. This was a simple one step cooling process. Since the 1950's a few continuous magnetic refrigerators operating at various temperatures from 1-300K have been constructed and tested. But most were inefficient and were run for only a few days at most. The discovery of the GMCE and the construction of the proof-of- principle magnetic refrigerator in 1997 by Astronautics Corporation of America/Ames Laboratory team of scientists have generated a lot of interest about both the MCE and magnetic refrigeration.

Fig 1.6 describes the principle of magnetic refrigeration. The spins are initially random in a zero magnetic field (Fig. 1.6) (a), upon adiabatic magnetization (b), the material heats up because of magnetic entropy decrease due to increasing magnetic order in the system and the heat is removed by a heat transfer fluid. Upon adiabatic demagnetization(c), the material cools down and it cools a load in a cold heat exchanger. Continuous refrigeration is achieved by repeating (b) and (c).

illustration not visible in this excerpt

Figure 1.6: (a) A magnetocaloric material (b) heats up when magnetized (c) When demagnetized and cooled (d) it experiences a sharp temperature drop. The property has promise as an everyday refrigerant. (Image: National Institutes of Standards and Technology)

The Astronautics/Ames Lab demonstrated the unit operating near room temperature using magnetic fields between 1.5 and 5T. This unit has run “maintenance free” for over 1500h. Several notable achievements were obtained with this demonstration unit have investigated B.F.Yu et al [28]:

i) A record cooling power of 600watts (about 100 times greater than previous near room temperature magnetic refrigerators);
ii) A coefficient of performance (COP), i.e. the cooling power divided by the input work, of 15 (typical gas compression cycle refrigeration’s have COPs between 2 and 6);
iii) A maximum efficiency of 60% of Carnot (the seal friction was subtracted off) compared to conventional vapour cycle refrigeration with a 40% of theoretical Carnot limit; and
iv) A maximum temperature span of 3 8K (the difference in the temperatures of the hot and cold heat exchangers).

Beside this, the magnetic refrigeration is an environmentally friendly cooling technology (figure 1.7 in details). It does not use ozone-depleting chemicals (such as chlorofluorocarbons), hazardous chemicals (such as ammonia), or greenhouse gases (hydrochlorofluorocarbons and hydrofluorocarbons) [30].

Most modern refrigeration systems and air conditioners still use ozone-depleting or global warming volatile liquid refrigerants. Magnetic refrigerators use a solid refrigerant (usually in a form of spheres or thin sheets) and common heat transfer fluids (e.g. water, water-alcohol solution, air, or helium gas) with no ozone -depleting and/ or global warming effects. Another important difference between vapour-cycle refrigeration’s and magnetic refrigeration’s is the amount of energy loss incurred during the refrigeration cycle. Even the newest most efficient commercial refrigeration units operate well below the maximum theoretical (Carnot) efficiency, and few, if any, further improvements may be possible with the existing vapour-cycle technology. However, magnetic refrigeration is rapidly becoming competitive with conventional gas compression technology because it offers considerable operating cost savings by eliminating the most inefficient part of the refrigerator: the compressor. Zimm et al [29] have reported that the cooling efficiency of magnetic refrigerators working with Gd has been shown[2,6,28,and 29] to reach 60% of the carnot limit, compared to only about 40% in the best gas-compression refrigerators[28]. However, with the currently available magnetic materials, this high efficiency is only realized in high magnetic fields of 5T.Therefore, research for new magnetic materials displaying larger MCE, which then can be operated in lower fields of about 2T that can be generated by permanent magnet, is very significant [28]. The heating and cooling that occurs in the magnetic refrigeration technique is proportional to the size of the magnetic moments and to the applied magnetic field. This is why research in magnetic refrigeration is at present almost exclusively conducted on superparamagnetic materials and on rare- earth compounds [28].

illustration not visible in this excerpt

Figure 1.7 Schematic representation of a magnetic-refrigeration cycle, which transports heat from the heat load to its surroundings. Yellow and green depict the magnetic material in low and high magnetic fields, respectively. Initially randomly oriented magnetic moments are aligned by a magnetic field, resulting in heating of the magnetic material. This heat is removed from the material to its surroundings by a heat-transfer medium. On removing the field, the magnetic moments randomize, which leads to cooling of the magnetic material below the ambient temperature. Heat from the system to be cooled can then be extracted using a heat-transfer medium. Depending on the operating temperature, the heat-transfer medium may be water (with antifreeze) or air, and for very low temperatures, helium. Taken from Ref [30]

Refrigeration in the subroom temperature (250-290) range is of practical interest because of potential impact on energy savings and environmental concerns.

Materials used in magnetic refrigerators should be soft ferromagnetic materials with large MCE and appropriate ordering temperature. Soft magnetic materials are used to reduce hysteresis losses. A large MCE value will increase COP of the refrigerator. Most of the research on the MCE has been associated with materials ordering from 4-77 К for applications such as helium and hydrogen liquefaction, or materials ordering near room temperature for applications such as conventional air conditioning and refrigeration. Materials to be applied refrigeration must present a series of properties:

i) A first-order field -induced transition around the working temperature, in order to utilise the associated entropy change.
ii) A high refrigerant capacity. Refrigerant capacity, q, is a measure of how much heat can be transferred between the cold and hot sinks in one ideal refrigeration cycle, and it is calculated as:
illustration not visible in this excerpt
Therefore, a large entropy change in temperature range as wide as possible is needed. Moreover it is easy to say that for any practical application it is the amount ofheat energy per unit volume transferred in one refrigeration cycle, which is the important parameter,i.e., the denser the magnetic refrigerant the more effective it is.[31]
iii) A low magnetic hysterisis, to avoid magnetic work losses due to the rotation of domains in a magnetic refrigeration cycle.
iv) A low heat capacity cp, since a high cp increases the thermal load and more energy is required to heat the sample itself and causes a loss in entropy, i.e., for a given AS, ATd will be lower.
v) Low cost and harmless. The main problem of the rare- earth -based compounds, which are usually the best magnetic refrigerants in the whole temperature range (including pure

Gd at room temperature) is their high cost. 3d-transition-metal compounds or ceramic manganites are a good alternative concerning the cost of the materials. In particular MnAs - based materials show good prospects [32, 33]. However, the presence of As in these compounds, which is poisonous, could make them be useless for commercial applications. Another type of compounds, an (FexSii_x)13 also presents a large MCE at room temperature, has a low cost and in this case all elements are harmless [34].

It was reported lately that American Astronautic Corporation combined with Ames laboratory developed 'the world first room temperature, permanent magnet; magnetic refrigerator' in September 2001(called a laboratory prototype MR). It is a rotary device using a C-shaped permanent magnet to generate the field. It achieved a no load cooling power of 95 W running at a frequency of 4 Hz. This MR runs on a 6 V motorcycle battery, and it can run continuously for 6 h before the battery needs researching[7,28].The idea of this prototype is based on utilizing a MCE material under field cycling, magnetization/ demagnetization using rotating permanent magnet. The refrigerator is schematically shown in fig (1.8).

illustration not visible in this excerpt

Figure 1.8: Astronautics Corporation of America

laboratory prototype permanent magnet, rotating bed magnetic refrigerator (a) schematic and (b) photograph [Ref 35].

1.5 Magnetocaloric Materials

There are several promising classes of materials with large MCEs and tunable Curie temperatures. All of them show great potential for room-temperature applications [7]. The properties that define a good performance of a magnetocaloric material may vary with the refrigeration system and desired temperature interval. The two parameters that provide a good basis for comparison are the adiabatic temperature change and the isothermal magnetic entropy change as was explained above. A general classification of magnetocaloric materials divides them into two types according to the nature of the magnetic phase transition used to exploit the MCE. Since the MCE is highest around magnetic phase transitions the behavior of the materials around these transitions are crucial to the performance of the refrigerant. They can be divided into first order magnetic transition (FOMT) materials and second order magnetic transition (SOMT) materials. This division into two classes is convenient since certain characteristics in general follow each class.

FOMT materials are currently thought to be the most promising candidates, since they show the highest measured values of the MCE. FOMT reach these high values of the MCE, since they have a very abrupt change in magnetization as function of temperature. The derivative of the magnetization is proportional to the magnetic entropy change and the very abrupt phase transitions therefore typically give a high MCE in a narrow temperature range. On the other hand, SOMT materials in general have broader transitions with lower values of the MCE. Since in technological applications several other material properties are of importance, the choice between FOMT and SOMT materials are therefore not as straightforward as it might seem. FOMT’s have in general slower kinetics than SOMT’s, since they are normally connected with structural changes. This leads to different hysteresis phenomena which are un- wanted in technological applications. Furthermore most of the known FOMT material systems contain either very expensive elements (e.g. Gd), toxic elements (e.g. As) or have very complicated and costly synthesis routes (e.g. La-Fe-Si-H). A variety of SOMT materials with expensive constituents or difficult synthesis routes has also been suggested. Due to several advantages in applications there is a continued interest in the class of mixed-valence manganites crystallizing in the perovskite structure despite the inferior MCE compared to many of the FOMT materials. Manganites are cheap, non-toxic, resistant to corrosion, easy to manufacture, have a decent MCE and have easily tunable transition temperatures. In the above classification they lie in between FOMT and SOMT materials depending on the compositions. Several extensive reviews on magnetocaloric materials exists [7, 28, 30]. Figure 1.4 has taken from [2] shows a comparison of some of the most investigated magnetocaloric materials in a plot of [illustration not visible in this excerpt] (T)ah = 0.5T versus transition temperature. It is quite evident from the figure that the FOMT materials have the highest MCE’s, however most prototype magnetic refrigerators use pure gadolinium (Gd) as refrigerant, which is a SOMT material.

Magnetocaloric materials have the most extensive MCEs around magnetic phase transitions. Figure (1.9) shows the behaviour of a magnetocaloric material is shown as function of temperature. The maximum MCE is obtained in the vicinity of magnetic phase transitions such as the ferro- to paramagnetic phase transition, which implies that magnetocaloric materials must have a magnetic phase transition temperature close to the working range of the refrigerator in order to be a potential candidate for magnetic refrigeration. In order to develop new refrigerants the MCEs has to be evaluated in a consistent manner in order to be able to compare the materials.

illustration not visible in this excerpt

Figure 1.9: The maximum magnetocaloric effect is obtained in the vicinity of magnetic phase transitions such as the ferro- to paramagnetic phase transition. The magnetization, M, the adiabatic temperature change, ATad, and the isothermal magnetic entropy change, [illustration not visible in this excerpt], are here shown as functions of temperature around the phase transition temperature T_c .

[...]