Magnetocaloric Effect

A thorough understanding of the magnetocaloric properties of existing magnetic refrigerant materials has been an important issue in magnetic refrigeration technology. This paper reviews a new class of magnetocaloric material, that is, the ferromagnetic perovskite manganites (R 1Àx M x MnO 3 , where R ¼ La, Nd, Pr and M ¼ Ca, Sr, Ba, etc.). The nature of these materials with respect to their magnetocaloric properties has been analyzed and discussed systematically. A comparison of the magnetocaloric effect of the manganites with other materials is given. The potential manganites are nominated for a variety of large-and small-scale magnetic refrigeration applications in the temperature range of 100-375 K. It is believed that the manganite materials with the superior magnetocaloric properties in addition to cheap materials-processing cost will be the option of future magnetic refrigeration technology. r

The magnetocaloric effect is calculated for superparamagnetic materials as a function of temperature, field and cluster size. Assuming classical behavior, a universal curve is calculated from which an optimum cluster moment may be found for maximum entropy change upon application of a given field H at a given temperature T. Quantum effects are shown to be small for temperatures above 10 K and fields less than a few tesla. A comparison with results for a spin -7 paramagnet such as GdaGasOx2 (GGG) is made, which indicates that superparamagnetic materials such as magnetic nanocompositeo -ffer the possibility of extending the upper useful temperature limit of paramag~etic materials for magnetic refrigeration.

In recent years, intensive studies on thermal control devices have been
conducted for the thermal management of electronics and computers as well as for applications in energy conversion, chemistry, sensors, buildings, and outer space. Conventional cooling or heating techniques realized using traditional thermal resistors and capacitors cannot meet the thermal requirements of advanced systems. Therefore, new thermal control devices are being investigated to satisfy these requirements. These devices include thermal diodes, thermal switches, thermal regulators, and thermal transistors, all of which manage heat in a manner analogous to how electronic devices and circuits control electricity. To design or apply these novel devices as well as thermal
control principles, this paper presents a systematic and comprehensive
review of the state-of-the-art of fluidic and mechanical thermal control devices that have already been implemented in various applications for different size scales and temperature ranges. Operation principles, working parameters, and limitations are discussed and the most important features for a particular device are identified.

Thermal management technology based on the magnetocaloric effect offers several advantages over conventional gas compression cooling. The efficiency of magnetic cooling systems can be much higher than conventional gas based cooling technologies. Additionally, ozone layer depleting chemicals are not used and there is reduced noise and vibrations. Iron and manganese based magnetocaloric materials (MCM) are promising due to the challenges surrounding the use of conventional rare earth based MCM. We review the recent progress in the development of iron and manganese based MCM. The magnetic phase transitions, processing techniques, performance , as well as applications of these materials are discussed. Critical analysis to determine the critical exponents and phase transition behavior of these MCM, using modified Arrot plot, critical isotherm plots, the Kouvel-Fisher method, Landau theory and the Bean-Rodbell model, is also presented.
Keywords: Magnetocaloric materials, Magnetocaloric effect, Fe based alloys, Mn based alloys, Modeling

Alloy ribbons of nominal composition MnNiGe1.05 were produced using the melt-spinning technique. As-quenched (aq) polycrystalline ribbons are single-phase showing the hexagonal Ni 2In-type crystal structure. After thermal annealing at 1148 K, the formation of the orthorhombic TiNiSi-type crystal structure by martensitic transformation is favored. However, XRD patterns for different temperatures indicate that the phase transition from hexagonal to orthorhombic structure is incomplete. The starting and finishing temperatures for the direct and reverse martensitic transformation for aq (annealed) samples determined by DSC were MS  = 264 (268) K Mf  = 235 (255) K, AS  = 259 (266) K, and Af  = 289 (276) K. Across this structural phase transition the annealed sample undergoes a drop in magnetization giving rise to a narrow temperature dependence of the magnetic entropy change with a peak value on heating (cooling) of 5.8 (4.8) Jkg−1K−1 for a field change of 5 T.

One of the challenges of modern societies consists in to increase the equipment energy efficiency, whereby reducing the energy consumption. In this sense, the magnetic solid-state refrigeration technology based on the magnetocaloric effect (MCE), attracts an enormous interest because of its potential to substitute the conventional liquid-gas refrigerant systems due to, among other advantages, its superior efficiency (up to 60% of Carnot's cycle) [1,2]. However, to be commercially competitive, this technology still needs cheap materials with enhanced refrigerant properties. Among the potential materials, metamagnetic shape memory alloys (mainly, Heusler-type Ni-Mn-based alloys) occupy a unique place because, alongside the shape memory effect and superelasticity, they exhibit large magnetocaloric effect due to the sharp change of the magnetization associated to the magnetostructural martensitic transformation (MT) [4].
We will present our recent studies of both the magnetocaloric effect and the influence of magnetic field on MT in metamagnetic Ni-Mn-In alloys doped by Cu and Cr. This doping mode allows a fine tuning of both the MT temperature around the room temperature (278-315 K) and magnetization drop at MT. The adiabatic MCE measurements have been performed using in-house made set-up [3]. An application of 1.9 T magnetic field results in a maximum inverse adiabatic temperature change of ~ -2 K caused by magnetic field-induced MT. Besides, the austenite phase undergoes a ferro-to-paramagnetic transition to which a direct adiabatic temperature change of almost the same amplitude as for inverse effect is associated. Furthermore, MT moves to lower temperatures (around 40 K for Cu-doped alloy) in magnetic fields up to 10 T accompanied by a decrease of the transformation entropy change.
References:
1. M.-H. Phan and S.-C. Yu, J. Magn. Magn. Mater. 308, 325 (2007).
2. V. Franco, J.S. Blázquez, B. Ingale, and A. Conde, Annu. Rev. Mater. Res. 42, 305 (2012).
3. V.A. Chernenko et al., J. Magn. Magn. Mater. 324, 3519 (2012).
4. P. Álvarez-Alonso et al., Key Eng. Mater. 644, 215–218 (2015).

The magneto-caloric effect (MCE) of arc-melted bulk and 10 h ball-milled nanostructured Pr2Fe17 powders has been investigated. The maximum value for the magnetic entropy change, |ΔSM|, in the milled alloy is 4.5 J kg−1 K−1 for μ0H = 5 T, at around room temperature. The full width at half maximum, δTFWHM, of |ΔSM|(T) for the nanostructured powders is about 60% greater than that of the starting bulk alloy, thus giving rise to large relative cooling power values of 573 J kg−1 (4.5 J cm−3) for μ0H = 5 T estimated from the product of |ΔSM|max × δTFWHM. These results have been compared with those of well-known magnetic materials that exhibit a large or giant MCE effect. The potential for using these low-cost iron based nanostructured Pr2Fe17 powders in magnetic refrigeration at room temperature is also discussed.

The main goal of this study was an experimental comparison of six different active 12 magnetic regenerators (AMRs) with gadolinium as the magnetocaloric material. The analysis was 13 carried out for three different parallel-plate AMRs (with different porosities and different 14 orientations of the plates in the magnetic field) and three different packed-bed AMRs (filled with 15 spheres, powders and cylinders). Since the operation of an AMR is strongly affected by the 16 operating conditions, the experiments were performed at different mass-flow rates and at 17 different operating frequencies. These were required in order to define the optimum 18 corresponding operating conditions for the analyzed AMRs. As comparative criteria the 19 maximum temperature span, the cooling capacity and the experimentally predicted COP were 20 taken into consideration. 21 2 The experimental analysis was performed on a new prototype of magnetic refrigerator 22 designed as an experimental device. Its operation is based on the linear movement of a 23 permanent-magnet assembly over a static AMR. The magnet assembly provides a measured 24 magnetic field of about 1.15 T.

Magnetic refrigeration is an emerging technology that exploits the magnetocaloric effect found in solid-state refrigerants. The combination of solid-state refrigerants, water-based heat transfer fluids, and high efficiency will lead to environmentally desirable products with minimal contributions to global warming. Among the numerous applications of refrigeration technology, air conditioning applications provide the largest aggregate cooling power and use the greatest quantity

The efficiency of bio-molecular motors stems from reversible interactions ∼ k B T; weak bonds stabilizing intermediate states (enabling direct conversion of chemical into mechanical energy). For their (unknown) origins, we suggest that a magnetically structured phase (MSP) formed via accretion of superparamagnetic particles (S-PPs) during serpentinization (including magnetite formation) of igneous rocks comprising the Hadean Ocean floor, had hosted motor-like diffusion of ligand-bound S-PPs through its ‘template’-layers. Ramifications range from optical activity to quantum coherence. A gentle flux gradient offers both detailed-balance breaking non-equilibrium and asymmetry to a magnetic dipole, undergoing infinitesimal spin-alignment changes. Periodic perturbation of this background by local H-fields of templatepartners can lead to periodic high and low-template affinity states, due to the dipole’s magnetic degree of freedom. An accompanying magnetocaloric effect allows interchange between system-entropy and bath temperature. We speculate on a magnetic reproducer in a setting close to the submarine hydrothermal mound-scenario of Russell and coworkers that could evolve bio-ratchets.

La 1Àx Pb x MnO 3 (x ¼ 0:1; 0.2, 0.3, 0.4, and 0.5) perovskites were prepared by a solid-state reaction. Except for x ¼ 0:5 (cubic) and x ¼ 0:4 (rhombohedral), the structure of the other compositions was pseudo-rhombohedral with P1 symmetry. The particle size of the grains is depending on the Pb content of the samples. The Curie temperature T c increases from 235 K for x ¼ 0:12310 K for x ¼ 0:2 and is almost constant (about 360 K) for xX0:3: The field-cooled and zero-field-cooled thermomagnetic curves measured at low field show a split below a so-called irreversibility temperature T r ; which is somewhat smaller than T c except for x ¼ 0:1; where it is 270 K. From a series of magnetic isotherms the magnetic entropy changes DSðTÞ were determined for a field step of 500 Oe. The maximum value of DS max increases with increasing x till x ¼ 0:3 and then decreases with further increasing x: The conductivity of perovskites is metallic at low temperatures and semiconducting at high temperatures. Magnetoresistance measurements have been performed. r

The temperature dependence of the isothermal magnetic entropy change, ΔSM, and the magnetic field dependence of the refrigerant capacity, RC, have been investigated in a composite system xA + (1 − x)B, based on Fe87Zr6B6Cu1 (A) and Fe90Zr8B2 (B) amorphous ribbons. Under a magnetic field change of 2 T, the maximum improvement of the full-width at half maximum of ΔSM(T) curve (47% and 29%) and the RC (18% and 23%), in comparison with those of the individual alloys (A and B), is observed for x ≈ 0.5. Moreover, a flattening over 80 K in the ΔSM(T) curve around room temperature range is observed, which is a key feature for an Ericsson magnetic refrigeration cycle.

This work investigates the magnetic and the intrinsic exchange bias (EB) properties of the sol-gel synthesised LaMnO3 and LaFeO3 perovskite compounds. The x-ray diffraction (XRD) has proved the high homogeneity of both compounds, which are crystallised in the orthorhombic Pnma structure (as proved by Rietveld refinement). The field cooling-zero field cooling magnetisation dependent temperature (M(T)) indicates the antiferromagnetic (AFM) nature of these compounds. Nevertheless, an anomalous ferromagnetic (FM) behaviour is observed, which is more likely, arises from the spin canting effect in the Mn3+ and Fe3+ ions. The thermal variation of the magnetisation reciprocal (M−1) has confirmed the presence of this FM component below 110K in LaMnO3 and the Curie-Weiss behaviour above 160K. Also, the magnetic hysteresis loops below 110K are corresponding to the FM-like behaviour. The spin of the FM component couples with the AFM phase spins, leading to the EB effect in both compounds that show maximum values of −1124Oe and 2343 Oe for the LaMnO3 and LaFeO3 compounds, respectively. It is observed that the EB effect is suppressed with the partial substitution of La3+ by Ba2+ due to the dominance of the FM phase and the absence of the FM/AFM coupling. Also, the influence of the FM-AFM phases co-existence on the magnetocaloric(MCE) properties of the LaMnO3 compound were studied. Where the LaMnO3 compound shows a magnetic entropy change (ΔS) of 0.42 J/kg.K with an adiabatic temperature change (ΔTad) of 0.1k that is improved in the La0·8Ba0·2MnO3 compound to 1.3 J/kg.K and 0.4K, respectively.

Single-phase polycrystalline samples of La 0.7 Sr 0.3 Mn 1-x Cr x O 3 with nominal composition of x ¼ 0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature T C decreases with increasing x and the maximum magnetic entropy change (ÀDS M ) for x ¼ 0.20 is $1.203 and $2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.

Conventional and anisotropic magnetocaloric effects were studied in cubic rare earth RNi2 (
R=Nd,Gd,Tb) ferromagnetic intermetallic compounds. These three compounds are representative of small, null, and large magnetocrystalline anisotropy in the series, respectively. Magnetic measurements were performed in polycrystalline samples in order to obtain the isothermal magnetocaloric data, which were confronted with theoretical results based on mean field calculations. For the R=Tb case, we explore the crystalline electrical-field anisotropy to predict the anisotropic magnetocaloric behavior due to the rotation of an applied magnetic field of constant intensity. Our results suggest the possibility of using both conventional and anisotropic magnetic entropy changes to extend the range of temperatures for use in the magnetocaloric effect.

The magnetic phase transition and the magnetic entropy change (ÀDS M ) in the La 0.67 Ba 0.33 Mn 0.9 Cr 0,1 O 3 manganite were investigated by measuring the magnetization as a function of temperature. The maximum magnetic entropy change (ÀDS M ) and the relative cooling power (RCP) are found to be, respectively, 4.20 J kg À1 K À1 and 238 J kg À1 for a 5-T field change, making of this material a promising candidate for magnetic refrigeration near room temperature.

Mn-rich Ni-Mn-Sn metamagnetic shape memory alloys exhibiting magnetostructural transformation are of a great potential as the base materials for solid-state refrigeration. With the aim of fine tuning of the transformation characteristics and improving functional properties, in the present work we have fabricated polycrystalline Ni 50-x Fe x Mn 40 Sn 10 (x = 0, 2, 4, 6, 8 at.%) melt-spun ribbons, starting from the base alloy with x = 0, which is weakly magnetic in both austenitic and martensitic phases. By exploring martensitic transformation (MT) and magnetic behaviors as a function of Fe doping and magnetic field, we have found that Fe and/or magnetic field reduce the MT temperature and Curie temperature of austenite phase, becoming closer to each other as the Fe-content increases, accompanied by an increase of the magnetic moment of austenite, magnetization jump at MT, transformation volume, and magnetic contribution, ∆S M , to the total entropy change at MT. The ribbons present moderate values of ∆S M equal to 11 J kg-1 K-1 at 5 T for x = 8, moderate thermal hysteresis (10-14 K) nearly independent of Fe doping or magnetic field, and adjustable structural and magnetic transition temperatures close to room temperature.

Structural, magnetic, magnetocaloric and magnetoresistance (MR) studies on La0.7Sr0.3Mn0.95Cu0.05O3 (No. 1) and La0.7Sr0.3Mn0.9Cu0.1O3 (No. 2) perovskites are reported. The crystal structure of the samples is rhombohedral with a change of the lattice constants depending on the Cu content. FC and ZFC thermomagnetic measurements for both compositions at low field indicate that a spin-glass-like state (or cluster glass) occurs at low temperatures and a very sharp change of magnetization around the phase-transition point. The Curie temperature, TC, does almost not depend on the content of Cu substitution. A maximum magnetic-entropy change, ΔSmax, of 1.96 and 2.07J/kgK at 13.5kOe and 350K is observed for sample No. 1 and No. 2, respectively. Therefore, they can be considered as active magnetic refrigerant materials for room-temperature applications. Electrical-resistance measurements show that both samples are metallic conductor for T<TC and semiconductor for T>TC moreover, the MR is maximal around TC.

Using high-energy ball milling, nanostructured Pr2Fe17 powders can be obtained from their arc-melted bulk alloys. High-resolution X-ray and neutron powder diffraction experiments reveal that the Th2Zn17-type rhombohedral crystal structure is maintained, after milling for 10 h, with almost unchanged values for both crystalline lattice parameters (Δa; Δc < 0.05%) and vanishing mechanically induced microstrain (<0.1%). Although the mean crystalline size decreases down to 20 ± 3 nm, magnetovolume anomalies observed in pure Pr2Fe17 are still present with a significant volume decrease on heating from 5 up to 320 K. After the milling, a significant increase in the magnetic anisotropy, due to the drastic reduction in crystalline size, is observed, while the value of the magnetic moment seems to be increased slightly (5%). In addition, the magnetocaloric effect of bulk and nanostructured Pr2Fe17 has been investigated. The magnetic entropy change, |ΔSM|, decreases from 6.3 to 4.5 J kg−1 K−1 under an applied magnetic field μ0H = 5 T after the milling process. However, the width of the |ΔSM|(T) curve is substantially enlarged and hence the refrigerant capacity is enhanced. These findings make the iron-based nanostructured Pr2Fe17 powders interesting for applications in magnetic refrigeration at around room temperature.

In this article, we investigate the influence of the first-order ferromagnetic-paramagnetic phase transition, on the magnetocaloric effect, under the combined effect of external magnetic field, pressure and magnetoelastic deformation. An application is made to Gd 5 (Si x Ge 1Àx ) 4 , for x ¼ 0:43 and 0.5. The obtained result leads to a good theoretical adjustment of the experimental data for isothermal magnetic entropy change. r

Recent investigations in R2Fe17 intermetallic compounds have evidenced that these materials present a moderate magnetocaloric effect (MCE) near room temperature. A series of accurate magnetization measurements was carried out to show that the value of the demagnetizing factor has a significant influence on the absolute MCE value of Er2Fe17. In addition, the critical exponents determined from heat capacity and magnetization measurements allow us to describe the field dependence of the observed MCE around the Curie temperature.

The influence of Ti-doping on the magnetic and magnetocaloric properties of La0.67Ba0.33Mn0.98Ti0.02O3
perovskite is investigated. La0.67Ba0.33Mn0.98Ti0.02O3 sample was prepared by ceramic route at 1400 ◦C.
It is a cubic Pm–3m single phase and exhibits a sharp ferromagnetic–paramagnetic (FM–PM) transition
at a Curie temperature TC (314 K) which is very close to room temperature. Above TC the data
follow a Curie–Weiss law with a shift between experimental and calculated effective paramagnetic
moment. The associated experimental magnetic entropy change (SM) and the relative cooling power
(RCP) have been determined. The observed field dependence of SM is explained reasonably well by
the Landau theory of second order phase transition. The maximum entropy change |Smax
M
| exhibits
a linear dependence with the applied magnetic field. |Smax
M
| and RCP are respectively 3.21 J kg−1 K−1
(21.48 mJcm−3 K−1) and 307 J kg−1 (2054 mJcm−3) at 5 T, which are about 30% of pure Gd. Our results
on the magnetocaloric effect (MCE) are compared favourably with reported values for other doped
manganites, thus concluding that our sample can be used as a magnetic refrigerant around room
temperature.

The study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isothermal entropy change and direct adiabatic temperature change, is not straightforward since huge differences between these two quantities have often been reported. Here we put in relation the direct and indirect measurements on the first order magnetostructural martensitic transformation occurring in Ni-Co-Mn-Ga alloys. In order to complete the characterization of the MCE and to find an explanation of these mismatches, differential scanning calorimeter measurements have been performed at different applied magnetic fields.

The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group ) exhibiting a saturation magnetization and Curie temperature of M S = 157 ± 2 A m2 kg−1 and T C = 21.5 ± 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change |ΔS M peak| = 13.5 J kg−1 K−1, and a refrigerant capacity RC = 209 J kg−1. Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications. F d 3 ¯ m

The magnetic entropy change for two-ribbon (amorphous) composite materials is investigated.The conditions to obtain a table-like shape of the magnetic entropy change are specified.We give the essential ingredients to maximize the effective refrigerant capacity and the efficiency.Our findings could be used in other magneto-caloric materials to tune the temperature range for the table-like behavior.We show a systematic study of the magneto-caloric response carried out on a series of FeZrB(Cu) amorphous ribbons with different Curie temperature values in the 210–320 K interval. The main aim of the work is to investigate the conditions to obtain, from the isothermal magnetic entropy change vs. temperature curves, ΔSM(T), a table-like behavior of the entropy using two-ribbon composites. Even though the maximum value of ΔSM for the composite is lower than those of the single components, the existence of a table-like behavior maximizes the effective refrigerant capacity, reaching values around 80 J/kg for an applied magnetic field change of 2 T. Furthermore, we discuss how the temperature range for such a table-like behavior can be tuned and the refrigerant capacity enhanced in terms of energy efficiency.

We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powder diffraction show that the crystal structure is rhombohedral with View the MathML source space group (Th2Zn17-type). The investigated compound exhibits a broad isothermal magnetic entropy change ΔSM(T) associated with the ferro-to-paramagnetic phase transition (TC ≈ 290 K). The |ΔSM| (≈2.3 J kg−1 K−1) and the relative cooling power (≈100 J kg−1) have been calculated for applied magnetic field changes up to 1.5 T. A single master curve for ΔSM under different values of the magnetic field change can be obtained by a rescaling of the temperature axis. The results are compared and discussed in terms of the magneto-caloric effect in the isostructural R2Fe17 (R = Y, Pr and Nd) binary intermetallic alloys.

The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compositions and Curie temperatures between 230 and 300 K has been studied. Albeit the maximum entropy change is low (|ΔSM|max ∼ 3 J kg−1 K−1 under an applied magnetic field change from 0 to 50 kOe), the magneto-caloric effect (MCE) spreads out over a broad temperature interval (ΔT ∼ 200 K). The estimated values for the relative cooling power are close to that of pure Gd. From the applied magnetic field dependence of the magnetic entropy change a master curve valid for these Fe-rich FeZrB amorphous alloys has been found. However, two reference temperatures of the |ΔSM|(T) are needed in order to get a collapsing curve for all the maximum applied magnetic field values. The latter suggests that the striking magnetic behaviour of these compounds below 200 K influences the MCE effect. Moreover, the collapse into the same curve for different rescaled temperature dependences of the magnetic entropy change in the three samples implies the existence of similar magnetic interactions in these compounds.

A detailed investigation of structure, critical behaviour and magnetocaloric properties of Ni 50 Mn 30 Sn 20 (Sn20) and Ni 50 Mn 30 In 20 (In20) alloys has been investigated by means of X-ray diffraction and magnetic measurements. Ni 50 Mn 30 Sn 20 alloy shows a cubic austenite L2 1 structure and undergoes a second order magnetic transition at a Curie temperature of T A c;1 Sn20 ð Þ¼333 K. However, the Ni 50 Mn 30 In 20 alloy exhibits a mixture of cubic L2 1 and B2 austenite structures having Curie temperatures of T A c;2 In20 ð Þ¼ 285 K and T * c In20 ð Þ¼ 330 K, respectively. The modified Arrott plots, Kouvel-Fisher curves and critical isotherm analysis have been used to estimate the critical exponents (β, γ and δ) around the Curie temperature. For Sn 20 alloy, the reliable exponents are consistent with the mean field model, revealing long-range ferromagnetic interactions. Nevertheless, the critical exponents of In 20 alloy around 330 K cannot be arranged into any of the universality classes of well-known classical standard models. The maximum entropy change under 5 T of Sn20 (ΔS max M ¼ 2:43 J kg :K) is slightly higher than that of In20 (ΔS max M ¼ 2:05 J kg :K). The experimental results of entropy changes are in good agreement with those calculated using Landau theory.

We report the effect of carrier doping via partial substitution of La 3 þ for Sr 2 þ on the structural, magnetic and magnetocaloric properties of Sr 2 FeMoO 6 double perovskite. Polycrystalline Sr 2 À x La x FeMoO 6 (x ¼0.0, 0.1, 0.2, 0.3) samples were prepared using the conventional solid state reaction method. Using the X-ray diffraction (XRD) analysis it was established that all the samples crystallized in a tetragonal structure with I4/mmm space group. An increase in the La doping lead to an increase in the lattice parameter 'a' and the volume of the unit cell. The lattice parameter 'c' however remained unchanged. The temperature variation in magnetization and Arrott analysis suggested a second order of ferromagnetic phase transition in all samples with Curie temperature, T C increasing from 358 K for x ¼ 0.0–365 K for x ¼0.3. A gradual increase in magnetization was also observed with the increasing La content up to x ¼ 0.2. The magnetic entropy change was calculated from the measurement of isothermal magnetization versus magnetic field at different temperatures. The tunability of magnetization and T C simply by adjusting the concentration of La and synthesis conditions makes Sr 2 FeMoO 6 an attractive material for magnetic refrigeration at desired temperature.

The magnetocaloric effect is studied at the transition to saturation in the antiferromagnetic spin-1/2 Heisenberg model on the simplest two-dimensional lattices, namely the square and the triangular lattice. Numerical results are presented for the entropy which are consistent with identical universal properties. However, the absolute values of the entropy are bigger on the geometrically frustrated triangular lattice than on the non-frustrated square lattice, indicating that frustration improves the magnetocaloric properties.

The critical properties of the manganese perovskite La 0.67 Ba 0.33 Mn 0.98 Ti 0.02 O 3 around the paramagnetic-ferromagnetic phase transition were investigated through various techniques such as modified Arrott plot, KouveleFisher method and critical isotherm analysis based on the data of static magnetic measurements recorded around the Curie temperature T c . The magnetic data analyzed in the critical region using the above methods yield the critical exponents b ¼ 0.551 AE 0.008 with T c ¼ 310.47 K AE 0.10 (from the temperature dependence of the spontaneous magnetization below T C ) and g ¼ 1.020 AE 0.024 with T C ¼ 310.11 K AE 0.14 (from the temperature dependence of the inverse initial susceptibility above T C ) and d ¼ 2.826, determined separately from the isothermal magnetization at T C . These critical exponents fulfill the Widom scaling relation d ¼ 1 þ g/b, implying that the obtained values of b and g are reliable. Based on these critical exponents, the magnetizationefieldetemperature (MeH eT) data around T c collapse into two curves obeying the single scaling equation

The behavior of active magnetic regenerators (AMR) can be described by means of a one-dimensional set of equations. A lot of geometrical, thermo-physical and magnetic parameters influence the behavior and performance of an AMR, together with external controls and operating parameters. Literature data on AMR performances, both experimental and numerical, are given in different conditions and configurations, so that a consistent comparison among different results is not straightforward. In this paper the most relevant parameters are outlined, by means of a dimensionless derivation of the AMR equations. The set of parameters is proposed as a set of information that should be reported together with experimental or numerical results, in order to specify meaningful and consistent operating conditions.

A theoretical model including both crystal-field and exchange interactions that considers the effect of magnetic fluctuations is developed to evaluate the temperature dependence of the isothermal magnetic entropy changes in ferromagnetic rare-earth-based intermetallic compounds. The Green's functions are derived from their equation of motion. The magnetic moment correlation functions are determined beyond the random phase approximation by incorporating a measure of magnetic spontaneous fluctuations in a way that ensures self-consistency with regard to the fluctuation-dissipation theorem. In particular, the exact magnitude of the entropy change without magnetic moment fluctuations depends on the ratio of both the crystal-field first-and the crystal-field third-order magnetic susceptibilities at the Curie temperature, T(C). These theoretical predictions are compared with experimental data on cubic RM(2) (R = rare earth and M = Al and Ni) compounds, where the principal crystal-field and exchange parameters are well known.

We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, for comparing the effect of thickness and substrate on their magnetic and magneto-thermal properties, with particular focus on the nature of the magneto-structural transition between the ferromagnetic hexagonal a-phase and the paramagnetic orthorhombic b-phase. The nature of this transformation depends on the presence of strain and turns out to be of second-order type for epitaxial-crystalline MnAs/(0 0 1)GaAs films and closer to a first-order type for polycrystalline MnAs/SiO 2 /Si films. The magnetocaloric effect at the transition has been estimated for thin film samples, resulting appreciably lower than that of bulk MnAs.

ErNi 2 ribbons were produced by rapid solidification using the melt spinning technique. Their structural, magnetic and magnetocaloric properties in the as-solidified state were studied by X-ray diffraction, scanning electron microscopy, magnetization and specific heat measurements. Samples are single phase with the MgCu 2-type crystal structure, a Curie temperature T C of 6.8 K and a saturation magnetization at 2 K and 5 T of 124.0 A$m 2 /kg. For a magnetic field change m 0 DH of 5 T (2 T) ribbons show a maximum magnetic entropy change jDS M peak j of 24.1 (16.9) J/(kg$K), and an adiabatic temperature change DT ad max of 8.1 (4.4) K; this is similar to the previously reported literature for bulk alloys that were processed through conventional melting techniques followed by prolonged thermal annealing. In addition, the samples also show slightly wider DS M (T) curves with respect to bulk alloys leading to a larger refrigerant capacity.

The effects of non-magnetic Ti 4+ substitution on the structural, electrical and magnetic properties of La 0.67 Ba 0.33 Mn 1−x Ti x O 3 (0 ≤ x ≤ 0.1) are investigated and compared to those existing in La 0.67 Ba 0.33 Mn 1−x Cr x O 3 (magnetic Cr 3+ ). The structural refinement by the Rietveld method revealed that Ti-doped samples crystallize in the cubic lattice with space group Pm3m, while samples with Cr crystallize in the hexagonal setting of the rhombohedral R3C space group for identical contents of dopant. The most relevant structural features are an increase of the lattice parameters, of the cell volume and of the inter-ionic distances with increasing Ti doping level. Both series of samples show a decrease of the paramagnetic-ferromagnetic transition temperature when the amount of chromium or titanium increases. Transport measurements show that when increasing the metal doping, the resistivity increases whereas the metallic behavior of the parent compound La 0.67 Ba 0.33 MnO 3 is destroyed. For a substitution higher than 5 at.% of Ti and 10 at.% of Cr, the samples exhibit a semiconducting behav-M. Oumezzine ( ) · S. Kallel · N. Kallel · M. Oumezzine ior in the whole range of temperature, for which the electronic transport can be explained by variable range hopping and/or small polaron hopping models.

The magnetocaloric effect in TbNi2 alloy ribbons synthesized by rapid solidification was investigated. This material crystallizes in a superstructure of the cubic Laves phase structure type C15 (space group F-43m). The saturation magnetization and Curie temperature are MS = 134 ± 2 A m2 kg−1 and TC = 37 ± 1 K, respectively. For a magnetic field change of 5 T, the material shows a maximum magnetic entropy change |ΔSMpeak| = 13.9 J kg−1 K−1, with a full-width at half-maximum δTFWHM = 32 K, and a refrigerant capacity RC = 441 J kg−1. The RC value is similar to those reported for other magnetic refrigerants operating within the temperature range of 10-80 K. Finally, it is worth noting that the use of rapid solidification circumvents the necessity for long-term high-temperature homogenization processes normally needed with these RNi2 alloys.

Using X-ray diffraction, DSC and magnetization measurements, a magnetostrutural map of the (Mn,Fe)3(Si,P) system was assembled and reported in the current paper. Besides the already known cubic phase for Mn3−xFexSi system and the tetragonal and orthorhombic phases for the Mn3−xFexP system, a novel hexagonal phase has been observed for Mn3−xFexSi1−yPy, within the approximate range of 0.2 < x < 2.0 and 0.2 < y < 0.9. Magnetization measurements both confirm and further detail the already known properties of the Mn3−xFexSi and Mn3−xFexP systems.