The magnetocaloric effect in Nd(Co1−xFex)12B6 alloys

The first-order phase transitions in NdFe₁₂B₆ and PrFe₁₂B₆ alloys give rise to giant values of magnetic entropy changes in relatively low field. However, the metastable nature of these alloys associates with a special procedure of preparation and considerable amount of impurities inevitably. By alloying NdFe₁₂B₆ with the iso-structural compound of NdCo₁₂B₆ appropriately, a Nd(Co_1−xFe_x)₁₂B₆ system which possesses the stable SrNi₁₂B₆-type structure can be obtained directly via the standard casting-and-annealing method. Remarkably improved thermal and magnetic reversibility are observed in the present system. The second-order phase transitions in NdCo₁₂B₆ alloy give rise to the relative cooling power, which is comparable with that of NdFe₁₂B₆ alloy around the ordering temperature.     

Entropy changes accompanying the magnetic phase transitions in low Si-doped Ce2Fe17−xSix Alloy

A summary of the results of ac susceptibility and isothermal magnetization measurements on polycrystalline samples of Ce₂Fe_17−xSi_x with nominal composition of x=0.0, 0.1, 0.2 and 1.0 is presented. These data reveal that the substitution of small amounts of Si for Fe produce a significant increase in temperature at which ferromagnetism appears, to the extent that, at x=1, characteristics of the anti ferromagnetic to paramagnetic transition (at temperature T_N) have disappeared completely. The nature of the various magnetic phase transitions — identified through the use of Arrott plots — and the accompanying magnetic entropy change, ΔSm, are both affected significantly by small amounts of Si substitution. In particular, while the peak entropy change is modest (occurring at x=0.1), the temperature interval over which a substantial entropy change occures is significant, approaching 150 K, an important criterion for improving the overall effectiveness of such materials for magnetic refrigeration.     

Phase transitions and the magnetocaloric effect in Mn rich Ni–Mn–Ga Heusler alloys

In Mn rich polycrystalline Heusler alloys, Ni₅₀Mn_25+−xGa_25−x, prepared by Arc melting, it is found that the structural/first-order magnetic transition temperature T_m increases as the Mn content increases. The Curie temperature T_c is higher than that of Ni rich alloys (Ni_50+xMn_25−xGa₂₅ ) of the same series, and is less affected by composition x. Magnetic entropy change of |ΔS_M| also increases as Mn content increases, while behaviour of the field dependence of ΔS_M is similar to that of single crystal Ni_52.6Mn_23.1Ga_24.3.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Heat capacity and magnetocaloric effect in manganites (La1−yEuy)0.7Pb0.3MnO3 (y:0.2; 0.6)

Heat capacity and intensive magnetocaloric effect (MCE) in manganites (La_1−yEu_y)_0.7Pb_0.3MnO₃ [y=0.2; 0.6] (LEPM) were investigated by means of adiabatic calorimeter. The heat capacity anomaly as well as the values of both the intensive (ΔT_AD) and the extensive (ΔS_MCE) MCE were found to decrease upon increased replacement of La with nonmagnetic Eu. However, because of widening of the MCE peaks, the LEPM compounds show the relative cooling power, RCP/ΔH, comparable to other solid solutions of manganites. Owing to strong effect of Eu→La substitution on the Curie temperature, LEPM might have potential as the solid state refrigerants in multi-element cooling apparatus operating in a wide temperature range.     

Reduction of hysteresis loss and large magnetocaloric effects in substituted compounds of itinerant-electron metamagnets La(FexSi1−x)13

The maximum value of hysteresis loss E_h^MAX due to the itinerant-electron metamagnetic (IEM) transition of La(Fe_xSi_1−x)₁₃ and the partially substituted compounds La_1−zCe_z(Fe_0.86Si_0.14)₁₃ and La_1−z′Pr_z′(Fe_0.86Si_0.14)₁₃ increases when the magnetocaloric effects (MCEs) become large. It should be noted that the reduction of E_h^MAX without the decrease of large MCEs is achieved in La_1−zCe_z(Fe_0.86Si_0.14)₁₃ and La_1−z′Pr_z′(Fe_0.86Si_0.14)₁₃. For both the compound systems mentioned above, the critical temperature T₀ for the IEM transition decreases and the difference between T₀ and the Curie temperature T_C becomes larger with decreasing T_C. These results are consistent with the magnetic phase diagram of La(Fe_0.86Si_0.14)₁₃ under hydrostatic pressure. Consequently, the reduction of E_h^MAX in La_1−zCe_z(Fe_0.86Si_0.14)₁₃ and La_1−z′Pr_z′(Fe_0.86Si_0.14)₁₃ is closely related with the magnetovolume effects.     

Influence of sample geometry on determination of magnetocaloric effect for Gd60Co30Al10 glassy ribbons using direct and indirect methods

Rare-earth based metallic glasses with high saturation magnetization show a sizeable magnetocaloric effect (MCE) and are subject of extensive research concerning magnetic refrigeration materials. In this work, the magnetic phase transition from paramagnetic to ferromagnetic of Gd₆₀Co₃₀Al₁₀ metallic glass has been characterized and three different methods were applied for the determination of its magnetocaloric specific parameters: (a) direct measurement of the adiabatic temperature change by exposing the material to an adiabatically applied magnetic field; (b) determination of the magnetization M(H,T) and calculation of the temperature dependent magnetic field induced entropy change ΔS_m by application of the Maxwell relation and (c) measuring the total heat capacity C_p(H,T) in zero and non-zero magnetic field. Gd₆₀Co₃₀Al₁₀ glassy ribbons were prepared by melt spinning, a technique that offers very high cooling rates due to the low dimensionality of the sample. Depending on the particular method of measurement, pieces of these glassy ribbons form samples with different appropriate total volume and dimensions. We show that the combination of the pronounced two-dimensionality of the ribbon pieces (aspect ratio ∼100) together with the very high magnetic permeability principally can cause strong internal demagnetizing fields that cannot be neglected when evaluating the intrinsic MCE parameters obtained from different methods.     

Magnetic transitions in delafossite CuFeO2: A magnetocaloric effect study

CuFeO₂ was synthetized by a solid-state reaction and its low temperature magnetic properties were investigated using the magnetocaloric effect. Magnetic susceptibility measurements show that there are two magnetic transition temperatures at about 16 and 11 K. Measurement of isothermal magnetization curves for different applied magnetic fields near these temperatures show a reversal in the magnetization trend around 16 K, and Arrott plots indicate they are accompanied by second- and first-order magnetic phase transitions, respectively. Both normal and inverse magnetocaloric effects are observed, and the maximum magnetic entropy change is obtained at 11 K.     

Table-like magnetocaloric effect and enhanced refrigerant capacity in crystalline Gd55Co35Mn10 alloy melt spun ribbons

Gd₅₅Co₃₅Mn₁₀ ribbons were prepared by melt-spinning and subsequent crystallization treatment. Crystallization resulted in the precipitation of the Gd₃Co-type and Gd₁₂Co₇-type phases in the amorphous matrix. Under a magnetic field change of 0–5 T, a table-like magnetocaloric effect, with a maximum magnetic entropy change ${(?\mathrm{\Delta }{S}_{\mathrm{M}})}^{\max }$ of $5.46\text{J}{\text{kg}}^{?1}{\text{K}}^{?1}$ in the temperature range of 137–180 K and enhanced refrigerant capacity (RC) of $536.4\text{J}{\text{kg}}^{?1}$, was achieved in Gd₅₅Co₃₅Mn₁₀ ribbons crystallized at 600 K for 30 min. The table-like ${(?\mathrm{\Delta }{S}_{\mathrm{M}})}^{\max }$ feature and enhanced RC values make Gd₅₅Co₃₅Mn₁₀ crystallized ribbons promising for Ericsson-cycle magnetic refrigeration in the temperature range from 137 to 180 K.     

Metamagnetic transition and magnetocaloric effect in antiferromagnetic TbPdAl compound

Magnetic properties and the magnetocaloric effect of the compound TbPdAl are investigated. The compound exhibits a weak antiferromagnetic (AFM) coupling, and undergoes two successive AFM transitions at T_N=43 K and T_t=22 K. A field-induced metamagnetic transition from AFM to ferromagnetic (FM) state is observed below T_N, and a small magnetic field can destroy the AFM structure of TbPdAl, inducing an FM-like state. The maximal value of magnetic entropy change is −11.4 J/kg K with a refrigerant capacity of 350 J/kg around T_N for a field change of 0-5 T. Good magnetocaloric properties of TbPdAl result from the high saturation magnetization caused by the field-induced AFM-FM transition.     

Enhancement of magnetocaloric effects in La0.8R0.2(Fe0.919Co0.081)11.7 Al1.3 (R=Pr, Nd) compounds

The magnetic properties and magnetocaloric effects (MCEs) in La_0.8R_0.2(Fe_0.919Co_0.081)_11.7Al_1.3 (R=Pr, Nd) compounds have been investigated. When Pr and Nd substitute for La, the Curie temperature of compounds decreases. The values of the MCEs are enhanced significantly by a partial substitution of Nd for La. Under the field change of 2 and 5 T, the maximum magnetic entropy changes for La_0.8Nd_0.2(Fe_0.919Co_0.081)_11.7Al_1.3 compound are −4.56 and −9.46 J/Kg K, respectively. It can be exploited for room temperature magnetic refrigeration material.     

Field induced sign reversal of magnetocaloric effect in Gd2In

We report the magnetocaloric effect in the metamagnetic compound Gd₂In obtained from magnetization measurement. Gd₂In was previously reported to have two magnetic transitions: (i) a paramagnetic to ferromagnetic transition below 190 K and (ii) a ferromagnetic to an antiferromagnetic state below 105 K. The low temperature antiferromagnetic state is unstable under an applied magnetic field and undergoes metamagnetic transition to a ferromagnetic like state. We observe conventional positive magnetocaloric effect (the magnetic entropy change, ΔS_M<0) around 190 K at all applied fields. The magnetocaloric effect is found to be inverse (negative) at low fields around 105 K (ΔS_M>0), however it turns positive at higher fields (ΔS_M<0). The observed anomaly is found to be related to the field induced transition which drives the system from an antiferromagnetic to a ferromagnetic state.     

Phase transition in multiferroic YMnO3 and its solid solution YMn(0.93)Fe(0.07)O3

Ceramics of YMnO₃ and its Fe substituted YMn_(0.93)Fe_(0.07)O₃ solid solution were synthesized by solid state reaction of the oxides at 1200 °C. Hexagonal phase was identified in both cases by X-ray powder diffraction. Rietveld refinement of cell parameters showed an increase of the parameter values for the solid solution. Dielectric permittivity measurements versus temperature showed a phase transition at 655 °C for yttrium manganite, however, for the solid solution no phase transition was detected on heating up to 700 °C. Dielectric loss measurements showed higher slope changes and better defined local maxima for the solid solution than for the pure phase.     

Direct measurement of the magnetocaloric effect in La0.67Ca0.33MnO3

Polycrystalline perovskite La_0.67Ca_0.33MnO₃ was synthesized by a sol–gel method. Its adiabatic temperature change ΔT_ad induced by a magnetic field change was measured directly. At 268 K, near its Curie temperature T_C, ΔT_ad of La_0.67Ca_0.33MnO₃ induced by a magnetic field change of 2.02 T reaches 2.4 K. The latent heat Q and magnetic entropy change −ΔS_M induced by a magnetic field change were calculated from the temperature dependence of ΔT_ad and zero-field heat capacity C_p. The maximum values of Q and −ΔS_M in La_0.67Ca_0.33MnO₃ induced by a magnetic field change of 2.02 T are 1.85 J g⁻¹ and 6.9 J kg⁻¹ K⁻¹, respectively. The former is larger than the phase transition latent heat of heating or cooling, which is about 1.70 J g⁻¹.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Influence of boron on the giant magnetocaloric effect of La(Fe0.9Si0.1)13

The influences of boron addition on the phase formation, Curie temperature and magnetic entropy change of the NaZn₁₃-type La(Fe_0.9Si_0.1)₁₃ compound have been investigated. Eight boron containing La(Fe_0.9Si_0.1)₁₃B_x samples were prepared with x=0, 0.03, 0.06, 0.1, 0.2, 0.3, 0.5 and 0.6, respectively. Experimental results show that a small amount of B addition in La(Fe_0.9Si_0.1)₁₃ forms the solid solution NaZn₁₃-type structure phase by substituting B for Si or doping B into interstitial position of the lattice, preserves its giant magnetocaloric effects due to their first-order structural/magnetic transition, as well as increase its Curie temperature T_c slightly. The maximum magnetic entropy changes in the magnetic field change of 0–1.6 T are around 20 J kg^–1 K^–1 for the samples with Boron addition less than 0.3, while improving the Curie temperatures by 2 K.     

Properties of magnetocaloric La(Fe,Co,Si)13 produced by powder metallurgy

We present a comprehensive study of the magnetocaloric materials series La(Fe_1−xCo_x)_11.9Si_1.1 with 0.055<x<0.122. The ferromagnetic samples were manufactured using a novel powder metallurgy process by which industrial scale production is feasible. This new production method makes the materials more attractive as magnetic refrigerants for room temperature magnetic refrigeration. The Curie temperature of the compounds can be easily tuned by altering the Co content and all samples have little magnetic anisotropy and present a second-order magnetic transition so that thermal and magnetic hysteresis is absent. For all seven samples, we have calculated the magnetic entropy change, ΔS_M, from initial curve measurements and measured the adiabatic temperature change, ΔT_ad, directly. In addition, for two of the samples, we determined the heat capacity as a function of applied magnetic field and the thermal conductivity. Where relevant, the results are compared with those of Gd, the benchmark material for room temperature magnetic refrigeration.     

Magnetocaloric effect in the La0.8Ce0.2Fe11.4−xCoxSi1.6 compounds

The effects of substitution of Co for Fe on the magnetic and magnetocaloric properties of La_0.8Ce_0.2Fe_11.4−xCo_xSi_1.6 (0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds have been investigated. X-ray diffraction shows that all compounds crystallize in the NaZn₁₃-type structure. Magnetic measurements show that the Curie temperature (T_C) can be tuned between 184 and 294 K by changing the Co content from 0 to 1. A field-induced methamagnetic transition occurs in samples with x=0, 0.2 and 0.4. The magnetic entropy changes of the compounds have been determined from the isothermal magnetization measurements by using the Maxwell relation.     

Anomalous phase transformation in magnetostrictive Fe81Ga19 alloy

In this work, we have investigated the room-temperature phase constitution of heat-treated Fe₈₁Ga₁₉ alloys cooled from 800 °C at different rates. Results show that at cooling rates in the range from 0.43 to 0.26 °C/min, in addition to the A2 matrix, an fcc phase also can be observed in Fe₈₁Ga₁₉ samples at room temperature. To investigate the precipitation of the fcc phase out of A2 matrix, a systematic study of phase constitution was carried out on the samples quenched from different temperatures during cooling from 800 °C at 0.32 °C/min, which reveals an anomalous phase transformation between A2 and fcc. Precipitation of the fcc phase from A2 matrix occurs at 500 °C and its volume fraction exhibits a sharp increase at 400 °C. However, it begins to dissolve when further decreasing the temperature and only a minor fcc phase can be retained at room temperature, which suggests that the fcc phase is metastable below 400 °C. Magnetic measurements indicate that the precipitation of fcc phase deteriorates the saturation magnetization of Fe₈₁Ga₁₉.