Field-induced structural transition and the related magnetic entropy change in Ni43Mn43Co3Sn11 alloy

Magnetic properties and magnetic entropy change ΔS were investigated in Heusler alloy Ni₄₃Mn₄₃Co₃Sn₁₁. With decreasing temperature this alloy undergoes a martensitic structural transition at T_M=188 K. The incorporation of Co atoms enhances ferromagnetic exchange for parent phases. Austenitic phase with cubic structure shows strong ferromagnetic behaviors with Curie temperature T_C^A at 346 K, while martensitic phase shows weak ferromagnetic properties. An external magnetic field can shift T_M to a lower temperature at a rate of 4.4 K/T, and a field-induced structural transition from martensitic to austenitic state takes place at temperatures near but below T_M. As a result, a great magnetic entropy change with positive sign appears. The size of ΔS reaches 33 J/kg K under 5 T magnetic field. More important is that the ΔS displays a table-like peak under 5 T, which is favorable for Ericsson-type refrigerators.     

GREAT MAGNETIC ENTROPY CHANGE IN La(Fe, M)13 (M=Si, Al) WITH Co DOPING

Very large magnetic entropy change Δ S_M, which originates from a fully reversible second-order transition at Curie temperature T_C, has been discovered in compounds La(Fe, Si)₁₃, La(Fe, Al)₁₃ and those with Co doping. The maximum change ΔS_M\approx19 J·kg^-1·K^-1, achieved in LaFe_11.4Si_1.6 at 209K upon a 5T magnetic field change, exceeds that of Gd by more than a factor of 2. The T_C of the Co-doped compounds shifts to higher temperatures. ΔS_M still has a considerable large magnitude near room temperature. The phenomena of very large ΔS_M, convenience of adjustment of T_C, and also thesuperiority of low cost, strongly suggest that the compounds La(Fe, M)₁₃ (M=Si, Al) with Co doping are suitable candidates for magnetic refrigerants at high temperatures.     

Magnetic properties and magnetocaloric effect of Sr-doped Pr0.7Ca0.3MnO3 compounds

In this work, we pointed out that Sr substitution for Ca leads to modify the magnetic and magnetocaloric properties of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds. Analyzing temperature dependence of magnetization, M(T), proves that the Curie temperature (T_C) increased with increasing Sr content (x); T_C value is found to be 130–260 K for x = 0.0–0.3, respectively. Using the phenomenological model and M(T,H) data measured at several applied magnetic field, the magnetocaloric effect of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds has been investigated through their temperature and magnetic field dependences of magnetic entropy change ΔS_m(T,H) and the change of the specific heat change ΔC_P(T,H). Under an applied magnetic field change of 10 kOe, the maximum value of -ΔS_m is found to be about 3 J/kg·K, and the maximum and minimum values of ΔC_P(T) calculated to be about ±60 J/kg·K for x = 0.3 sample. Additionally, the critical behaviors of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds around their T_C have been also analyzed. Results suggested a coexistence of the ferromagnetic short- and long-range interactions in samples. Moreover, Sr-doping favors establishing the short-range interactions.     

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.     

La0.8-xCa0.2MnO3纳米颗粒的居里温度与磁热效应

采用溶胶凝胶法制备了系列La_0.8-xCa_0.2MnO₃多晶样品,用X射线衍射分析确定了样品的钙钛矿结构,用透射电子显微镜观察了样品的形貌及粒径分布情况,用PAR155型振动样品磁强计测量了样品的磁性随外场和温度的变化,确定样品的居里温度并计算了各样品的磁熵变.磁测量及计算结果表明制备的各样品的居里温度在180—260K的范围内且随焙烧温度和La³⁺离子空位浓度的不同而变化,不同温度焙烧的样品均有较大的磁熵变值,其中1100℃焙烧的La_0.77Ca_0.2MnO₃,多晶样品在240.5K,H=1.0T的外场下的磁熵变达3.76J/kg·K,对实验结果做了定性的分析.该材料具有较高的居里温度和较大的磁熵变,所需外场强度适中,电阻率高,性能稳定,适合做高温磁制冷材料. 关键词： 钙钛矿 居里温度 磁热效应     

Influence of Dy addition on the magnetocaloric effect of La0.67Ca0.33Mn0.9V0.1O3 ceramics

The influence of partial substitution of La by Dy on the magnetocaloric response of (La_1−xDy_x)_0.67Ca_0.33Mn_0.9V_0.1O₃, where x=0.03, 0.15 and 0.25 is studied. Rietveld refinement of X-ray diffraction pattern using GSAS method shows that the compounds adopt the orthorhombic structure with Pnma space group. The systematic change in lattice parameters and magnetic phase transition indicates the substitution effect of Dy. From the magnetization isotherms at different temperatures, magnetic entropy change close to their respective transition temperatures (T_C) has been evaluated. The maximum value of entropy change near T_C is found to be about 4.8 J/kg K at 187.5 K for LCMVDy_0.03, 2.45 J/kg K at 107.5 K for LCMVDy_0.15 and 2.15 J/kg K at 92.5 K for LCMVDy_0.25 at 4 T. Dy addition produces a reduction in T_C and in magnitude of the magnetic entropy change. Even though the entropy change decreases with increasing Dy substitution the refrigerant temperature range, ΔT, is found to be 10 K for LCMVDy_0.03, 31 K for LCMVDy_0.15 and 35 K for LCMVDy_0.25 compounds [90%] at 4 T. The field dependence of the magnetic entropy change is also analyzed showing the power law dependence, ΔS_M∞Hⁿ where n=0.75(2) for LCMVDy_0.03, n=0.80(4) for LCMVDy_0.15 and n=0.92(8) for LCMVDy_0.25 compounds at their respective transition temperatures. The relative cooling power and its field dependance are also analyzed.     

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.     

Structure,magnetic properties and magnetostriction of laves compounds Nd1−xPrx(Fe0.35Co0.55B0.1)2

The C15 Laves phases with composition Nd_1−xPr_x(Fe_0.35Co_0.55B_0.1)₂ (0?x?1) have been synthesized by arc melting and subsequent annealing. The Curie temperature T_c and the saturation magnetizations M_s at 5 and 295 K decrease with increasing Pr content. The linear anisotropic magnetostriction λ_a=λ_∥−λ_⊥ at room temperature for Nd_1−xPr_x(Fe_0.35Co_0.55B_0.1)₂ alloys with 0?x?0.4 initially reaches a negative minimum, then increases and changes its sign with increasing magnetic field H, and the λ_a for the alloys with x?0.6 is positive and increases as magnetic field H increases.     

Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Large reversible magnetocaloric effect in spinel MnV2O4 with minimal Al substitution

Magnetocaloric effect of MnV_1.95Al_0.05O₄ was studied by the magnetization and heat capacity measurements. MnV_1.95Al_0.05O₄ is a cubic spinel structure with ferromagnetism of second order in nature and performs reversible magnetic entropy around the magnetic transition temperature. The large magnetic entropy changes −ΔS_M∼5.2 and 8.2 J/kg K and the adiabatic temperature changes ΔT_ad∼1.5 and 2.6 K are revealed for the magnetic field changes of 2 and 4 T near the Curie temperature (T_C) of 59.6 K, respectively. The relative cooling power (RCP) are about 82.2 and 177.2 J/kg K for magnetic field changes 2 and 4 T, respectively. Compared with the parent compound, although the −ΔS_M and ΔT_ad become smaller, the refrigeration working temperature span and the RCP have been improved.     

Magnetocaloric effect at room temperature in manganese perovskite La0.65Nd0.05Pb0.3MnO3 with double resistivity peaks

Series of polycrystalline manganese perovskite oxides La_0.7−xNd_xPb_0.3MnO₃ (x=0, 0.05, and 0.1) are prepared by the sol-gel technique, La_0.65Nd_0.05Pb_0.3MnO₃ were representatively investigated because the peculiar double resistivity peaks were found; the maximum magnetic entropy change ΔS_H=−2.03 J/kg K and its good refrigerant capacity 71.05 J/kg around room temperature were obtained under 9 kOe magnetic field variation. The expected double peaks of magnetocaloric effect had not occurred since magnetic entropy change originated from the differential coefficient of magnetic moment to temperature; the relatively well refrigerant capacity possibly results from the faint magnetic inhomogeneity mixed in the double exchange strong magnetic signal.     

Simultaneous enhancements of Curie temperature and magnetocaloric effects in the La1−xCexFe11.5Si1.5Cy compounds

The effects of introducing Ce and C atoms on the Curie temperature (T_C), the magnetic entropy change (ΔS_M) and the hysteresis loss have been investigated in the NaZn₁₃-type LaFe_11.5Si_1.5 compound. Partial replacement of La with Ce leads to a decrease in T_C and an increase in ΔS_M; however, the introduction of interstitial C atoms can adjust T_C to high temperature. The itinerant-electron metamagnetic transition is weakened after carbonization, which results in a reduction of both the hysteresis loss and magnetocaloric effect (MCE). The maximum value of ΔS_M for La_0.8Ce_0.2Fe_11.5Si_1.5C_0.2 is found to be −28 J/kg K at T_C=207 K with an effective refrigeration capacity of 420 J/kg for a field change from 0 to 5 T. Our study reveals that the enhancements of both T_C and MCEs can be achieved simultaneously in the La_1−xCe_xFe_11.5Si_1.5C_y compounds by adjusting the concentrations of Ce and C atoms appropriately.     

Mn5Ge2.7M0.3 (M=Ga，Al，Sn) 化合物的磁性和磁熵变

研究了Mn₅Ge_2.7M_0.3(M=Ga，Al，Sn)化合物的磁性和磁熵变. x射线衍射实验表明，研究的化合物均呈六角Mn₅Si₃型结构. 三种原子对Ge原子的替代，使得平均Mn原子磁矩下降，但居里温度没有明显的变化. 由于磁矩的降低，导致磁熵变值的下降，在磁场变化为4.0×10⁶A·m^-1时，对应于M=Ga，Al和Sn的样品，最大磁熵变值ΔS^max_Ｍ分别为6.1，6.3和5.3J·kg^-1K^-1，但磁熵变峰值的半高宽ΔＴ_ＦＷＨＭ有所增加. 另外，Mn₅Ge_2.7M_0.3(M=Ga，Al，Sn)化合物在高于居里温度的Arrott曲线上出现了一个不连续点，即样品在一定温度下的顺磁磁化率在某一临界磁场下发生了突变，临界磁场与温度几乎呈正比关系.这可能是由于样品在加一定磁场时3d带的费米能级发生了变化，使得有效电子数的减少所致. 关键词： 居里温度 平均Mn原子磁矩 磁熵变 Arrott图     

Structural,magnetic and magnetocaloric properties of AMn1−xGaxO3 compounds with 0≤x≤0.2

Structural, magnetic and magnetocaloric properties of manganites series with the AMn_1−xGa_xO₃ (A=La_0.75Ca_0.08Sr_0.17 and x=0, 0.05, 0.1 and 0.2) composition have been investigated to shed light on Ga-doping influence. Solid-state reaction method was used for preparation. From XRD study, all samples are found single phase and crystallize in the orthorhombic structure with the Pnma space group. The variation of the magnetization M vs. temperature T, under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The experimental results indicate that T_C decreases from 336 to 135 K with increasing Ga substitution. Magnetocaloric effect (MCE) was estimated, in terms of isothermal magnetic entropy change (−ΔS_M), using the M(T, μ₀H) data and employing the thermodynamic Maxwell equation. The maximum entropy change and Relative Cooling Power (RCP) show non-monotonic behaviors with increasing the concentration of Gallium. In fact, the maximum value of ΔS_Mmaxof AMn_1−xGa_xO₃ for x=0.00 and 0.2 samples is found to be, respectively, 2.87 and 1.17 J/kg/K under an applied magnetic field change of 2 T. For the same applied magnetic field (μ₀H=2 T), the RCP values are found to vary between 97.58 and 89 J/kg.     

Magnetic entropy change in perovskite manganites La0.65Nd0.05Ca0.3Mn0.9B0.1O3 (B=Mn,Cr, Fe)

We have investigated the effect of B-site dopant on magnetic entropy change in perovskite manganite samples of La_0.65Nd_0.05Ca_0.3MnO₃, La_0.65Nd_0.05Ca_0.3Mn_0.9Cr_0.1O₃, and La_0.65Nd_0.05Ca_0.3Mn_0.9Fe_0.1O₃ prepared by sol–gel technique. The maximum ΔS_H is in the order of −1.68 J/kg K and peaks at Curie temperature for La_0.65Nd_0.05Ca_0.3MnO₃ upon 10 kOe applied field change. For the sample with B-site dopant, a decrement of the maximum magnetic entropy change has been observed.     

Polyol synthesis and magnetic study of Mn3O4 nanocrystals of tunable size

Nanosized manganese oxide particles were prepared by the so-called polyol process. The average diameter of the particles was controlled by the growth time. X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photon spectroscopy (XPS) show that the particles are well crystallized, pure, stoichiometric Mn₃O₄ single crystals of uniform size ranging from about 5 to 12 nm. The variation of their dc-magnetization, M, as a function of the magnetic field, H, and temperature, T, clearly corresponds to ferromagnetic ordering at low temperature, with a Curie temperature slightly higher than 40 K. The evidence for superparamagnetism in these particles, due to their very small size, has been discussed in the light of their M(H) and M(T) for zero-field-cooled (ZFC) and field-cooled (FC) plots.     

Magnetocaloric effect in high Ni content Ni52Mn48−xInx alloys under low field change

Ni-rich Heusler alloys Ni₅₂Mn_48−xIn_x (x=15.5, 16 and 16.5) were prepared by the arc melting method. X-ray diffraction analysis revealed that the martensite has orthorhombic structure (S.G. Pmm2) at room temperature. The only alloy with x=15.5 has structural transmission from martensite to austenite without any magnetic transmission. The temperature dependence and the field dependence of the magnetization measurement indicated that the magnetization increased with the decreasing of the concerntration of Mn. The lesser the Mn atoms located in the In atom sites, the weaker the total AFM interaction in the system. Giant entropy changes ΔS_M(T, H) were found in Ni₅₂Mn_48−xIn_x alloys with the maximum ΔS_M value of 22.3 J kg K for the sample with x=16.5 at 270 K under the magnetic field change of 1.5 T.     

Magnetocaloric properties of the La0.7Pr0.3Fe13−xSix compounds

Magnetic entropy change ΔS_m, hysteresis loss and refrigerant capacity of NaZn₁₃-type La_0.7Pr_0.3Fe_13−xSi_x (1.5?x?2.0) compounds have been investigated. The Curie temperature T_C increases linearly with the increase of silicon concentration. Although the maximum value of ΔS_m under a field change of 0−5 T decreases from 30.5 to 11.4 J/kg K as x increases from 1.5 to 2.0, the hysteresis loss at T_C reduces remarkably from 89.2 J/kg for x=1.5 to zero for x=2.0 because the increase of Si content can weaken the itinerant electron metamagnetic transition. The effective refrigerant capacity RC_eff is maintained at high values of 362−439 J/kg for a field change of 0−5 T. This implies that a large ΔS_m and a high RC_eff can be achieved simultaneously in the La_0.7Pr_0.3Fe_13−xSi_x compounds.     

Structural, magnetic and magnetocaloric properties of La0.7−xEuxBa0.3MnO3 perovskites

Polycrystalline perovskite manganites La_0.7−xEu_xBa_0.3MnO₃(x=0.05, 0.1 and 0.15) were prepared by sol-gel method. The prepared samples remain single phase with a perovskite structure, revealed by X-ray diffraction. The structure refinement of La_0.7−xEu_xBa_0.3MnO₃(x=0.05, 0.1 and 0.15) samples was performed in the hexagonal setting of the R3¯c space group. The dependence of magnetization M on applied magnetic field H and temperature T was measured carefully near the Curie temperature T_C for all the samples. With the increasing Eu content, both the unit cell volume and Curie temperature T_C of 298 K has been detected with a maximum of magnetic entropy |ΔS_M^max| for the La_0.7−xEu_xBa_0.3MnO₃ with x=0.15, reaching a value of 2.3 J/kg K when a magnetic field of 10 kOe was applied and the relative cooling power (RCP) is 46 J/kg. These results suggest that the material may be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Magnetic properties of amorphous Co75W25

Saturation magnetization M(T), spin wave stiffness D and Curie temperature T_c of amorphous Co₇₅W₂₅ were determined by magnetic measurements. Tungsten reduces these quantities more than metalloids or 3d-transition metals do. T_c is below room temperature. The ratio D/T_c, however, is equal to that observed on a fcc single crystal of Co₉₂Fe₈ and on amorphous Co-Ti alloys.