Anisotropic magnetocaloric effect in TbNiAl

We present study of the anisotropic magnetocaloric effect in TbNiAl crystallizing in the hexagonal ZrNiAl-type structure. TbNiAl orders antiferromagnetically below T_N = 47 K and undergoes second magnetic phase transition to another antiferromagnetic structure at T₁ = 23 K. Magnetization and specific heat measurements on single crystal revealed strongly anisotropic magnetocaloric effect. The large effect occurs for field applied along the hexagonal c-axis whereas the entropy change is almost zero for the perpendicular field direction. Plateau-like character of the determined temperature change is observed between T_N and T₁.     

Entropy and magnetocaloric effect in ferromagnets and antiferromagnets

Analytic expressions for the entropy of a ferromagnet and an antiferromagnet that make it possible to calculate the magnetic-field dependence of the entropy in a wide temperature range have been obtained in the mean-field approximation for the Heisenberg model. It has been shown that for Fe, Ni, and Gd the calculated magnitude of the magnetocaloric effect is in satisfactory agreement with experiment. The results of calculations of the dependence of the entropy of an antiferromagnet on the magnitude and direction of a magnetic field have been discussed.     

Rotating magnetocaloric effect in HoAl2 single crystal

In this paper we theoretically discuss the rotating magnetocaloric effect in HoAl₂ single crystal. In order to do that, we use a model Hamiltonian of interacting magnetic moments including a term to account for the crystal electric field. Our theoretical calculations of the entropy changes are in a reasonable agreement with the available experimental data. Moreover, we predict the existence of an anomalous rotating magnetocaloric effect for some directions of the magnetic field rotation.     

Specific features of magnetocaloric effect in isotropic antiferromagnets

The molecular-field method has been used to study the magnetocaloric effect and the dependence of the magnetic entropy on an external magnetic field in a two-sublattice isotropic antiferromagnet. It has been shown that, for an isotropic antiferromagnet, the magnetization of a canted phase that arises after a spin-flop transition in an applied magnetic field is temperature-independent. Therefore, the magnetic entropy of the phase is independent of the strength of an external field and no magnetocaloric effect is observed. The dependence of the magnetic entropy on the magnetic field and the normal magnetocaloric effect appear only after the spin-flip transition of the antiferromagnet into a magnetic-field-induced ferromagnetic phase. In moderate magnetic fields, this occurs in fact only in the temperature range of T ≥ T _N, where T _N is the Néel temperature.     

Magnetic property and magnetocaloric effect in TmCoAl compound

A large reversible magnetocaloric effect accompanied by a second order magnetic phase transition from paramagnetic (PM) to ferromagnetic (FM) has been observed in TmCoAl intermetallic compound. For the magnetic field change of 5 T, the maximum value of magnetic entropy change (−ΔS_M^max) and the value of refrigerant capacity (RC) are evaluated to be 18.2 J/kg K and 211 J/kg, respectively. In particular, a large −ΔS_M^max (10.2 J/kg K) is achieved at 7.5 K under a low magnetic field change from 0 to 2 T with no thermal hysteresis and magnetic hysteresis loss. The large reversible magnetocaloric effect (both the large −ΔS_M and the high RC) indicates that TmCoAl is one of a promising material for magnetic refrigeration in low temperature.     

Giant magnetocaloric effect in Tb3Rh

Tb₃Rh exhibits a giant magnetocaloric effect (GMCE) around ordering temperature of 88 K. The maximum magnetic entropy change ΔS_m obtained at 2 T, 3 T, 5 T and 7 T amounts −11.6 J/(K kg), −14 J/(K kg), −17.9 J/(K kg) and −21 J/(K kg) respectively. The origin of an enhanced MCE is related to a first-order magnetic transition. Magnetic frustration and short range correlation are very important in understanding the properties of Tb₃Rh.     

Investigation of the magnetocaloric effect in correlated metallic systems with Van Hove singularities in the electron spectrum

In this work, the magnetic contribution to the isothermal entropy change ΔS upon switching on a magnetic field has been investigated in correlated metallic ferromagnets within the Hubbard nondegenerate model. The analytical expression ΔS for obtained in the mean-field approximation depends substantially on the electronic structure (density of electron states), which presents new ways to increase the absolute value of ΔS relative to the known result obtained within the Heisenberg model. The temperature dependence of ΔS has been calculated at different values of the Coulomb interaction U and the number of electrons n for the Bethe infinite-dimensional lattice and square lattice with allowance for transfer integrals in the first (t) and the second (t') coordination shells. It has been found that the presence of Van Hove singularities in the electronic spectrum near the Fermi level makes it possible to considerably increase |ΔS| at a fixed magnetic field. The possibility of first-order magnetic phase transitions depending on the model parameters has been analyzed.     

Magnetic properties and magnetocaloric effect of FeCrNbYB metallic glasses with high glass-forming ability

The influences of Cr addition on the Curie temperature (T_C), glass-forming ability (GFA), and magnetocaloric effect were investigated in FeCrNbYB metallic glasses. It was found that the addition of Cr element slightly decreases the GFA and saturation magnetization, whereas effectively modulates T_C. By the method of copper mold casting, bulk metallic glasses (BMGs) with critical diameters up to 5 mm can be obtained in Fe_68−xCr_xNb₄Y₆B₂₂ (x = 2–6) alloys. The resulting metallic glasses exhibit T_C of 271–367 K and excellent magnetocaloric properties, including magnetic entropy change of 0.76–1.05 J/kg K, and refrigerant capacity of 83–93 J/kg under a low field change of 1.5 T. In addition, they exhibit a wide supercooled liquid region of 116–135 K. The successful synthesis of the FeCrNbYB BMGs with near room-temperature magnetocaloric properties is encouraging for the future development of Fe-based BMGs as a new magnetic refrigerant in magnetic cooling system.     

Specific heat and magnetocaloric effect of the Mn5Ge3 ferromagnet

The specific heat C_p(T) of the Mn₅Ge₃ ferromagnet has been studied in a wide temperature range of 2–400 K. The ferromagnetic ordering has been confirmed at T_C = 297.4 K, however the behaviour of C_p(T,H) in small magnetic fields suggests a presence of a weak antiparallel component in the magnetic structure. The combination of the C_p(T,H) and magnetization M(T,H) measurements enabled the determination of not only the isothermal magnetic entropy change ΔS_M but the adiabatic temperature change ΔT_ad as well. It has been also found that the mechanical milling process leads only to a moderate drop of the magnetocaloric effect. The reduction of the grain size by 50% decreases the relative cooling power by 28%.     

Dependence of the magnetocaloric effect in superparamagnetic nanocomposites on the distribution of magnetic moment size

The magnetocaloric effect of superparamagnetic nanocomposites was studied by evaluating their magnetic entropy changes ΔS from magnetization data sets. Influence of distribution of the magnetic moment size on ΔS was evaluated. It is pointed out that a narrow size distribution is favorable for high performance of the magnetocaloric effect.     

B掺杂对La-Fe-Si合金微观结构及磁热效应的影响

研究了小剂量B元素的掺杂及热处理对La-Fe-Si快淬条带微观结构及磁热性能的影响。结果表明:B的掺杂促进了La(Fe,Si)_(13)相的形成,相比于未掺杂B的La-Fe-Si合金,La-Fe-Si-B合金在热处理前就已经获得较多的相,当B的掺杂量为0.08,快速凝固过程中获得的La(Fe,Si)_(13)相的相对含量最多。B的掺杂有利于快速凝固过程中微观组织结构的细化,这些细化的微观组织缩短了热处理过程中进行包析反应(α-Fe+LaFeSi→ La(Fe,Si)_(13)的元素扩散路径,有利于快速反应生成La(Fe,Si)_(13)相。在热处理过程中,B的掺杂优化了合金的微观组织结构,与没有掺杂B的La-Fe-Si合金相比,其居里温度和磁热效应得到提高。当B的掺杂量由0增加到0.08,合金快淬条带的居里温度由210K提高到233K,最大等温磁熵变由5.47J/kg·K提高到9.40J/kg·K。     

Influence of carbon on the giant magnetocaloric effect of LaFe11.7Si1.3

1. Introduction Since the discovery of the giant magnetocaloric effect (GMCE) in Gd5Si2Ge2 by Pecharsky and Gschneidner [1] in 1997, the materials with magne- tocaloric effect (MCE) for magnetic refrigerators have been investigated intensively. Giant magneto- caloric effects around room temperature have been found in some materials, such as Gd5(Si1?xGex)4 [1-2], MnFeP0.45As0.45 [3], MnAs1?xSbx [4], and La(FexSi1?x)13 [5], as reported by different research groups. Further effort is ne…     

The influence of quadrupolar interaction on the magnetocaloric effect in PrMg2

We report the magnetocaloric properties, the isothermic entropy change and adiabatic temperature change, in the PrMg₂. This compound was fully investigated using a model Hamiltonian which includes the exchange magnetic interaction, the crystalline electrical field anisotropy and the quadrupolar interaction. All theoretical results were obtained using the proper model parameters, found in the literature. The quadrupolar interaction strong influences the lowest doublet crystal field level, Γ₃ changing the order of magnetic phase transition. The magnetocaloric properties were systematic studied in term of the model parameters.     

Large reversible magnetocaloric effect in the RECoC2 (RE=Ho and Er) compounds

The crystal structure, magnetic properties and magnetocaloric effect (MCE) of HoCoC₂ and ErCoC₂ have been investigated. They both crystallize in orthorhombic CeNiC₂-type structure with Amm2 space group and a second-order paramagnetic to ferromagnetic phase transition around the Curie temperature T_C ∼11 K and ∼14 K occurred in them. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 15.6 J/kg K, 183 J/kg, 242 J/kg for HoCoC₂ and 17.2 J/kg K, 243 J/kg, 375 J/kg for ErCoC₂, respectively.     

Crystal structure,magnetocaloric effect and magnetovolume anomalies in nanostructured Pr2Fe17

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, |ΔS_M|, 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 |ΔS_M|(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.     

Giant magnetocaloric effect in a Heusler Mn50Ni40In10 unidirectional crystal

A modified high-pressure optical zone-melting technique was used to grow a Mn-rich Heusler Mn₅₀Ni₄₀In₁₀ unidirectional crystal. Experimental results showed that the produced unidirectional crystal underwent a magnetic transition in austenite, followed with a martensitic transformation from a ferromagnetic austenite to a ferromagnetic martensite upon cooling. Under a magnetic field change of 30 kOe, the total effective refrigeration capacities (RC_total) reached as high as 231.58 J/kg when the magnetic field was applied along parallel to the crystal growth direction, or 246.79 J/kg when the magnetic field was applied along perpendicular to the crystal growth direction. It was suggested that this unidirectional crystal growing technique may provide an effective approach to enhance the magnetocaloric effect of Mn-rich Heusler materials.     

Effect of gallium doping on the magnetocaloric effect of LaFe11.2Co0.7Si1.1

The lattice parameter and magnetocaloric properties of three samples of LaFe11.2Co0.7Si1.1-xGax with x = 0,0.03 and 0.05 have been investigated by X-ray powder diffraction and magnetization measurements.The lattice parameter increases slightly and the Curie temperature increases somewhat with increasing gallium content.However,a small amount of Ga doping into the sample decreases the magnetic entropy change of the sample.All the samples remain in the first-order magnetic phase transition.The most striking effect of the Ga doping is that the cooling capacity in the samples increases significantly.The maximum magnetic entropy change,-△SM,max,and the cooling capacity of the sample LaFe11.2Co0.7Si1.07Ga0.03 are 11.9 J·kg-1.K-1 and 254.8 J.kg-1,respectively.