The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni43Mn46Sn11−xAlx alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni₄₃Mn₄₆Sn_11−xAl_x (x=0-2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.     

Sequence of structural and magnetic transitions in Ni48Co2Mn39Sn11 shape memory alloy

The structural and magnetic transitions in Ni₄₈Co₂Mn₃₉Sn₁₁ shape memory alloy were systematically investigated. During cooling, the paramagnetic austenite transforms into paramagnetic martensite at T_M∼375 K, followed by a gradual transition from paramagnetic to superparamagnetic martensite around T_S∼320 K. Upon further cooling through T_P∼100 K, the superparamagnetic clusters collectively freeze into a superspin glass state as corroborated by aging, rejuvenation, and memory effects. Consequently, the unique transition sequence of paramagnetic austenite→paramagnetic martensite→superparamagnetic martensite→superspin-glass martensite is disclosed.     

Determination of the magnetocaloric effect associated with martensitic transition in Ni46Cu4Mn38Sn12 and Ni50CoMn34In15 Heusler alloys

This paper presents a study of the inverse magnetocaloric effect (MCE) corresponding to martensitic transition using various experimental approaches for Ni46Cu4Mn38Sn12 and Ni50CoMn34In15 Heusler alloy. Through heat capacity measurements,it is found that the "giant inverse MCE" upon martensitic transition evaluated by the Maxwell relation in these alloys are unphysical results. This is due to the coexistence of both martensitic and austenitic phases,as well as thermal hysteresis during martensitic transition. However,careful study indicates that the spurious results during martensitic transition can be removed using a Clausius-Clapeyron equation based on magnetization measurements.     

Microstructure and magnetic properties of Ni-rich Ni54Mn25.7Ga20.3 ferromagnetic shape memory alloy thin film

A Ni₅₄Mn_25.7Ga_20.3 ferromagnetic shape memory alloy thin film has been fabricated by using the RF magnetron-sputtering technique. The structure and magnetic properties of the film were systematically investigated. The results show that the film is in ferromagnetic martensite state at room temperature with the Curie temperature (T_c) of about 370 K. The saturation magnetization (M_s) of the film reaches 45 emu/g at 300 K, which is about 80% as large as that of Ni–Mn–Ga bulk material. The magnetization hysteresis loops significantly depend on temperatures. The residual magnetization (M_r) and the coercive force (H_c) increase with decreasing temperatures. The grains homogeneously distribute in the film. The microstructure of the film consists of martensite plates. The interface between the martensite variants is clear and straight, indicating a good mobility.     

哈斯勒合金Ni50Mn35In15的马氏体相变及其磁热效应

利用电弧炉熔炼了Ni₅₀Mn₃₅In₁₅多晶样品,根据磁性测量对其马氏体相变和磁热效应进行了系统研究.结果表明,随着温度的降低,样品在室温附近先后发生了二级磁相变与一级结构相变特征的马氏体相变,导致它的磁化强度产生突变. 同时通过低温下的磁滞回线的测量发现样品存在交换偏置行为,表明低温下马氏体相中铁磁和反铁磁共存. 此外,根据Maxwell方程,计算了样品在马氏体相变温度附近的磁熵变,当温度为309K,磁场改变5 T时,样品的磁熵变可达22.3J/kgK. 关键词： 哈斯勒合金 50Mn₃₅In₁₅')" href="#">Ni₅₀Mn₃₅In₁₅ 马氏体相变 磁热效应     

Magnetic indication of the stress-induced martensitic transformation in ferromagnetic Ni–Mn–Ga alloy

A quantitative study of the stress-induced martensitic transformation in Ni_49.7Mn_29.1Ga_21.2 magnetic shape memory alloy has been carried out in two different ways: the first way is based on the measurements of saturation magnetization under variable mechanical stress and the second one is founded on the quantitative theoretical treatment of experimental stress–strain loops. A functional dependence between the volume fraction of transformed martensite and applied stress has been determined from both magnetization and strain values. A quantitative agreement between the functions determined in two different ways has been observed, and hence, the effectiveness of the magnetic indication of the stress-induced martensitic transformations has been proved. This method can be used to monitor stress-induced transformations in martensitic films, needles and small specimens.     

Effect of Sn doping on the martensitic and premartensitic transitions in Ni2MnGa

The effect of Sn doping at the Ga site of Ni₂MnGa is investigated through magnetic and magneto-transport measurements. Clear signatures of martensitic and premartensitic transitions are observed in the pure as well as in 5% Sn doped alloy. For 10% Sn doping, the martensitic transition vanishes, while the premartensitic transition remains visible at low temperature. All the samples are found to have a ferromagnetic ground state with saturation moment decreasing with increasing Sn concentration. The magnetocaloric effect near the martensitic transition in the pure and 5% Sn doped samples is found to be positive. However, the entropy change is found to decrease with increasing magnetic field, which is particularly prominent in the undoped sample. The samples also show negative magnetoresistance with anomalies at the martensitic and premartensitic transition points.     

Magnetic and magnetocaloric properties of perovskite manganite Pr0.55Sr0.45MnO3

In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganite Pr_0.55Sr_0.45MnO₃. It shows a sharp paramagnetic-ferromagnetic phase transition at 291 K and possesses a moderate magnetic entropy change near room temperature. In addition, a large relative cooling power (143.64 J/kg) and a wide temperature range (84 K) have been found in this material. Compare with the Landau model, we find that the itinerant electrons mainly contribute the larger magnetic entropy change at paramagnetic region.     

Magnetic transitions and thermomagnetic properties of GdCu6

We report results of dc magnetization and specific heat studies focusing on the paramagnetic to antiferromagnetic transition in GdCu₆. These results clearly reveal the evidences of multiple magnetic transitions in GdCu₆. In addition, a marked thermomagnetic irreversibility is observed in the temperature dependence of magnetization in low applied magnetic fields. Nature of the magnetic response changes with the increase in applied magnetic field in the temperature regime around the paramagnetic-antiferromagnetic transition temperature and also well inside the antiferromagnetic state. Experimentally measured specific heat in GdCu₆ is quite large in the temperature regime below 20 K, which indicates to the potential of GdCu₆ as a magnetic regenerator material for cryocooler related applications. Isothermal magnetic entropy change estimated from the results of magnetization and specific heat measurements shows a change in sign at the antiferromagnetic ordering temperature.     

The influence of gallium on magnetocaloric effect in Gd60Tb40 alloys

The influences of gallium substitution for terbium in Gd₆₀Tb₄₀ on the phase formation, Curie temperature and magnetic entropy change have been investigated. A series of Gd₆₀Tb_40−xGa_x with x=0, 1, 3 and 5 alloys were prepared by arc-melting method. The X-ray diffraction (XRD) analysis reveals that a small amount of Ga substitution for terbium in Gd₆₀Tb₄₀ can form the solid solution (Gd, Tb). The Curie temperature (T_c) increases from 270 K for Gd₆₀Tb₄₀ to 297 K for Gd₆₀Tb₃₇Ga₃, while the maximum magnetic entropy changes ΔS_{M, max} decreases from 5.15 J/K kg for Gd₆₀Tb₄₀ to 3.32 J/K kg for Gd₆₀Tb₃₇Ga₃ with increasing the Ga content.     

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.     

Magnetic properties of Mn-rich Rh2Mn1+xSn1−x Heusler alloys

X-ray powder diffraction and magnetization measurements have been carried out on Rh₂Mn_1+xSn_1−x (0≤x≤0.3) alloys. The alloys, which crystallize in the L2₁ structure, were found to exhibit ferromagnetic behavior. The lattice constant a at room temperature decreases with increasing x, whereas the Curie temperature T_C decreases linearly. At 5 K the magnetic moment per formula unit first increases with increasing x and then saturates for x≥0.2. The experimental results are discussed in terms of the influence of the Mn-Mn exchange interactions between the Mn atoms on the Sn and Mn sites.     

掺杂对Ni51.5Mn25Ga23.5相变行为和磁性的影响

通过往母合金Ni_51.5Mn₂₅Ga_23.5掺入7种IVA, VA和VIA 过渡族元素得到系列掺杂合金Ni_51.5Mn₂₃M₂Ga_{23 .5}.M为掺杂元素.实验结果表明，掺杂效应一般引起马氏体相变温度的下降，其中，W 的掺杂是7种元素中唯一使相变温度升高的特例，且出现了中间马氏体相变.同时，在价电子 浓度不变的情况下，相变更敏感于原子的尺度效应.实验发现，Ti，Zr，Hf，V四种非磁性元 素的掺杂使Mn原子磁矩减小，而Nb，Ta，W三种非磁性元素的掺杂却可以明显地增大Mn原子 的磁矩.在考察掺杂效应时，不能忽略马氏体相变引起的晶格变化对材料磁性的影响. 关键词： NiMnGa 掺杂 马氏体相变 磁性     

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.     

一级磁结构相变材料Mn0.6Fe0.4NiSi0.5Ge0.5和Ni50Mn34Co2Sn14的磁热效应与磁场的线性相关性

磁熵变（△S_M）与磁场（μ₀H）的相关性已在很多二级相变材料中被研究并报道,但一级相变材料的磁热效应与磁场相关性还少有报道.本文在具有一级磁结构相变的Mn_0.6Fe_0.4NiSi_0.5Ge_0.5材料中研究发现△S_M与μ₀H存在线性相关性,并通过麦克斯韦关系式的数值分析详细讨论了这一线性相关性的来源.同时,进一步发现在低磁场时,△S_M近似正比于μ₀H的平方.该线性相关性同样在一级磁结构相变Ni₅₀Mn₃₄Co₂Sn₁₄材料中得到了印证.但由于一级磁弹相变LaFe_11.7Si_1.3材料相变温度具有更强的磁场依赖性,不具有△S_M的线性相关性,因此,本研究表明,当磁结构相变材料的相变温度具有弱磁场依赖性时,△S_M与μ₀H具有线性相关性.进而,在磁场未达到相变饱和磁场以下,利用△S_M与μ₀H的线性相关性可以有效推测更高磁场下的△S_M.     

Tuning the magnetocaloric properties of La0.7Ca0.3MnO3 manganites through Ni-doping

The effect of Ni²⁺ doping on the magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ manganites synthesized via the auto-combustion method is reported. The aim of studying Ni²⁺-substituted La_0.7Ca_0.3Mn_1 ? xNi_xO₃ ($x=0,0.02,0.07$, and 0.1) manganites was to explore the possibility of increasing the operating temperature range for the magnetocaloric effect through tuning of the magnetic transition temperature. X-ray diffraction analysis confirmed the phase purity of the synthesized samples. The substitution of Mn³⁺ ions by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice was also corroborated through this technique. The dependence of the magnetization on the temperature reveals that all the compositions exhibit a well-defined ferromagnetic to paramagnetic transition near the Curie temperature. A systematic decrease in the values of the Curie temperature is clearly observed upon Ni²⁺ doping. Probably the replacement of Mn³⁺ by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice weakens the Mn³⁺–O–Mn⁴⁺ double exchange interaction, which leads to a decrease in the transition temperature and the magnetic moment in the samples. By using Arrott plots, it was found that the phase transition from ferromagnetic to paramagnetic is second order. The maximum magnetic entropy changes observed for the $x=0,0.02,0.07$, and 0.1 composites was 0.85, 0.77, 0.63, and 0.59 J/kg?K, respectively, under a magnetic field of 1.5 T. In general, it was verified that the magnetic entropy change achieved for La_0.7Ca_0.3Mn_1 ? xNi_xO₃ manganites synthesized via the auto-combustion method is higher than those reported for other manganites with comparable Ni²⁺-doping levels synthesized via standard solid state reaction. The addition of Ni²⁺ increases the value of the relative cooling power as compared to that of the parent compound. The highest value of this parameter (～60 J/kg) is found for a Ni-doping level of 2% around 230 K in a field of 1.5 T.     

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.     

Ni47Mn32Ga21多晶合金的磁熵变和磁感生应变

对定向凝固方法制备的Ni₄₇Mn₃₂Ga₂₁多晶合金,通过扫描电镜、金相、电子能谱等手段研究其组份和组织形貌,通过对合金磁化强度与温度关系、等温磁化曲线及磁感生应变曲线等的测量分析,研究了合金结构相变和磁相变过程中的磁熵变及不同压力下的磁感生应变. 研究结果表明:合金组份与设计组份基本一致,室温下合金大部分为马氏体相. 升温过程中合金的磁熵变在居里温度(365 K)附近有最大值,并有较大的磁熵变峰值半高宽,747 kA/m的磁场下该磁熵变最大值为-1.45 J/kg ·K,磁熵变峰值的半高宽为21 K. 合金在室温(298 K)下有较好的双向可恢复磁感生应变,480 kA/m磁场下,无压力时合金的磁感生应变值达到-670×10^-6,并趋饱和;而在与磁场方向平行的27.3 MPa外压力作用下合金的磁感生应变值增大到-1300×10^-6,且未饱和. 关键词： Ni-Mn-Ga 铁磁形状记忆合金 磁熵变 磁感生应变     

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni 2 MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni 2 MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.