磁性形状记忆合金Mn_2NiGa性质的第一性原理研究

运用基于密度泛函理论的第一性原理,对Hg_2CuTi型Mn_2NiGa分别在奥氏体态和马氏体态下的晶体结构、磁性、电子结构以及Hg_2CuTi型Mn_2NiGa的压力响应、四方变形等进行了计算.计算结果表明:i)在奥氏体态和马氏体态下,Mn原子均是Mn_2NiGa总磁矩的主要贡献者,但Mn(A)、Mn(B)原子磁矩的值不等且呈反平行耦合,因而Mn_2NiGa合金均表现为亚铁磁结构;ii)在奥氏体态和马氏体态下,Mn(A)-d、Mn(B)-d的投影态密度在费米面附近交叠都很少,说明Mn(A)、Mn(B)原子之间的d-d直接交换作用均很弱,而此时Mn的原子内交换作用很强.这是Mn_2NiGa在奥氏体态和马氏体态下均呈现亚铁磁结构的成因;iii)理论计算出在四方变形中,在c/a约为1.27处出现一个稳定的马氏体相,这与实验结果相符.     

磁性形状记忆合金Mn2NiGa band Jahn-Teller效应的第一性原理研究

运用第一性原理的方法,研究了磁性形状记忆合金Mn₂NiGa在马氏体相变中晶格结构、磁结构、Mn原子d电子结构的变化.研究表明,伴随Mn₂NiGa马氏体相变的发生,形成了一个由两根长键及四根短键组成的拉长八面体结构,即产生了沿z轴拉长的Jahn-Teller畸变;在相变时,位于八面体中心的Mn原子的磁矩发生显著的变化,而作为配体的Ni、Ga原子的磁矩变化很微小;Jahn-Teller畸变的发生,是由于晶体的畸变使配住场产生变化,导致Mn原子d电子态密度重新分布,从而使e_g和t_2g能级分裂所致.     

磁性形状记忆合金Mn2NiGa band Jahn-Teller效应的第一性原理研究

运用第一性原理的方法,研究了磁性形状记忆合金Mn2NiGa在马氏体相变中晶格结构、磁结构、Mn原子d电子结构的变化．研究表明,伴随Mn2NiGa马氏体相变的发生,形成了一个由两根长键及四根短键组成的拉长八面体结构,即产生了沿z轴拉长的Jahn–Teller畸变;在相变时,位于八面体中心的Mn原子的磁矩发生显著的变化,而作为配体的Ni、Ga原子的磁矩变化很微小;Jahn–Teller畸变的发生,是由于晶体的畸变使配位场产生变化,导致Mn原子d电子态密度重新分布,从而使eg和t2g能级分裂所致．     

Heusler合金Mn2NiGa的第一性原理研究

运用基于密度泛函理论的投影缀加波方法研究了Heusler合金Mn₂NiGa的四方变形,对立方和四方结构的磁矩、电子结构、弹性常数及声子谱进行了计算和分析.Mn原子是Mn₂NiGa总磁矩的主要贡献者,但Mn（A）、Mn（B）原子磁矩的值不等且呈反平行耦合,因而Mn₂NiGa合金在两种状态下均表现为亚铁磁结构.四方变形中,Mn₂NiGa在c/a=0.94和c/a=1.27处出现总能的局域极小值和局域最小值,分别对应一个稳定的马氏体.弹性常数的计算结果显示,Mn₂NiGa的立方结构不满足立方相稳定性判据,四方结构（c/a=0.94和c/a=1.27）的弹性常数满足相应的稳定性判据.立方结构声子谱中存在虚频,而四方结构（c/a=0.94和c/a=1.27）则不存在虚频,验证了Mn₂NiGa四方结构比立方结构稳定.c/a=1.27的四方结构Mn₂NiGa转变为c/a=1.0的立方结构的相变温度在315 K左右.     

形状记忆合金

我们知道，一般金属材料受到外力作用后，首先发生弹性变形，达到屈服点，就产生塑性变形，外力消除后留下永久变形．1932年，瑞典人奥兰德在金镉合金中首次观察到记忆效应（SME），即合金的形状被改变之后，一旦加热到一定的跃变温度时，它又可以魔术般地变回到原来的形状，人们把具有这种特殊功能的合金称为形状记忆合金（SMA）．     

Hyperfine fields in a new Heusler alloy Mn2CoSn

A new Heusler alloy Mn₂CoSn has been obtained. The distribution of hyperfine fields in Mn₂CoSn and Mn₂CoSn alloys by nuclear spin echo method has been investigated. The conclusion concerning ferrimagnetic structure of Mn₂CoSn alloy has been drawn.     

铁磁形状记忆合金Mn2NiGa中应力诱发马氏体相的结构和磁性

采用金刚石对顶砧高压装置(DAC)和同步辐射X射线光源法,对Heusler类型的磁性形状记忆合金Mn2NiGa的结构进行了原位高压X射线衍射测量,并对卸载后的受压样品进行了磁测量.实验观察到材料在室温下分别在0.77 GPa和20 GPa压力下发生了两次不可逆结构相变:马氏体相变和两种不同马氏体间的等结构相变.同时加压使马氏体结构中产生了大量的缺陷,造成了严重的晶格畸变,致使马氏体结构的矫顽力提高了近10倍,达到204 kA/m.结果发现,加压处理造成样品马氏体相饱和磁化强度的大幅度降低,显示出明显的缺陷效应.     

Dynamic Simulation for Hysteresis in Shape Memory Alloy under Tension

We demonstrate that the Suliciu model is capable to model the hysteresis phenomenon observed experimentally in NiTi shape memory alloy micro-tubes. This model allows a class of stationary phase interfaces. By a series of fully dynamic numerical simulations that mimic quasi-static loading and unloading, the nominal stress-train curve exhibits a big hysteresis loop, which quantitatively agrees with the experimental results.     

Automatic Alignment of Optical System Using Shape Memory Alloy

Properties of the shape memory alloy of 29 μm thick Ni-Ti foil are investigated as an actuator to align optical elements. Since the intrinsic properties of the Ni-Ti foil are not satisfactory in reproducibility and hysteresis, a simple feedback control is used. A pinhole integrated with a surrounding position sensor is proposed to realize a confocal optical system having an automatic alignment function. Large displacement (mm) and precision at the level of the optical wavelength are found to be possible.Present at 1996 International Workshop on Interferometry (IWI ‘96), August 27-29, Saitama, Japan.     

Martensite Transformation,Magnetotransport Properties,and Magnetocaloric Effect in Ni47Mn42In11 Alloy

Physics of the Solid State - The structure, electric properties, and magnetocaloric effect in Ni47Mn42In11 ferromagnetic alloy in which the martensite transformation temperature is close to room...     

Direct and Indirect Determination of the Magnetocaloric Effect in the Heusler Compound Ni1.7Pt0.3MnGa

Magnetic shape-memory materials are potential magnetic refrigerants, due the caloric properties of their magnetic-field-induced martensitic transformation. The first-order nature of the martensitic transition may be the origin of hysteresis effects that can hinder practical applications. Moreover, the presence of latent heat in these transitions requires direct methods to measure the entropy and to correctly analyze the magnetocaloric effect. Here, we investigated the magnetocaloric effect in the Heusler material Ni1.7Pt0.3MnGa by combining an indirect approach to determine the entropy change from isofield magnetization curves and direct heat-flow measurements using a Peltier calorimeter. Our results demonstrate that the magnetic entropy change ΔS in the vicinity of the first-order martensitic phase transition depends on the measuring method and is directly connected with the temperature and field history of the experimental processes.     

In Situ TEM Study of Phase Transformations in Nonstoichiometric Ni46Mn41In13 Heusler Alloy

Physics of the Solid State - This paper presents a study of the metamagnetostructural transition of the martensitic type in the Ni46Mn41In13 alloy with magnetic shape memory and inverse...     

Direct measurement of magnetocaloric effect in metamagnetic Ni43Mn37.9In12.1Co7 Heusler Alloy

The magnetocaloric effect in the metamagnetic Ni₄₃Mn_37.9In_12.1Co₇ Heusler alloy is directly studied experimentally under the adiabatic and quasi-isothermal conditions in a magnetic field with induction of up to 14 T.     

Possible Martensitic Transformation in Heusler Alloy Pt_2MnSn from First Principles

Using density functional theory calculations, we investigate the tetragonal distortion, electronic structure and magnetic property of Pt_2 Mn Sn. The results indicate that, when the volume-conserving tetragonal distortion occurs, the energy minimum appears at c/a(6 = 0.84, and the energy difference between the minimum and cubic phase is as high as 107 me V/f.u. Thus from the point of view of thermodynamics, martensitic transformation may occur in Pt_2 Mn Sn with decreasing the temperature. The electronic structure of its cubic and martensitic phases also approves this. Moreover, both the cubic and tetragonal phases of Pt_2 Mn Sn are ferromagnetic structures and their total magnetic moments are 4.26 μ_B and 4.12 μ_B, respectively.     

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.     

Heusler合金Mn2NiGe马氏体相变的带Jahn-Teller效应研究

运用第一性原理的方法,计算磁性形状记忆合金Mn₂NiGe的晶体结构、磁结构、电子结构在马氏体相变中的变化.结果表明:Mn₂NiGe在发生马氏体相变时,产生了c轴拉长而a和b轴缩短的Jahn-Teller畸变,形成一个由两根长键和四根短键组成的拉长的八面体;伴随Jahn-Teller畸变,处于八面体中心的Mn离子的磁矩发生了显著的变化,而作为配体的Ni和Ge离子的磁矩基本不变;Jahn-Teller效应中Mn离子的e_g和t_2g能级的分裂,源自于晶体畸变所产生的配位场的改变,导致Mn离子d电子态密度的重新分布,从而在费米能级两侧打开一个赝能隙.     

Possible Topological Hall Effect in a Spin Gapless Semiconducting Mn2CoAl Heusler Compound

Topological Hall effect (THE), induced by the interaction of charge carriers with mesoscopic or microscopic noncoplanar spin structures, holds promising applications in the field of spintronics. In the present study, a giant THE of about 2 μΩ cm at room temperature is reported in a bulk spin gapless semiconducting Mn₂CoAl cubic Heusler compound. Temperature-dependent investigation of magneto-transport data reveals that the Mn₂CoAl has the large THE over a wide temperature range of 2–300 K. The alternating current (AC) susceptibility as a function of magnetic field exhibits a smooth and continuous response rather than any kink or anomaly, suggests that the observed THE in the Mn₂CoAl compound results from the interaction of charge carriers with the microscopic noncoplanar spin texture. The observed THE as a function of temperature follows the same behavior as the magnetocrystalline anisotropy (MCA) of the cubic Mn₂CoAl, indicating the competition of the MCA with ferromagnetic and antiferromagnetic exchange interactions as the origin of the noncoplanar spin texture and hence THE. Micromagnetic simulations further support the emergence of noncoplanar spin structure as a result of the competition between different energies.     

Sn hyperfine field trends in the Heusler alloys Co2Mn(Al,Si)

Hyperfine Interactions - The Sn hyperfine field in the series of Heusler alloys Co2MnAl0.98−xSixSn0.02 (x=0.00, 0.20, 0.40, 0.58, 0.78, 0.98) has been measured at 77 K using119Sn...     

Heusler合金Mn2NiGe磁性形状记忆效应的第一性原理预测

运用基于密度泛函理论的第一性原理,对Hg₂CuTi型Mn₂NiGe的四方变形、晶体结构、磁性、电子结构、压力响应等进行了计算.计算结果表明: 1)在由立方结构至四方结构的转变中,在c/a约为1.34处存在一个稳定的马氏体相;2)在奥氏体态和马氏体态下,Mn原子均是Mn₂NiGe总磁矩的主要贡献者,但Mn(A),Mn(B)原子磁矩的值不等且呈反平行耦合,因而Mn₂N 关键词： 第一性原理 磁性形状记忆 四方变形 马氏体相变     

Magnetocaloric effects in Ni-Mn-X based Heusler alloys with X=Ga, Sb, In

The Mn-based Heusler alloys encompass a rich collection of useful materials from highly spin-polarized systems to shape memory alloys to magnetocaloric materials. In this work we have summarized our studies of magnetostructural transitions from paramagnetic austenite to ferromagnetic martesite phases at TMC in Ni₂MnGa-based alloys (Ni₂Mn_0.75Cu_0.25-xCo_xGa, Ni₂Mn_0.70Cu_0.30Ga_0.95Ge_0.05, Ni₂Mn_1-xCu_xGa, Ni_2+xMn_1-xGa, and Ni₂Mn_0.75-xCu_xGa), and martensitic transitions from the ferromagnetic austenite to the martesite state in off-stoichiometric Ni-Mn-(In/Sb) Heusler alloys. The phase transition temperatures and respective magnetic entropy changes (ΔS) depend on composition in these systems and have been determined from magnetization measurements in the temperature interval 5-400 K, and in magnetic fields up to 5 T. It is shown that, depending on the composition and doping scheme the “giant” ΔS=40-60 J/(kgK) (for a field change of 5 T) can be observed in the temperature range (300-360 K) for the Ga-based alloys. The interplay between or coupling of the various transitions in Ni₂Mn(Mn,X) systems with X=Sb and In leads to exchange bias effects, giant magnetoresistance, and both inverse and “normal” magnetocaloric effects.