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     

Hg2CuTi型Mn2NiGe合金的电子结构、磁性对压力、四方变形的响应

运用基于密度泛函理论的第一性原理方法,研究了Hg2CuTi型 Mn2NiGe合金的电子结构对外加压力的响应以及Mn2NiGe的电子结构、磁性对四方变形的响应．结果表明：i）随着外加压力的增加,因Ni、Mn原子间距的减小而导致杂化程度的增强,使得态密度整体向低能区域移动,同时,态密度幅度整体略有减小;ii）在由奥氏体相到马氏体相的变形中,同样因Ni、Mn原子间距的减小而导致杂化程度的增强,占据态的态密度向低能区域移动,体系的能量降低,同时,成键态向低能方向移动,反键态向高能方向移动,能带变宽,成键作用加强,最终导致在马氏体相中的稳定性增大;iii）在四方变形过程中,Mn2NiGe总磁矩的变化主要由Ni原子磁矩的变化所产生．     

Hg2CuTi型Mn2NiGe合金的电子结构、磁性对压力、四方变形的响应

运用基于密度泛函理论的第一性原理方法,研究了Hg2CuTi型 Mn2NiGe合金的电子结构对外加压力的响应以及Mn2NiGe的电子结构、磁性对四方变形的响应．结果表明：i）随着外加压力的增加,因Ni、Mn原子间距的减小而导致杂化程度的增强,使得态密度整体向低能区域移动,同时,态密度幅度整体略有减小;ii）在由奥氏体相到马氏体相的变形中,同样因Ni、Mn原子间距的减小而导致杂化程度的增强,占据态的态密度向低能区域移动,体系的能量降低,同时,成键态向低能方向移动,反键态向高能方向移动,能带变宽,成键作用加强,最终导致在马氏体相中的稳定性增大;iii）在四方变形过程中,Mn2NiGe总磁矩的变化主要由Ni原子磁矩的变化所产生．     

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.     

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.     

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.     

Heusler合金Ni2MnSi的电子结构、磁性、压力响应及四方变形的第一性原理研究

运用基于密度泛函理论的第一性原理的投影扩充波函数（PAW）方法,计算了化学计量的Ni₂MnSi的晶体结构、磁性、电子结构、压力响应以及四方变形. 计算结果表明,在Ni₂MnSi的总磁矩中,Mn原子对总磁矩的贡献最大;Ni₂MnSi的总态密度的低能部分主要由Si-s投影态密度决定,高能部分主要由Ni-d,Mn-d和Si-p的投影态密度决定;Ni₂MnSi在四方变形中,在095<c/a<11 关键词： 第一性原理 电子结构 压力响应 四方变形     

C掺杂Mn3Ge的电子结构和磁性

采用第一性原理计算方法研究C掺杂对Mn₃Ge的影响.对Mn_3-xGeC_x的晶体结构进行几何优化,发现C原子最稳定的掺杂位置在正八面体的中心位置.研究其电子结构和总磁矩随C掺杂量的变化,发现总磁矩随着C浓度的增加先减小后增大,其中Mn₃GeC_0.4总磁矩接近零,可以实现完全的磁性补偿.研究Mn₃GeC_0.4多层膜的磁性,给出总磁矩接近零的Mn₃GeC_0.4多层膜结构,为Mn₃Ge的实际应用提供参考.     

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．     

新型Heusler合金RuMn2Sn的态密度、磁性对压力、四方变形的响应

采用基于密度泛函理论的第一性原理方法,对新型Heusler合金RuMn2 Sn的态密度对外加压力的响应以及其态密度、磁矩对四方变形的响应进行了系统的研究。研究结果表明：1）随着外加压力的增大,态密度向低能区移动,幅度略有增加,原子之间的杂化程度增强,其原因是随外加压力的增大, Ru、Mn以及Sn原子之间的距离减小;2）在由奥氏体态到马氏体态相变的过程中,同样由于合金中各原子之间的距离减小导致态密度向低能区发生移动,体系的能量降低,同时在相变过程中,发现能带变宽,成键作用增强,从而说明马氏体态的稳定性增大;3）在上述相变的过程中, RuMn2 Sn总磁矩的变化主要由Ru原子磁矩的变化决定。     

Development of NiMnGa-based ferromagnetic shape memory alloy by rapid solidification route

The ferromagnetic shape memory alloy with nominal composition of Ni_52.5Mn_24.5Ga₂₃(at%) was developed by the melt-spinning technique. The as-spun ribbon showed dominant L2₁ austenitic (cubic) structure with splitting of primary peak in the X-ray diffractogram indicating existence of a martensitic feature. The quenched-in martensitic plates were revealed from Transmission electron microscopy (TEM). Increase of magnetisation at low-temperature rise indicates martensite to austenite transformation and its reverse with a drop in magnetisation during cooling cycle. The martensite to austenite transformation can be made spontaneous at higher magnetic field.     

Martensitic transformation in Cu-doped NiMnGa magnetic shape memory alloys

This paper studies the martensitic transformation in the Cu-doped NiMnGa alloys. The orthorhombic martensite transforms to L2₁ cubic austenite by Cu substituting for Ni in the Ni_50-xCu_xMn₃₁Ga₁₉ (x=2--10) alloys, the martensitic transformation temperature decreases significantly with the rate of 40 K per Cu atom addition. The variation of the Fermi sphere radius (k_F) is applied to evaluate the change of the martensitic transformation temperature. The increase of k_F leads to the increase of the martensitic transformation temperature.     

Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique

A ferromagnetic shape memory alloy with nomial composition Ni_52.5Mn_24.5Ga₂₃ (at%) was developed by a melt spinning technique. The as-spun ribbon showed dominant L2₁ austenitic (cubic) structure with a splitting of the primary peak in the X-ray diffractogram indicating the existence of a martensitic feature. The quenched-in martensitic plates were revealed in transmission electron microscopy. An increase of magnetization at low temperature indicated a martensite to austenite transformation and its reverse with a drop in magnetization during the cooling cycle. Higher magnetic fields propel martensite–austenite transformation spontaneously.     

Enhanced magnetoresistance through magnetic-field-induced phase transition in Ni2MnGa co-doped with Co and Mn

We report the observation of giant negative magnetoresistance up to −46% at 60 kOe magnetic field in Ni_1.68Co_0.32Mn_1.20Ga_0.80 alloy, which is about 5 times larger than that reported in Ni-Mn-Ga alloys. The significant change in resistivity during martensitic transformation originates from the altered electronic structure due to the change of magnetic state. The magnetic-field-induced phase transition from partially antiferromagnetic martensite to ferromagnetic austenite is responsible for the enhanced magnetoresistance.     

Hall effect in a martensitic transformation in Ni-Co-Mn-In Heusler alloys

The Hall effect, transverse magnetoresistance, and magnetization of Ni₄₈Co₂Mn₃₅In₁₅ Heusler alloys have been studied at T = 77–300 K in magnetic fields up to 15 kOe. It has been shown that a martensitic transformation is accompanied by a change in the sign of the constant of the ordinary Hall effect, which means a strong change in the electronic spectrum in the martensitic transformation, while the anomalous Hall effect (AHE) constant is positive in both the austenite and martensite phases. In both phases, there are no correlations between the AHE constant and the square of the resistivity, which are characteristic of the side jump mechanism in the AHE theory. In the near vicinity of the martensitic transformation, the field dependences of the Hall resistance are complex and nonmonotonic, indicating a change in the relative concentrations of the austenite and martensite phases in strong fields.     

Neutron spectroscopy study of magnons in the austenite and martensite phases of a Ni-Mn-Ga Heusler alloy

The low energy region of magnon excitations of an off-stoichiometric Ni_49.1Mn_29.4Ga_21.5 single crystal has been investigated by neutron spectroscopy. The lowest magnetic exchange stiffness constant D of 97±2 meV Å² has been found in the cubic austenite phase. In the two martensitic phases the exchange stiffness constants are significantly larger with values of 149±4 meV Å² in the tetragonal phase and 198±7 meV Å² in the low temperature martensite. The large value of D in the low temperature phase compared to the other phases cannot be explained solely by renormalization effects due to magnon-magnon interaction and is attributed to a stronger magnetic coupling. In both the martensitic phases a gap of magnon excitation at the Γ-point of about 0.2 meV was observed.     

Investigation of total and partial magnetic moments of Mn2NiAl with pressure at a several temperatures

ABSTRACT

The first principles calculations of structural and magnetic properties of the Heusler material Mn₂NiAl, have been studied using a full potential linearized augmented plane wave (FP-LAPW) within the density-functional theory (DFT). The phase stability of the cubic austenitic (L2₁₎ structure for Mn₂NiAl in both Cu₂MnAl (Fm3?m space group) and Hg₂CuTi (F43?m space group) type of structures with ferromagnetic and antiferromagnetic states, has been treated by applying the generalized gradient approximation proposed by Wu and Cohen (WC-GGA) alongside with the martensitic structure of Mn₂NiAl. The analysis of phase stability, cohesive energy and the calculated formation enthalpy of Mn₂NiAl reveal that the ferromagnetic MnMnNiAl is the most stable type of structure. Moreover, the calculated lattice parameters are found to be in good agreement with theoretical data. The variation of total magnetic moments M_T(μ_B), M_Mn (μ_B), M_Ni (μ_B) and M_Al (μ_B) in Mn₂NiAl with pressure at varying temperature (0, 273 and 344?K), have been calculated.