Magnetic entropy change involving martensitic transition in NiMn-based Heusler alloys

Our recent progress on magnetic entropy change（S） involving martensitic transition in both conventional and metamagnetic NiMn-based Heusler alloys is reviewed.For the conventional alloys,where both martensite and austenite exhibit ferromagnetic（FM） behavior but show different magnetic anisotropies,a positive S as large as 4.1 J·kg^-1·K^-1 under a field change of 0-0.9 T was first observed at martensitic transition temperature T M～197 K.Through adjusting the Ni：Mn：Ga ratio to affect valence electron concentration e/a,T M was successfully tuned to room temperature,and a large negative S was observed in a single crystal.The △S attained 18.0 J·kg^-1·K^-1 under a field change of 0-5 T.We also focused on the metamagnetic alloys that show mechanisms different from the conventional ones.It was found that post-annealing in suitable conditions or introducing interstitial H atoms can shift the T M across a wide temperature range while retaining the strong metamagnetic behavior,and hence,retaining large magnetocaloric effect（MCE） and magnetoresistance（MR）.The melt-spun technique can disorder atoms and make the ribbons display a B2 structure,but the metamagnetic behavior,as well as the MCE,becomes weak due to the enhanced saturated magnetization of martensites.We also studied the effect of Fe/Co co-doping in Ni 45（Co1-xFex）5 Mn36.6In13.4 metamagnetic alloys.Introduction of Fe atoms can assist the conversion of the Mn-Mn coupling from antiferromagnetic to ferromagnetic,thus maintaining the strong metamagnetic behavior and large MCE and MR.Furthermore,a small thermal hysteresis but significant magnetic hysteresis was observed around TM in Ni51Mn49-xInx metamagnetic systems,which must be related to different nucleation mechanisms of structural transition under different external perturbations.

Microwave ferromagnetic properties of as-deposited Co_2FeSi Heusler alloy films prepared by oblique sputtering

The Co2FeSi films are deposited on Si （100） substrates by an oblique sputtering method at ambient temperature. It is revealed that the microwave ferromagnetic properties of Co2FeSi films are sensitive to sample position and sputtering power. It is exciting that the as-deposited films without any magnetic annealing exhibit high in-plane uniaxial anisotropy fields in a range of 200 Oe-330 Oe （1 Oe = 79.5775 A.m ^-1）, and low coercivities in a range of 5 Oe-28 Oe. As a result, high self-biased ferromagnetic resonance frequency up to 4.75 GHz is achieved in as-deposited oblique sputtered films. These results indicate that Co2FeSi Heusler alloy films are promising in practical applications of RF/microwave devices.

First-principles calculations of Ni–(Co)–Mn–Cu–Ti all-d-metal Heusler alloy on martensitic transformation,mechanical and magnetic properties

The martensitic transformation,mechanical,and magnetic properties of the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) (x=0.125,0.25,0.375,0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5)[(x=0.125,y=0.125,0.25,0.375,0.5) and (x=0.125,0.25,0.375,y=0.625)]alloys were systematically studied by the first-principles calculations.For the formation energy,the martensite is smaller than the austenite,the Ni–(Co)–Mn–Cu–Ti alloys studied in this work can undergo martensitic transformation.The austenite and non-modulated (NM) martensite always present antiferromagnetic state in the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) (y<0.625) alloys.When y=0.625 in the Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) series,the austenite presents ferromagnetic state while the NM martensite shows antiferromagnetic state.Cu doping can decrease the thermal hysteresis and anisotropy of the Ni–(Co)–Mn–Ti alloy.Increasing Mn and decreasing Ti content can improve the shear resistance and normal stress resistance,but reduce the toughness in the Ni–Mn–Cu–Ti alloy.And the ductility of the Co–Cu co-doping alloy is inferior to that of the Ni–Mn–Cu–Ti and Ni–Co–Mn–Ti alloys.The electronic density of states was studied to reveal the essence of the mechanical and magnetic properties.

Formation of quaternary all-d-metal Heusler alloy by Co doping fcc type Ni_(2)MnV and mechanical grinding induced B2–fcc transformation

The structure of the all-d-metal alloy Ni_(50-x)Co_(x)Mn_(25)V_(25)(x=0–50)is investigated by using theoretical and experimental methods.The first-principles calculations indicate that the most stable structure of the Ni_2MnV alloy is face-centered cubic (fcc)type structure with ferrimagnetic state and the equilibrium lattice constant is 3.60A,which is in agreement with the experimental result.It is remarkable that replacing partial Ni with Co can turn the alloy from the fcc structure to the B2-type Heusler structure as Co content x>37 by using the melting spinning method,implying that the d–d hybridization between Co/Mn elements and low-valent elements V stabilizes the Heusler structure.The Curie temperature T_(C) of all-dmetal Heuser alloy Ni_(50-x)Co_(x)Mn_(25)V_(25)(x>37)increases almost linearly with the increase of Co due to that the interaction of Co–Mn is stronger than that of Ni–Mn.A magnetic transition from ferromagnetic state to weak magnetic state accompanying with grinding stress induced transformation from B2 to the dual-phase of B2 and fcc has been observed in these all-d-metal Heusler alloys.This phase transformation and magnetic change provide a guide to overcome the brittleness and make the all-d-metal Heusler alloy interesting in stress and magnetic driving structural transition.

Effect of local atomic disorder on the half-metallicity of full-Heusler Co_2FeSi alloy:a first-principles study

This paper investigates the effect of atomic disorder on the electronic structure, magnetism, and half-metallicity of full-Heusler Co2FeSi alloy by using the full-potential linearized augmented plane wave method within the generalized gradient approximation （GGA） and GGA-kU schemes. It considers three types of atomic disorders in Co2FeSi alloy： the Co-Fe, Co-Si, and Fe-Si disorders. Total energy calculations show that of the three types of disorders, the Fe-Si disorder is more likely to occur. It finds that for the Co Si disorder, additional states appear in the minority band-gap at the EF and the half-metallcity is substantially destroyed, regardless of the disorder level. On the other hand, the Co-Fe and Fe-Si disorders have little effect on the half-metallicity at a low disorder level. When increasing the disorder levels, the half-metallcity is destroyed at about 9 % of the Co-Fe disorder level, while that stays at 25 % of the Fe-Si disorder level.

Determination of the magnetocaloric effect associated with martensitic transition in Ni_(46)Cu_(4)Mn_(38)Sn_(12) and Ni_(50)CoMn_(34)In_(15) Heusler alloys

This paper presents a study of the inverse magnetocaloric effect （MCE） corresponding to martensitic transition using various experimental approaches for Ni46Cu4Mn38Sn12 and NisoCoMn34In]5 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.

Electronic structures and magnetisms of the Co_2TiSb_(1-x)Sn_x(x= 0, 0.25, 0.5) Heusler alloys: A theoretical study of the shape-memory behavior

The total energy, electronic structures, and magnetisms of the Al Cu2Mn-type Co2TiSb1-xSnx（x = 0, 0.25, 0.5） with the different lattice parameter ratios of c/a are studied by using the first-principles calculations. It is found that the phase transformation from the cubic to the tetragonal structure lowers the total energy, indicating that the martensitic phase is more stable and that a phase transition from austenite to martensite may happen at a lower temperature. Thus, a ferromagnetic shape memory effect can be expected to occur in these alloys. The Al Cu2Mn-type Co2TiSb1-xSnx（x = 0, 0.25, 0.5） alloys are weak ferrimagnets in the austenitic phase and martensitic phase.

Lattice Dynamics and Electronic Properties of Heusler Alloys Li_(2)AlX(X=Ga,In):A Comparison Study

The lattice parameters,bulk modulus,rst derivative of the bulk modulus,electronic band structures,phonon dispersion curves and phonon density of states calculations for Li_(2)AlGa and Li_(2)AlIn Heusler alloys are performed and compared in this study using density functional theory within the generalized gradient approximation.Computed lattice parameters display a good agreement with the literature.Obtained electronic band structures of both Heusler alloys show that they are in semi-metallic structure.Phonon dispersion curves and the phonon density of states graphs are also obtained in order to study the lattice dynamics of these Heusler alloys.It is noticed that Li_(2)AlGa and Li_(2)AlIn Heusler alloys are dynamically stable in the ground state.

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 Pt2MnSn. The results indicate that, when the volume-conserving tetragonal distortion occurs, the energy minimum appears at c/a = 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, martensitie transformation may occur in Pt2MnSn with decreasing the temperature. The electronic structure of its cubic and martensitic phases also approves this. Moreover, both the cubic and tetragonal phases of Pt2MnSn are ferromagnetic structures and their total magnetic moments are 4.26 μB and 4.12 μB, respectively.

同构合金化对P型ZrCoSb基高熵half-Heusler合金热电性能的影响

Half-Heusler(HH)合金由于其本身具有较为优异的力学性能和高温热稳定性,已成为目前最具有应用前景的中高温热电材料之一。然而,其本身较高的本征晶格热导率阻碍了热电性能的进一步提升。本文以P型Zr Co Sb0.85Sn0.15合金为研究对象,基于同构合金化具有优异P型热电性能的(Nb0.8Ta0.2)0.8Ti0.2Fe Sb,通过磁悬浮熔炼和放电等离子烧结设计并制备出一种(Zr Co Sb0.85Sn0.15)1-x[(Nb0.8Ta0.2)0.8Ti0.2Fe Sb]x(x=0,0.2,0.3,0.4,0.5)高熵HH合金。微观组织分析表明,同构合金化这一策略引入了大量多尺度多衬度的第二相,这将有效增强对声子的散射。其中,当同构合金化含量为0.3时,晶格热导率在923 K时从Zr Co Sb0.85Sn0.15的4.72 W·m-1·K-1降至3.07 W·m-1·K-1,降低了35%。然而,由于多位点合金化元素间存在较为复杂的掺杂效果,使其电导率和塞贝克系数同时降低,最终导致热电优值存在一定的降低。本研究工作表明,高熵合金设计思想是一种降低HH热电合金晶格热导率的有力措施。

Half-Heusler合金NiFeSb和NiMnSb的磁性及电子结构的第一性原理研究

用基于密度泛函理论的第一性原理方法,计算了half-Heusler合金NiFeSb和NiMnSb的晶体结构、磁性及电子结构.计算结果表明,磁性原子Fe和Mn在两种合金的总磁矩中贡献最大,在NiFeSb总磁矩中,Ni原子贡献比例接近在NiMnSb中的2倍,而Sb原子的贡献比例是在NiMnSb中的1/5;两种合金的自旋向上能带都具有明显的金属特征,而自旋向下能带有明显的差别;两种合金费米能级以下的总态密度(DOS)主要由Ni—3d和Fe—3d(Mn—3d)态决定,费米能级以上主要由Fe—3d(Mn—3d)自旋向下部分决定.

LiMgPbSb型四元Heusler合金CoFeTiSb的半金属性及其无序效应

基于密度泛函理论的GGA计算,我们研究了LiMgPbSb型Heusler合金CoFeTiSb电子结构,发现CoFeTiSb在费米面处存在100%的自旋极化率,并且具有2μB的原胞总磁矩.此外,我们考虑了Co-Fe、Co-Ti和Fe-Ti交换无序对CoFeTiSb合金电子结构的影响,发现这三种交换无序均使得CoFeTiSb完全丧失了半金属性,在Co-Fe交换无序下,CoFeTiSb合金具有85%的自旋极化率,而在Co-Ti无序下,CoFeTiSb合金的自旋极化率只有5%.

Half-Heusler合金NiMn_(1-x)Nb_xSb的磁性和半金属稳定性

基于密度泛函理论的第一性原理计算,我们系统地研究了half-Heusler合金NiMn_1-_xNb_xSb的电子结构、磁性和半金属稳定性.结果表明合金的晶格常数和磁性分别很好地符合Vegard定理和S1ater-Pauling规则;当掺杂浓度为25%时,合金的费米面恰好位于其自旋向下带隙的中部.从而呈现最稳定的半金属性.此外,当前的研究也显示Ni-Mn和Ni-Nb之间的d电子杂化依赖于RKKY间接交换机制,共同贡献于合金的总磁矩.

Heusler合金Co_2CrGa(100)的表面结构、磁性和自旋极化

基于第一性原理计算,我们系统地呈现了三元合金Co2CrGa(100)表面的原子弛豫、磁性、电子结构以及表面原子极化行为.结果显示,由于Co-Ga和Co-Cr成键的差异,表面的Co和Cr原子分别向内层收缩和向外部真空层伸展.与块体相比较,表面Co和Cr原子的自旋磁矩由于局域性的提升而明显增大.在研究的Co2CrGa(100)不同原子端面中,可以观察到块体中的半金属带隙在CoCo和GaGa原子端面被大量的表面态所破坏,仅仅在CrGa和CrCr原子覆盖的端面,检测到100%的理想极化,预测其在隧道结中可能具有较佳的应用潜力.

Full-Heusler合金Ni_2MnSn力学性能和电子性能的第一性原理

Heusler合金,是有序的三元金属间化合物。围绕full-Heusler合金Ni_2MnSn力学性能和电子性能展开计算,利用基于密度泛函理论的第一性原理方法,从量子力学角度考虑了电子的电荷特性和自旋特性。建立full-Heusler合金Ni_2MnSn的结构模型,计算最优化晶格常数、能带结构和态密度;测试其体模量、剪切模量、杨氏模量和泊松比。计算得到总磁矩为-4.34μB,其中Mn原子的磁矩对总磁矩贡献最大,并通过对Ni_2MnSn态密度及各个原子态密度的分析,发现磁性来源于Mn原子的能级劈裂。在几何优化的基础上,进行了Ni_2MnSn的力学性能计算,发现Ni_2MnSn具有很好的延展性和塑性。

Heusler合金Mn_2TiGe的电子结构与磁性研究

基于密度泛函理论(DFT),采用第一性原理赝势方法,结合广义梯度近似(GGA),对Mn2TiGe Heusler合金的电子结构、磁性及半金属特性进行了研究。结果表明,Mn2TiGe的稳定晶体结构是Mn2CuAl型晶体结构;由能带计算得到Mn2TiGe的自旋磁矩为1.97μB,其中Mn、Ti、Ge原子的自旋磁矩分别为0.71、-0.39、-0.03μB。

Co基Heusler合金:磁性、半金属性,sp元素掺杂及在磁隧道结中的应用和发展

首先致力于介绍Co基Heusler合金的结构特征、磁性来源以及独特的半金属能带结构,以揭示这些合金独特的电磁性质和在磁隧道结中的应用潜力。在此基础上,进一步讨论了sp元素掺杂对调整Heusler合金电子结构和稳定其半金属性的重要意义。最后,通过分析Co基Heusler合金磁隧道结的电子输运原理,描述了Co基Heu-sler合金磁隧道结的研究现状和发展方向。

第一性原理研究Hg_2CuTi型Heusler合金Ti_2FeB的磁性和半金属性

基于密度泛函理论的GGA计算,我们具体研究了Hg2CuTi型Heusler合金Ti2FeB的电子结构和磁性质,结果发现Ti2FeB合金在其费米面处存在100%的自旋极化,并在5.1～6.2晶格范围内被保留.Ti2FeB具有大约0.5eV的半金属带隙和1μB的原胞总磁矩,是一个稳定的半金属铁磁体.此外,我们的研究也表明RKKY型间接交换和d电子杂化在决定合金磁性质中起决定性作用.

Heusler合金Mn_2NiSi的形状记忆行为和磁性

采用基于密度泛函理论的第一性原理,对Heusler合金Mn2NiSi的电子结构和磁性进行了研究。计算结果表明:从立方结构到四方结构的相变降低了总能量,表明马氏体相是更加稳定的。随着温度的降低,Mn2NiSi经历了从奥氏体到马氏体的转变,体积几乎不变,表明了该合金具有形状记忆行为。磁基态是亚铁磁,Mn(A)和Mn(B)磁矩是反平行排列的、并且不等。奥氏体相和马氏体相的总磁矩分别是9.64×10-24A·m2和2.60×10-24A·m2。在这两种结构中,Mn(A)和Mn(B)是Mn2NiSi总磁矩的主要贡献者。根据态密度解释了马氏体相变和磁性的产生。

Inverse-Heusler合金Ti_2Co_(1-x)Ni_xGa的磁性和半金属性

利用第一性原理计算了Inverse-Heusler合金Ti_2Co_(1-x)Ni_xGa的半金属性、磁性和电子性质.计算结果表明Ti_2Co_(1-x)Ni_xGa的原胞总磁矩和晶格常数随着x的增加而增加.由于过渡金属原子之间的直接杂化和sp电子的调节,原胞总磁矩和晶格常数随掺杂浓度x的改变而改变,并与Slater Pauling规则形成一定的差异.当Ni的掺杂浓度x=0.5时,Ti_2Co_(1-x)Ni_xGa合金的费米面在自旋向下带隙的中间位置,因而可以判定Ti2Co0.5Ni0.5Ga将具有最佳的半金属稳定性.