Ni-X-In(X=Mn,Fe和Co)合金的缺陷稳定性和磁性能的第一性原理研究

Ni-Mn-In是一种新型的磁控形状记忆合金, 它通过磁场诱导逆马氏体相变实现形状记忆效应. 实验中常围绕化学计量比Ni₂MnIn合金进行成分调整, 以获得适宜的马氏体相变温度与居里温度, 在这个过程中必然会产生多种点缺陷. 本文使用量子力学计算软件包VASP, 在密度泛函理论的框架下通过第一原理计算, 系统地研究了非化学计量比Ni-X-In(X=Mn, Fe 和Co)合金的缺陷形成能和磁性能. 反位缺陷中, In和Ni在X亚晶格的反位缺陷(In_X和Ni_X)的形成能最低, Ni和X反位于Y的亚晶格(Ni_Y和X_Y)得到较高的形成能. 因此, In原子可以稳定立方母相的结构, 而X原子对母相结构稳定性的影响则相反; 空位缺陷中最高的形成能出现在In空位缺陷, 再次肯定了In原子对稳定母相结构的作用. 此外, 详细研究了点缺陷周围原子的磁性能以及电荷分布. 本文的计算结果在指导实验中的成分设计和开发新型磁控形状记忆合金方面具有重要意义.

Defect stabilities and magnetic properties of Ni-X-In (X= Mn,Fe and Co) alloys: a first-principle study

Ferromagnetic shape memory alloys (FSMAs) have received much attention as high performance sensor and actuator materials, since a large magnetic-field-induced strain by the rearrangement of twin variants in the martensitic phase was reported. Up to now, several FSMAs including Ni-Mn-Ga, Ni-Fe-Ga, Co-Ni-Ga, Ni-Mn-Al systems have been studied. Vast amount of knowledge accumulated at the properties of Ni-Mn-Ga Heusler alloys in the past decade can foresee the possibility of employing these alloys in device applications. However, the actuation output stress level of the Ni-Mn-Ga alloy is only less than 5 MPa, which represents a shortcoming of this alloy system. Recently, an unusual type of FSMAs Ni-Co-Mn-In Heusler alloy has been experimentally investigated. It shows magnetic-field-induced reverse martensitic transition (MFIRT), making it more attractive for practical application as magnetically driven actuator because it possesses a magnetostress level on the order of tens of MPa. An almost perfect shape memory effect associated with this phase transition is induced by a magnetic field and is called the metamagnetic shape memory effect. NiMnIn is the basic ternary alloy system of the NiMnInCo alloy, and possesses the same metamagnetic shape memory effect. Moreover, large magnetoresistance, large entropy change that generates giant reverse magnetocaloric effects (MCEs), giant Hall effect have been discovered in Ni-Mn-In alloys. Composition adjustment must be carried out around stoichiometric Ni₂MnIn in order to obtain the appropriate martensitic transformation temperature and Curie temperature. Therefore, a variety of point defects would be generated in this process. In this paper, the defect formation energy and magnetic properties of the off-stoichiometric Ni-X-In (X= Mn, Fe and Co) alloys are systematically investigated by the first-principle calculations within the framework of the density functional theory through using the Vienna ab initio software package. The In and Ni antisites at the site of the X sublattice (In_X and Ni_X) have the relatively low formation energies. For most cases of the site occupation, the excess atoms of the rich component directly occupy the site (s) of the deficient one (s), except for In-rich Ni-deficient composition. In the latter case, the defect pair (In_X+X_Ni) is energetically more favorable. The formation energy of Ni vacancy is the lowest and that of In vacancy is the highest in the vacancy-type defects. It is confirmed that the In constituent is dominant for the stability of the parent phase. The value of the Ni magnetic moment sensitively depends on the distance between Ni and X atoms. The smaller the distance, the larger the Ni magnetic moment will be. For the anti-site type point defect, when the extra X atom occupies a Ni site, most of the free electrons gather around the extra X atom; while the extra X occupies an In position, the charges are regularly distributed between Ni and extra-X atoms. Moreover, with the increase of the X atomic number, the number of the valence electrons increases, and the bonding strength between the extra X and its neighboring Ni is also enhanced. The results are particularly useful in guiding composition design and developing new type of magnetic shape memory alloy.