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 (InX and NiX ) 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 (InX+XNi) 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.
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