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Abstract

Ni2MnGa exhibits ideal ferromagnetic shape memory properties, however, brittleness and a low-temperature martensite transition hinder its technological applications motivating the search for novel materials showing better mechanical properties as well as higher transition temperatures. In this work, the crystal structure, phase transitions, and the magnetic properties of quaternary Ni2-x Pt-x MnGa(0 <= x <= 1) shape memory alloys were studied experimentally by x-ray diffraction, magnetization measurements, and neutron diffraction and compared to ab initio calculations. Compositions within 0 <= x <= 0.25 exhibit the cubic austenite phase at room temperature. The x approximate to 0.3 composition exhibits a seven-layer modulated monoclinic martensite structure. Within 0.4 <= x <= 1, the system stabilizes in the nonmodulated tetragonal structure. The martensite transition has very narrow thermal hysteresis 0 <= x <= 0.3, which is a typical characteristic of a shape memory alloy. By increasing x, the temperature of the martensite transition increases, while that of the magnetic transition decreases. The x = 1 composition (NiPtMnGa) in the martensite phase undergoes a para-to-ferrimagnetic transition. The saturation magnetization exhibits a nontrivial behavior with increasing up to x approximate to 0.25, above which, it suddenly decreases. Powder neutron diffraction reveals the presence of antisite disorder, with about 17% of the original Ga sites being occupied by Mn. Computations suggest that the antisite disorder triggers an antiferromagnetic coupling between two Mn atoms in different crystallographic positions, resulting into a sudden drop of the saturation magnetization for higher x.