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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.
