We have performed a quantum-mechanical study of two ferrimagnetic Ni2MnSn structures, a stoichiometric one with Mn-Ni swaps and an off-stoichiometric (Ni7Mn8Sn1) alloy. We used 16-atom supercells with the atomic positions related to those in the full Heusler structure, i.e. austenite phase (unless distorted by internal distribution of point defects as in the case of Ni2MnSn with swaps). We have determined thermodynamic, magnetic and structural properties of both pressure-free states as well as those corresponding to hydrostatic pressures of a few GPa. The atomic and magnetic configurations of the studied states are found to exhibit anomalous pressure-induced changes in the total magnetic moment. In particular, the total magnetic moments per 16-atom supercells increase with increasing hydrostatic pressure. Despite this peculiar trend in the total magnetic moment, the magnitudes of local magnetic moments of Mn atoms, that are decisive for the value of the total magnetic moment, decrease with increasing hydrostatic pressure (as is common in majority of magnetic systems). The identified phenomena may be related to an interplay of a few contributing mechanisms. First, the magnetic moments of Mn atoms, that are either parallel or antiparallel to the orientation of the total magnetic moment of the supercells, nearly compensate each other due to the ferrimagnetic nature of the studied magnetic states. Second, the swapped and off-stoichiometric atoms lead to different local atomic environments of Mn atoms and, consequently, to different local magnetic moments of these atoms as well as their different response to hydrostatic pressures. Importantly, the local magnetic moments of Mn atoms, that are antiparallel to the orientation of the total magnetic moment, are more sensitive to the applied pressures. Regarding thermodynamics, the studied states are excited ones.Keywords: Ni-Mn-Sn, magnetism, pressure, ferrimagnetic, austenite, quantum-mechanical, defects
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