Pressure induced structural transformation in Ni₂Mn_2-xIn_x shape memory alloys

The metamagnetic Heusler alloy Ni₂Mn_2-xIn_x (0.32<x<0.64) has garnered significant interest thanks to several technologically significant properties that result from a coupled magnetostructural phase transformation. These properties include the magnetocaloric and barrocaloric effects, shape memory effect, and large magnetoresistance. While the vast majority of studies have investigated the effects of alloy composition on the magnetostructural phase transformation in Ni₂Mn_2-xIn_x, analysis of the effects of pressure on the transformation are also relevant for practical utilization in real devices. We report the results of high energy synchrotron x-ray diffraction coupled with a diamond anvil cell to study the pressure induced magnetostructural transformation in Ni₂Mn_2-xIn_x. A composition of x=0.61 was chosen and both single crystal and powder samples were used. All measurements were carried out in collaboration with HPCAT at the Advanced Photon Source of Argonne National Laboratory. In the isothermal case, the sample pressure was increased incrementally from 2GPa up to 50GPa. A clear structural transformation from L21 austenite to a modulated phase was observed near 7GPa, with an additional transformation possible near 16GPa. We report on the structural phases that emerge under pressure, the transformation pathways, and the effects of hysteresis, and we present first-principles analysis of the role of magnetostructural coupling on the observed transformation behavior.