Large Barocaloric Effect with High Pressure-Driving Efficiency in a Hexagonal MnNi_0.77Fe_0.23Ge Alloy

The hydrostatic pressure is expected to be an effective knob to tune the magnetostructural phase transitions of hexagonal MM'X alloys (M and M' denote transition metals and X represents main group elements). We perform magnetization measurements under hydrostatic pressure on an MM'X martensitic MnNi_0.77Fe_0.23Ge alloy. The magnetostructural transition temperature can be efficiently tuned to lower temperatures by applying moderate pressures, with a giant shift rate of -151 K/GPa. A temperature span of 30 K is obtained under the pressure, within which a large magnetic entropy change of -23 J\cdotkg^-1K^-1 in a field change of 5 T is induced by the mechanical energy gain due to the large volume change. Meanwhile, a decoupling of structural and magnetic transitions is observed at low temperatures when the martensitic transition temperature is lower than the Curie temperature. These results show a multi-parameter tunable caloric effect that benefits the solid-state cooling.