Effects of heat treatments on magneto-structural phase transitions in MnNiSi-FeCoGe alloys

A first-order magneto-structural transition from a ferromagnetic orthorhombic TiNiSi-type martensite phase to a paramagnetic hexagonal Ni2In-type austenite phase was observed in (MnNiSi)0.62(FeCoGe)0.38. In this work, we demonstrate that the first-order magneto-structural transition temperature for a given composition is tunable over a wide temperature range through heat treatment and hydrostatic pressure application. The first-order transition temperature decreased by over 150 K as the annealing/quenching temperature went from 700 to 1050 °C. The largest maximum magnetic-field-induced isothermal entropy change with μ0ΔH=7 T reached −36.2 J/kg-K for the sample quenched at 700 °C, and the largest effective refrigeration capacity reached 344.5 J/kg for the sample quenched at 800 °C. Similar to the influence of annealing temperatures, the first-order martensitic transition temperatures decreased as the applied hydrostatic pressure increased until they were completely converted to second order. Our results suggest that the class of MnNiSi-based alloys is a promising platform for tailoring working temperature ranges and associated magnetocaloric effects through heat treatment or application of hydrostatic pressure.