Tuning martensitic transitions in (MnNiSi)0.65(Fe2Ge)0.35 through heat treatment and hydrostatic pressure

Abstract

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)0.65(Fe2Ge)0.35. 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. The first-order transition temperature was reduced by over 100 K as the annealing temperature went from 600 to 900 °C, and this first-order transition was converted to second order when the sample was annealed at 1000 °C. The maximum magnetic-induced isothermal entropy change with μ0ΔH=7 T reaches −58 J/kg K for the sample annealed at 600 °C, and the relative cooling power reaches 558 J/kg for the sample annealed at 700 °C. Similar to the influence of annealing temperatures, the first-order martensitic transition temperatures were reduced as the application of hydrostatic pressure increased until they were converted to second order. Our results suggest that the (MnNiSi)0.65(Fe2Ge)0.35 system is a promising platform for tuning magneto-structural transitions and the associated magnetocaloric effects. Furthermore, a similar heat treatment methodology or application of hydrostatic pressure can be applied to MnNiSi-based shape memory alloys to tailor their working transition temperatures.