Effect of heat treatment on coupled magnetostructural transition and magnetocaloric effect of MnNiGe system

Material with coupled magnetic and structural transition is of significant interest in the field of energy-efficient technology. In this work, a sharp first-order magneto-structural transition (MST) from a ferromagnetic orthorhombic TiNiSi-type martensite phase to a paramagnetic hexagonal Ni ₂In-type austenite phase is observed in hexagonal Mn _0.85Fe _0.15NiGe compound through the heat treatment technique. Through heat treatment, the first-order MST for Mn _0.85Fe _0.15NiGe sample is observed to decrease drastically from 279.1 K to 211.8 K for the elevated annealing temperatures from 873 K to 1173 K. Furthermore, total entropy change (ΔS) is synergistically associated with the magnetic and structural entropy change in this system. This synergic feature governs a large ΔS ∼ 62.7 Jkg ^-1K ⁻¹, while magnetic entropy change is about 31.3 Jkg ^-1K ⁻¹ (∼52.1%) and the lattice part contributes the remaining ∼48.7% for the sample annealed at 1073 K due to the field change of 5T. In addition, a large effective refrigerant capacity reaches 226.7 Jkg ⁻¹ at 226 K for the sample due to the field change of 5T. Henceforth, this work shows an effective and efficient scheme to control the MST temperature of hexagonal phase-transition families using heat treatment method without changing the chemical composition of the sample. Our results based on heat treatment methodology provide a feasible platform for tuning MST and the associated caloric properties.