Structural origin of hysteresis for hexagonal (Mn,Fe)2(P,Si) magneto-caloric compound

We performed in-situ transmission electron microscope observations on the unconventional ferromagnetic transition in hexagonal (Mn,Fe)(2)(P,Si) magneto-caloric compounds in order to understand the origin of the large thermal hysteresis that is detrimental to magnetic refrigeration. We find that the ferromagnetic transition is coupled to a martensitic-like transformation. Although crystal structure is the same for both paramagnetic (PM) and ferromagnetic (FM) phases, large lattice distortion occurs during the PM-FM transition, which induces energy barrier of about 13.6 kJ/mol. Supercooling or superheating is therefore needed to overcome the energy barrier, which causes the thermal hysteresis for the ferromagnetic transition. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.