X. F. Miao

Thermal-history dependent magnetoelastic transition in (Mn,Fe)2(P,Si)

The thermal-history dependence of the magnetoelastic transition in (Mn,Fe)2(P,Si) compounds has been investigated using high-resolution neutron diffraction. As-prepared samples display a large difference in paramagnetic-ferromagnetic (PM-FM) transition temperature compared to cycled samples. The initial metastable state transforms into a lower-energy stable state when the as-prepared sample crosses the PM-FM transition for the first time. This additional transformation is irreversible around the transition temperature and increases the energy barrier which needs to be overcome through the PM-FM transition. Consequently, the transition temperature on first cooling is found to be lower than on subsequent cycles characterizing the so-called “virgin effect.” High-temperature annealing can restore the cycled sample to the high-temperature metastable state, which leads to the recovery of the virgin effect. A model is proposed to interpret the formation and recovery of the virgin effect.

Thermal-history dependent magnetoelastic transition in (MN, FE) 2 (P, SI)

(Mn, Fe)2(P, Si)-type compounds are, to date, the most promising materials for refrigeration and energy conversion applications due to the combination of highly tunable giant magnetocaloric effect (GMCE) and low material cost.[1, 2] The GMCE of these compounds originates from the first-order magnetoelastic transition around the magnetic phase-transition temperature TC. However, the phase-transition temperature shows a peculiar thermal-history dependence in these compounds. As-prepared (Mn, Fe)2(P, Si) displays a significantly lower TC upon first cooling than on second and subsequent cooling processes. Since this behavior is only observed in as-prepared samples it is called the “virgin effect”. The difference in TC between the first and second cooling processes of the as-prepared sample, hereafter referred to as ΔTC0, is taken as a measure of how strong the virgin effect is. The virgin effect is not exclusive to (Mn, Fe)2(P, Si) compounds being observed in other GMCE materials[3, 4], however its origin was for a long time unknown. In this study, we report our high-resolution neutron diffraction experiments that finally shed light on the origin of the virgin effect. Additionally, recovery of the virgin effect induced by thermal activation was observed experimentally.