First-order magnetostructural transition in Ni-Mn-Sn powder - A matter of particle and grain size

Abstract

In Ni-Mn-based Heusler alloys, the magnetocaloric effect originates from a first-order magnetostructural transition (FOMST). However, the influence of particle and grain size on thermal hysteresis, transition width, and nucleation and growth mechanism remains poorly understood, as mechanical grinding can introduce residual stresses and processing-induced defects. In this study, we examined the size dependence of the FOMST in gas-atomized Ni-Mn-Sn powder by combining microstructural characterization with temperature-dependent XRD, magnetometry, calorimetry and single-particle measurements, enabling decoupling of size dependence from processing-related influences. We observed a pronounced size dependence of the transition ranges (Ms-Mf) and (Af-As), which decrease with decreasing particle size and approach zero for single-crystalline particles below ~20-30 micrometer. The thermal hysteresis decreases with increasing particle size, reaching a lower limit of 17 K for large polycrystalline particles above 100 micrometer. EBSD reveals a nearly constant grain size, implying a transition from polycrystalline to single-crystalline particles as particle size decreases. In polycrystalline particles, multiple grains transform gradually over an extended temperature interval because grain boundaries act as barriers to the progression of transformation. In contrast, single-grain particles lack preferred nucleation sites, resulting in larger hysteresis, while the absence of barriers to transformation propagation causes the entire particle to transform in a narrow range. These results demonstrate that the size dependence of the FOMST is governed by the interplay between particle size, grain boundaries, and the free particle surface.