Direct Imaging of Composition-driven Magnetoelastic Phase Transformations in Bulk Fe–Rh

Fe–Rh alloys exhibit world-leading multicaloric responses associated with a magnetoelastic first-order magnetic phase transition between antiferromagnetic (AFM) and ferromagnetic (FM) states. Here, we demonstrate that polarized neutron imaging provides a powerful technique to deciphering the fundamental mechanisms governing the phase transformation in bulk Fe _100–x Rh _x alloys (x = 50, 51, and 52 at.%) by resolving the three-dimensional spatial pathways, and magnetic domain coexistance. Despite identical synthesis route, near-equiatomic Fe _100–x Rh _x alloys exhibit markedly different transition temperatures and transition temperature widths upon small variations in Rh content. Polarized neutron-contrast radiography reveal that the equiatomic alloy undergoes a strongly heterogeneous transformation, characterized by the extended coexistence of AFM and FM domains over broad temperature ranges and spatially distributed local transition temperatures. This behaviour is accompanied by multiple, spatially separated nucleation-and-growth regimes, consistent with a defect-mediated transformation mechanism. In contrast, Rh-rich compositions exhibit a more uniform and abrupt magnetic transition, with reduced AFM–FM coexistence, indicating that excess Rh acts as an effective nucleation catalyst that homogenizes the phase transformation across the entire alloy volume. These results expose the hidden composition-driven control of AFM–FM phase coexistence and transformation pathways in Fe–Rh, and establish polarized neutron imaging as a unique mesoscale probe of magnetoelastic phase transformations in bulk functional magnetic materials.