Direct observation of the phase transition in FeRh alloy under temperature cycling

•Magnetic domain evolution of FeRh alloy in AFM-FM phase transition process is directly observed.•Magnetic domain evolution differences under temperature cycles for FeRh alloy are revealed.•The remnant magnetic domain at 300 K after temperature cycles are quite different.•The effect of M and A on magnetic domain evolution under temperature cycles are studied. The magnetocaloric effect of CsCl-type ordered FeRh alloy (space group Pm-3m, No. 221, cP2) almost cut in half with temperature cycling, limiting their application in magnetic refrigeration. This behavior is related to its phase transition dynamics that remain unveil. In this work, the phase transition dynamic process of FeRh alloy with entropy change of 10 J/kgK (0–3 T) were studied by directly observing the temperature-dependent magnetic domain evolution using in-situ LTEM. Significant differences of magnetic domain evolution behavior under temperature cycles occur in FeRh alloys. Specially, a virgin effect appears for the remnant magnetic domain at 300 K, where the number of magnetic vortexes reach 5 and remain stable at 300 K after the first temperature cycle instead of the original large irregular domains. Combined with micromagnetic simulation, we demonstrated that the domain wall motion induced change of magnetization M and exchange stiffness A should be responsible for the reduction of magnetic refrigeration performance under temperature cycles. These results help to understand the physical origin of the cyclability performance of magnetocaloric effect and antiferromagnetic to ferromagnetic phase transition in FeRh alloy.