Magnetic and structural dynamics of first-order phase transition in giant magnetocaloric Fe₄₉Rh₅₁

Near-equiatomic FeRh alloys have been studied over the past years due to their remarkable magnetoelastic phase transition, making them promising candidates for technological applications. This is particularly relevant for magnetic refrigeration, one of the most viable solid-state cooling technologies, which requires high operating frequencies (1-10 Hz) to achieve improved device performance. With this in mind, assessing the kinetics of the phase transitions is a necessary step to foster further opportunities in material performance and device optimization. This study investigates the relaxation dynamics of FeRh's AFM to FM magnetoelastic phase transition, employing SQUID magnetometry and in-situ synchrotron X-ray diffraction to systematically characterize the behavior of its order parameters (magnetization and lattice parameter) under stepwise magnetic field variations. The distinct responses to field application and removal highlight the presence of asymmetric kinetic behavior, with higher relaxation effects observed during field application. We obtained relaxation times in the order of 50 s for the magnetization and 80 s for the lattice parameter. The relaxation time is observed to have a maximum near the critical field, being 1.5 T for the lattice parameter evolution and 1.1 T for the magnetization evolution. Lower sweep rates and temperatures closer to the transition temperature, T_t, tend to result in higher relaxation effects. This work, therefore, represents an important step not only towards optimizing magnetic refrigeration devices, but also in advancing the fundamental understanding of magnetoelastic phase transitions. Given its unique properties, FeRh serves as an excellent model system, and the insights gained from this study may be extended to other materials with similar characteristics.