Study of the phase transition of a FeRh film at different temperature regimes

The global energy transition is confronted with the need to rationalize energy consumption and to valorize lost resources. Today, cold production is energy-hungry, representing approximately 10 % of global electricity production and contributing up to 7,8 % to greenhouse gas emissions. At the same time, the potential of lost energy, notably in the form of waste heat (estimated at 100 TWh by ADEME), remains under-exploited. To face these crucial challenges, the development of high-efficiency refrigeration solutions and energy recovery systems is essential. A particularly promising path lies in the exploitation of magnetocaloric materials.These materials are characterized by the magnetocaloric effect (EMC), a reversible change of temperature induced by the variation of an external magnetic field. They offer a clean and efficient alternative to conventional vapor-compression refrigeration systems, and can also serve as the basis for thermo-magnetic conversion devices allowing the valorization of part of the available waste heat deposit. First-order phase transition materials appear to be the most promising because of their large magnetocaloric effects. However, these transitions inevitably come with dynamic effects and hysteresis which manifest as a delay between the thermal effect and the applied field, thus limiting the efficiency and power density of magnetocaloric systems under high-frequency operation.To lift this technological lock, this work focuses on the in-depth study of the dynamics of phase transitions. The intermetallic compound FeRh was chosen as a model material because of its complex and intriguing first-order phase transition, involving a strong coupling between electronic, structural and magnetic degrees of freedom. This phenomenological richness is the source of a significant EMC, but also of a complex transition dynamics and hysteresis phenomena that it is imperative to decipher. The objective is to characterize the fundamental physics that governs the kinetics of these transitions.To this end, we adapted a device originally dedicated to modulated thermoreflectance measurements. This tool allows the study of the static hysteresis properties of a thin film (195 nm) FeRh at an extremely local spatial scale, close to the correlation length of the transition. Furthermore, the large good passband of the device used allows us to also study the dynamics of the phase transition at this scale thanks to the good dynamics of the optical systems.This device will allow us to study thermal hysteresis and its properties at a very local scale. By studying the temperature by inducing rapid temperature variations on the order of 10^7K/s, we highlight the stochastic aspect of the FeRh phase transition to the statistics and its characteristics. Finally, we will show the relevance of using this phase transition dynamics and its properties in neuromorphic computing systems proposing a new promising path for magnetocaloric materials.