Ultrafast X-ray sonography reveals the spatial heterogeneity of the laser-induced magneto-structural phase transition in FeRh

Even on the nanoscale, phase transitions are governed by both intrinsic and extrinsic heterogeneities, yet capturing their spatio-temporal dynamics remains a challenge. While ultrafast techniques track phase changes on femtosecond timescales, the spatial complexity and stochastic nature of the processes often remain hidden. Here, we present an experimental approach that combines established ultrafast hard-X-ray diffraction with a propagating strain pulse as a universal and noninvasive structural marker. This ultrafast X-ray sonography identifies the spatio-temporal structural phase heterogeneity on nanometer length- and picosecond time scales by tracking the phasespecific strain response. It offers a versatile platformfor investigating a wide range of phase transitions accompanied by structural changes in typical nanometer thin films. We apply this approach to the antiferromagnetic-to-ferromagnetic magneto-structural phase transition in FeRh. Modeling the phase-specific strain response identifies the nucleation of domains of approximately 20 nm height exhibiting the structural signatures of the ferromagnetic phase.