Femtosecond x-ray nanodiffraction imaging of structural phase transition in FeRh

First-order phase transitions, characterized by discontinuities in the order parameter, proceed through nucleation and growth under equilibrium conditions. However, at ultrafast timescales, before thermal equilibrium is established, the underlying nonequilibrium and nanoscale structural dynamics remain poorly understood. In this work, we implemented a newly developed nano-focusing capability at a free-electron laser (Linac Coherent Light Source) to investigate the spatiotemporal dynamics of the laser-induced antiferromagnetic-to-ferromagnetic (AFM–FM) transition in a FeRh film grown on MgO, which accompanies an ultrafast lattice expansion. Spatial inhomogeneity of the ultrafast transition was examined using two complementary approaches: scanning X-ray nanodiffraction imaging to probe sub-micrometer-scale structural dynamics, and time-dependent speckles arising from coherent X-ray diffraction to assess nanometer-scale structural dynamics. The measurements revealed ultrafast evolution of in-plane structural heterogeneities within a few picoseconds and their interplay with out-of-plane strain wave propagation through the film. Local defective regions, such as those arising from non-uniform growth, exhibit distinct dynamics. The ultrafast nanoscopic insights revealed by femtosecond x-ray nanodiffraction are important for optimizing heat-assisted magnetic recording at the nanoscale.