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

TL;DR

This paper introduces ultrafast X-ray sonography, a novel non-invasive technique combining ultrafast X-ray diffraction and strain pulses, to visualize the detailed spatio-temporal heterogeneity of phase transitions in FeRh.

Contribution

The study demonstrates the application of ultrafast X-ray sonography to reveal the spatial heterogeneity of the magneto-structural phase transition in FeRh, providing new insights into nucleation and growth processes.

Findings

Ferromagnetic phase nucleates at the surface as 30 nm domains.

Reconciles conflicting experimental results on FeRh.

Versatile platform for studying structurally changing phase transitions.

Abstract

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 well-established ultrafast hard-X-ray diffraction with a propagating strain pulse as a universal and non-invasive probe. This ultrafast X-ray sonography can capture the spatio-temporal phase heterogeneity in great detail by resolving the phase-specific strain response. We apply this approach to the antiferromagnetic-to-ferromagnetic magneto-structural phase transition in FeRh and identify the ferromagnetic phase to nucleate at the surface as narrow columnar domains of approximately $30\,\text{nm}$ diameter. Besides reconciling the diverse experimental results in the literature on FeRh, X-ray sonography offers a versatile platform for investigating a wide range of phase transitions accompanied by structural changes.