FeRh is an archetypal system for the investigation of ultrafast behaviour in coupled transitions due to its meta-magnetic phase transition occurring around 380 K 1. In this coupled phase transition, the electronic structure transforms lowering the resistivity by ≈ 33%, the lattice expands isotropically with a volumetric expansion of ≈ 1%, and the magnetic order changes from a G-type antiferromagnet (AF) to a ferromagnet (FM) 1, 2. Previous x-ray diffraction (XRD) studies have indicated that the lattice expands with first-order dynamics within 10-30 ps 3, with long-range AF order throughout the transition 4. The sub-ps capabilities of the SACLA free-electron laser allowed for investigation of the ultrafast behaviour of the FeRh lattice upon laser excitation. This shows new dynamics at high fluences which were compared to the quasi-static behaviour of the Bragg peaks as measured using heated XRD. We describe the lattice temperature (see Fig. 1a) and expansion as a function of pump-probe delay. We have observed a perturbation to the expected dynamics above fluences of 5 mJ cm-2 where the lattice initially contracts before finally expanding as predicted. We demonstrate that a model (see Fig. 1b) using a transient lattice state 5 can explain the observed behaviour. Our model suggests that the transient state is paramagnetic, reached by a subset of the phonon bands which are preferentially coupled to the electronic system 6. A complete description of the FeRh structural dynamics requires consideration of coupling strength variation across the phonon frequencies.

References
1 J. S. Kouvel and C. C. Hartelius, J. Appl. Phys., 33, 1343, (1962). 2 J. S. Kouvel, J. Appl. Phys., 37, 1257–1258, (1966). 3 S. O. Mariager, F. Pressacco, G. Ingold, et al., Phys. Rev. Lett., 108, 087201, (2012). 4 M. Grimes, N. Gurung, H. Ueda, et al., AIP Adv., 12, 035048, (2022). 5 B. Mansart, M. J. G. Cottet, G. F. Mancini, et al., Phys. Rev. B, 88, 054507, (2013). 6 M. Grimes, H. Ueda, D. Ozerov, et al., Sci. Rep., 12, 8584 (2022)