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VIDEO DOI: https://doi.org/10.48448/fkkw-d617

technical paper

MMM 2022

November 07, 2022

Minneapolis, United States

Determination of sub ps lattice dynamics in FeRh thin films.

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.

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)


Transcript English (automatic)

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