B2-ordered FeRh undergoes a thermally activated first-order phase transition between antiferromagnetic (AF) and ferromagnetic (FM) order upon heating, with a transition temperature between 350 - 380 K 1, making it an ideal candidate for use in a wide range of magnetic storage architectures 2. The evolution of magnetic domains through the transition has been well-characterized 1-5. However, behaviours such as the domain nucleation mechanism and subsequent evolution of the magnetic domains along the sample thickness through the phase transition remain unclear 1,3-5. Here, we present three-dimensional (3D) reconstructions of the magnetic structure through the sample volume at various temperatures through the phase transition, achieved using magnetic laminography 6-7. Imaging cross-sections through the sample thickness reveal that domains for both types of magnetic order nucleate at the surface and travel into the bulk of the sample with changing temperature. Creating a complex 3D magnetic structure, in which the evolution of the magnetic state along the sample thickness with temperature is non-uniform. We also observe quasi-uniform FM domains at 310 K when heating, in contrast to the expected flux-closed state 1, as well as AF regions where FM domain walls are expected. Both observations can be attributed to the exchange coupling between adjacent AF and FM regions, which plays a key role determining the FM configuration in this material.

References:

1 T. P. Almeida et al., Phys. Rev. Materials 4, 034410 (2020).
2 R. C. Temple et al., Phys. Rev. Materials 2, 104406 (2018).
3 C. Gatel et al., Nat. Comms. 8, 15703 (2017).
4 V. Uhlir et al., Nat. Comms. 7, 13113 (2016).
5 M. A. de Vries et al., Appl. Phys. Lett. 104, 232407 (2014).
6 C. Donnelly et al., Nature Nanotech. 15, 356 (2020)
7 K. Witte et al., Nano Lett. 20, 1305 (2020).