Field-orientation-dependent magnetic phases in GdRu2Si2 B. M. Huddart, A. Hernández-Melián, G. D. A. Wood, D. A. Mayoh, M. Gomilšek, Z. Guguchia, C. Wang, T. J. Hicken, S. J. Blundell, G. Balakrishnan, T. Lancaster

Centrosymmetric GdRu2Si2 exhibits a variety of multi-Q magnetic states as a function of temperature and applied magnetic field, including a square skyrmion-lattice phase. The material's behavior is strongly dependent on the direction of the applied field, with different phase diagrams resulting for fields applied parallel or perpendicular to the crystallographic c axis. Here, we present the results of muon-spin relaxation (μ+SR) measurements on single crystals of GdRu2Si2. Our analysis is based on the computation of muon stopping sites and consideration of quantum zero-point motion effects of muons, allowing direct comparison with the underlying spin textures in the material. The muon site is confirmed experimentally, using angle-dependent measurements of the muon Knight shift. Using transverse-field μ+SR with fields applied along either the [001] or [100] crystallographic directions, we distinguish between the magnetic phases in this system via their distinct muon response, providing additional evidence for the skyrmion and meron-lattice phases, while also suggesting the existence of RKKY-driven muon hyperfine coupling. Zero-field μ+SR provides clear evidence for a transition between two distinct magnetically ordered phases at 39 K.