Enhanced non-collinear magnetic phase in mixed anisotropy antiferromagnet*

Antiferromagnetic (AFM) materials and non-collinear spin textures have recently become popular for their possible technological advantages, such as THz dynamics and topological protection. We have grown AFM epitaxial thin films of (110) Fe_xNi_1-x F₂ on MgF₂ substrates using molecular beam epitaxy (MBE). FeF₂ has a strong uniaxial anisotropy along its tetragonal c-axis, whereas NiF₂ has an anisotropy in the a-b plane. Orthogonal anisotropies such as these can be described by the Dzyaloshinskii-Moriya interaction (DMI) and are responsible for interesting spin textures, such as helices and skyrmions. DMI is enhanced via coupling to a high spin-orbit material. We observe an enhancement of a secondary phase close to the AFM Néel ordering temperature in our Pt capped films. We also measured temperature-dependent X-ray resonant magnetic scattering that scales well with the magnetisation. We argue that an AFM skyrmionic-like phase is stabilised in this mixed anisotropy system which is further enhanced upon proximity to a high spin orbit heavy metal.