Spin-Canting at the Frontiers of NiCo₂O₄ Thin Films

NiCo₂O₄(001) thin films have been shown to possess perpendicular magnetic anisotropy (PMA) [1], using bulk-sensitive measurements. The spin anisotropy at the surface and interface could differ from the anisotropy in the bulk of the thin film. From angle-resolved x-ray magnetic circular dichroism (XMCD), performed focusing on surface region, the hysteresis loops at the Ni and Co absorption edges show that saturation magnetization (M_S) decreases for NiCo₂O₄(001) thin film but increases for NiCo₂O₄(111) thin film, as the angle of incident polarized x-ray increases with respect to the surface normal of the respective thin films. The maximum XMCD M_S is observed at the angle of 60⁰ for NiCo₂O₄(111) indicating spin-canting by 60⁰ with respect to [111] normal. For NiCo₂O₄(001), the maximum M_S is observed at 10⁰, and angular dependency of M_S is observed to have some deviation from what is expected for ideal PMA. The spins at NiCo₂O₄(001) surface are thus slightly canted by 10⁰ with respect to [001] normal, although the bulk NiCo₂O₄(001) is reported to have PMA. Such small spin-canting is further supported by in-plane spin-polarized unoccupied states of NiCo₂O₄(001), using spin-polarized inverse photoemission spectroscopy (SPIPES). When a thin Pt layer is deposited on NiCo₂O₄(001) thin film, the interface of Pt/NiCo₂O₄(001) is found to have large spin-canting by 40⁰ with respect to [001] normal. Comparative analysis of the angle-resolved XMCD hysteresis loops among NiCo₂O₄(111), NiCo₂O₄(001) and Pt/NiCo₂O₄(001) suggested that the interface of Pt/NiCo₂O₄(001) could host skyrmions. The possiblility of hosting skyrmions is also corroborated by observation of the topological Hall effect in the Pt/NiCo₂O₄(001) [2]. Furthermore, magnetization reversal in NiCo₂O₄(001) is qualitatively governed by Kondorsky model, as suggested by the angular dependence of XMCD coercivity. Our results provide better fundamental insights into the properties of NiCo₂O₄ thin films and make these systems more attractive for both fundamental science and device applications.

[1] C. Mellinger et al., Phys. Rev. B 101, 014413 (2020).

[2] B. Giri et al., Appl. Phys. Lett. 127, 052403 (2025).