Observation of Topological Hall Effect and Magnetic Textures in Ferrimagnetic (110) NiCo₂O₄ Films

We report the observation of topological Hall effect (THE) and magnetic textures in 16 nm (110) NiCo₂O₄ (NCO) films. Epitaxial NCO films are deposited on (110) MgAl₂O₄ (MAO) substrates via off-axis RF magnetron sputtering. The films are ferrimagnetic with Curie temperature T_C above 370 K and possess in-plane magnetic anisotropy. The saturation magnetization is about 2.2 μ_B/formula unit. We observe anomalous Hall effect (AHE) in out-of-plane magnetic field H, which exhibits negligible hysteresis and saturates at the high magnetic fields. Below 100 K, prominent THE signal occurs within a narrow magnetic field range close to where AHE saturates. At 4 K, magnetic force microscopy (MFM) imaging shows magnetic bubble domains with an average size of 150 nm emerge at H = 3.0-3.7 kOe, which correlates well with the magnetic field range for the THE. The gradient of the bubble domain suggests the existence of magnetic textures. We also compare the THE and MFM data with the magnetization hysteresis (M-H) curves. Our results suggest the emergence of spin bimerons/merons as the spins rotate from in-plane to out-of-plane orientations in (110) NCO film.