Anomalous Hall Effect and Perpendicular Magnetic Anisotropy in Ultrathin NiCo₂O₄ Films

The high Curie temperature (T_C) and high spin polarization of ferrimagnetic spinel NiCo₂O₄ (NCO) make it a promising material candidate for spintronic applications.¹ We report the magnetotransport study of 1.5 uc (unit cell) to 5 uc (1.2-4 nm) epitaxial NCO thin films deposited on (001) MgAl₂O₄ substrates. The 3 uc and thicker NCO films exhibit metallic conduction, while the 1.5 and 2 uc films become totally insulating. Samples of all thicknesses exhibit anomalous Hall effect (AHE) below T_C, with the AHE switching hysteresis revealing strong perpendicular magnetic anisotropy. For all films, the coercive field H_C increases exponentially with decreasing temperature, which can be attributed to thermally activated domain wall depinning. The AHE resistance shows a nonmonotonic temperature dependence and reverses sign at low temperature. The scaling between the AHE conductivity σ_xy and longitudinal conductivity σ_xx points to the dominating dirty metal behavior. We also discuss the topological Hall effect and magnetic force microscopy studies of the ultrathin NCO films.