Anomalous Hall Effect and Topological Hall Effect in NiCo₂O₄ Thin Films and Free-Standing Membranes

The inverse spinel ferrimagnetic NiCo₂O₄ possesses high Curie temperature and high spin polarization, making it a promising material candidate for spintronic applications. The magnetic state of NiCo₂O₄ films depends sensitively on the strain and disorder effects, which can lead to unconventional spin transport. In this talk, I will discuss our recent studies of the magnetotransport anomalies in epitaxial NiCo₂O₄ thin films and free-standing NiCo₂O₄ membranes, including linear magnetoresistance, anomalous Hall effect (AHE), and topological Hall effect (THE). NiCo₂O₄ films deposited on (001) MgAl₂O₄ substrates exhibit strong perpendicular magnetic anisotropy down to 1.2 nm (1.5 unit cell) thickness, while the (110) films possess in-plane magnetic anisotropy. Films with high crystallinity show quasi-linear magnetoresistance in magnetic fields up to 17 T with weak temperature dependence. The AHE exhibits a nonmonotonic temperature dependence and sign reversal driven by both temperature and film thickness, revealing the intricate interplay between the impurity spin scattering, band intrinsic Berry curvature, and correlation effect. THE has been observed in (110) NiCo₂O₄ films and (001) NiCo₂O₄ membranes at low temperatures. The former has been correlated with magnetic bubble domain formation, as revealed via magnetic force microscopy, suggesting the emergence of bimeron spin textures. The latter can be sensitively tuned by magnetic field cooling. Our study provides effective material strategies for designing spin transport in NiCo₂O₄ via strain and disorder, paving the path for its technological implementation.<br/>This work was primarily supported by NSF through Grant No. DMR-1710461 and EPSCoR EQUATE Award No. OIA-2044049.