Magnetotransport Anomaly in NiCo₂O₄ /SrTiO₃ thin films

We report the studies of anisotropic magnetoresistance (AMR), planar Hall effect (PHE), and topological Hall effect (THE) in 30 nm ferrimagnetic NCO thin films deposited on (001) SrTiO₃ substrates via off-axis RF magnetron sputtering. The samples show a Curie temperature T_C above 340 K. Below T_C, we observe anomalous Hall effect with slanted hysteresis, pointing to perpendicular magnetic anisotropy with spin canting. The samples exhibit insulating temperature dependence. The in-plane AMR at 5 T shows the cos2θ dependence above 200 K with θ the angle between current and magnetic field directions, consistent with the coherent rotation model. An additional cos4θ component emerges below 200 K, which can be attributed to strain-induced tetragonal magnetocrystalline anisotropy. The PHE at 5T shows the sin2θ dependence above 20 K, with higher harmonics emerges at low temperatures. We also observe THE upon magnetic field cooling below 140 K. The THE signal changes sign when the cooling magnetic field is reversed. The magnetotransport anormaly in NCO thin films points to the intricate roles of strain, disorder, and magnetic multipoles, and can promote NCO-based spintronic devices.