Anisotropic magnetoresistance and planar Hall effect in NiCo₂O₄ thin films and membranes

Due to the high Curie temperature and spin polarization, epitaxial NiCo₂O₄ (NCO) films are promising candidates for spintronic applications. We report the studies of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) in 30 nm ferrimagnetic NCO thin films and membranes. Epitaxial NCO thin films are deposited on (001) LaAlO₃ (LAO) and Sr₃Al₂O­₆ (SAO) buffered LAO substrates via off-axis RF magnetron sputtering. Free-standing NCO membranes are achieved by water etching of the SAO buffer layer. All samples exhibit T_C above 300 K. Above 200 K, the AMR of NCO films on LAO at 5 T exhibit cos2θ dependence on the angle θ between current and in-plane magnetic field direction, consistent with the coherent rotation model. Below 200 K, an additional cos4θ component emerges and increases with decreasing temperature. Similar transistions have been observed in NCO films on SAO buffered LAO and suspended NCO membranes, while the cos4θ component emerges at 50 K. At 10 K, both the AMR and PHE show hysteresis behaviors in the angular dependence at magnetic fields below 2 T, suggesting that the in-plane magnetization is lagging behind the magnetic field due to magnetic anisotropy. Our results suggest that the suspended NCO membranes can retain the strain state and thus the magnetic anisotropy of the epitaxial NCO films, which make them promising for designing fexible spintronic devices.