Thin Films at Ultralow Temperatures*

The inverse spinel NiCo₂O₄ (NCO) is a room temperature ferrimagnetic material that is promising for developing nanoscale energy and spintronic applications. In this work, we report the unusual thickness-dependent magnetotransport properties of high quality epitaxial NCO thin films. We have characterized the longitudinal magnetoresistance (MR) and Hall effect in NCO films with thickness varying from 5 unit cell (~4.1 nm) to 30 uc (24.6 nm) at ultralow temperatures down to 100 mK. The 10 and 30 uc films exhibit an intrinsic linear negative magnetoresistance, which persists up to 17 T without showing any saturation or upturn, while the 5 uc sample exhibits a positive MR that is possibly due to 2D weak antilocalization (WAL). We also observe robust anomalous Hall effect in the 10 and 30 uc samples, while the switching hystereses are in opposite directions. We discuss the effects of the Berry curvature, disorder scattering, and dimensionality crossover on the observed unusual thickness effect.