Orientation-dependent magnetic properties and anomalous Hall effect in half-metallic NiCo2O4 thin films

The proliferation of spintronic devices necessitates the advancement of novel material platforms, particularly those featuring ferrimagnetic and antiferromagnetic properties. Among them, NiCo2O4 (NCO), a ferrimagnetic material, stands out for its half-metallic band structure and remarkable attributes like robust perpendicular magnetic anisotropy, a high Curie temperature, and tunable film characteristics through conductivity modulation. These unique features render NCO a prime candidate for spintronics applications. Here, we focus on the orientation-dependent magnetic properties and transport behavior of epitaxial NCO films grown on (001)-, (110)-, and (111)-oriented MgAl2O4 substrates. Notably, NCO films with different epitaxial orientations exhibit significant differences in magnetic anisotropy and transport behavior. Specifically, the (110)-oriented NCO samples present a field-driven spin reorientation transition in the out-of-plane direction, resulting in a split hysteresis loop and corresponding longitudinal magnetoresistance and anomalous Hall curves. Importantly, the anomalous Hall effect (AHE) of (100) and (111)-oriented NCO films displays an opposite sign, while the AHE of (110)-oriented NCO films exhibits an unusual sign reversal at approximately 150 K, distinguishing them from other magnetic oxides. With these findings, NCO films offer a novel platform for fundamental AHE research, featuring tunable AHE signs and intriguing physical mechanisms. Additionally, by introducing an antiferromagnetic NiO layer, we achieve a significant exchange-bias effect and enhanced coercive field, holding immense potential for future spintronics applications, such as ferrimagnetic high-density memories.