Revealing the reversal of the anomalous hall effect and the exchange bias-like effect in single-phase perpendicularly magnetized NiCo2O4 epitaxial films

Transition metal oxides (TMOs) with perpendicular magnetic anisotropy (PMA) and metallic behavior have promising potential for application in the development of new generation spintronic devices with high density, low power consumption, and nonvolatility. Although much progress has been made, the simultaneous coexistence of robust PMA and excellent metallicity at room temperature or higher temperatures in TMOs remains a huge challenge, limiting their practical application. Herein, high-quality NiCo2O4 (NCO) epitaxial films are reported, which have low resistivity, strong room temperature PMA with highly tunable coercive field, a sign-reversible anomalous Hall effect (AHE), as well as an exchange bias (EB)-like effect. The AHE sign reversal in higher substrate temperature (Tsub.) samples is attributed to the competition between the intrinsic mechanism contributed by the Berry curvature and the extrinsic mechanism dominated by impurity scattering. The EB-like effect in higher Tsub. samples is caused by the presence of non-stoichiometric NCO in films, which in turn leads to local antiferromagnetic coupling. Finally, a perpendicular magnetic tunnel junction based on the NCO homojunction is designed. This work reveals the relevance of cations in NCO to the crystal structure, magnetism and transport properties, which opens up new opportunities for the use of NCO films in the design of novel materials and devices.