Valence reconstruction driven metal-insulator transition and enhanced anomalous hall effect in NiCo2O4 epitaxial films induced by CaH2 reduction

NiCo2O4 is a promising magnetic oxide for spintronic applications due to its high Curie temperature, large spin polarization, and robust perpendicular magnetic anisotropy. However, it remains an outstanding challenge to disentangle the intrinsic roles of cation valence and oxygen stoichiometry from extrinsic factors. In this work, CaH2-assisted reduction is employed to precisely modulate the oxygen stoichiometry in epitaxial NiCo2O4 thin films, as well as the Ni2+/Ni3+ and Co2+/Co3+ ratios. This post-growth electron-doping approach achieves precise chemical tuning while preserving high crystallinity, atomic surface smoothness, and uniform film thickness, thereby decoupling the intrinsic valence effects from extrinsic structural perturbations such as epitaxial strain. Intensified reduction is found to progressively suppress Ni3+ at octahedral sites, enhancing carrier localization and triggering a transition from metallic to insulating behavior. Concurrently, within a certain range of moderate reduction, the process reinforces localized magnetic moments and defect-induced domain-wall pinning, resulting in a pronounced enhancement in both anomalous Hall resistance and coercive fields. These findings establish CaH2 reduction as a powerful tool for disentangling the intrinsic roles of valence states and oxygen vacancies, providing a versatile strategy to engineer the physical properties of oxide-based spintronic materials.