Maintaining high conductivity in NiCo2O4 epitaxial thin films to 625 °C via an overlayer stabilization approach

Spinel materials offer excellent physical, optical, and biomedical properties, particularly in epitaxial form. However, spinel electrode materials for these epitaxial films are limited. While NiCo2O4 (NCO) is a high-performing conductive spinel, it loses conductivity above 400 °C, making it unsuitable as an electrode for overgrown functional spinel layers, which typically require growth above 600 °C. Here, we demonstrate the stabilization of conducting NCO through the overgrowth of a functional spinel CoFe2O4 (CFO) layer. Notably, when the NCO is capped with 625 °C-grown CFO, it retains most of the conductivity of 350 °C-grown films. The overgrowth approach of this work shows a path for NCO to be used as a conducting electrode in epitaxial spinel-based epitaxial devices.