Sensitive metallic behavior in epitaxial NiCo2O4 films regulated by the film thickness

The transition metal oxide NiCo2O4 has aroused extensive research interest due to its metallic conductivity, but the precise nature of the conductivity remains elusive. In this study, the dependence of the metallic behavior in NiCo2O4 epitaxial films on temperature and film thickness are investigated for film thicknesses in the range of 7–155 nm. Our results show that a subtle lattice distortion caused by the substrate is necessary for metallic behavior in NiCo2O4 films. The widest temperature window for the metallic behavior is obtained for a 31-nm thick NiCo2O4 film. The NiCo2O4 films thinner than 31 nm are quasi-two-dimensional, and are affected by substrate clamping. In these films, conduction is constrained by electron–electron and electron–magnon scattering. However, NiCo2O4 films thicker than 31 nm are three-dimensional, exhibiting long-range structural order, with electrical transport constrained mainly by electron–phonon scattering, as well as electron–electron and electron–magnon scattering. Our study demonstrates that the metallic behavior in NiCo2O4 films can be tuned using the film thickness, which is of great significance for application of NiCo2O4 in development of alternative ferroelectric metal electrodes in magnetic tunnel junctions.