American Institute of Physics, AIP Advances, 9(3), p. 092116
DOI: 10.1063/1.4821548
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We have measured the terahertz-frequency optical conductivity of the epitaxial inverse spinel NiCo2O4 films grown at different temperatures. The low-temperature-grown film exhibits a metallic behavior with ferrimagnetic ordering, while the high-temperature-grown film shows greatly suppressed magnetization and insulating behavior. Both films exhibit band-like coherent conduction at intermediate temperatures, albeit with very different carrier densities consistent with the proposed models of cation valencies in this mixed-valence material. Both films also display a crossover to incoherent transport at low temperatures, indicating a disorder-induced tendency toward localization.