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Despite the low resistivity (~ 1 mohm cm), the metallic electrical transporthas not been commonly observed in the inverse spinel NiCo2O4, except in certainepitaxial thin films. Previous studies have stressed the effect of valencemixing and degree of spinel inversion on the electric conduction of NiCo2O4films. In this work, we have studied the effect of microstructure by comparingthe NiCo2O4 epitaxial films grown on MgAl2O4 (111) and on Al2O3 (0001)substrates. Although the optimal growth condition and the magnetic propertiesare similar for the NiCo2O4/MgAl2O4 and the NiCo2O4/Al2O3, they show metallicand semiconducting electrical transport respectively. Despite similartemperature and field dependence of magnetization, the NiCo2O4/Al2O3 show muchlarger magnetoresistance at low temperature. Post-growth annealing decreasesthe resistivity of NiCo2O4/Al2O3, but the annealed films are stillsemiconducting. The correlation between the structural correlation length andthe resistivity suggests that the microstructural disorder, generated by thedramatic mismatch between the NiCo2O4 and Al2O3 crystal structures, may be theorigin of the absence of the metallic electrical transport in NiCo2O4. Theseresults reveal microstructural disorder as another key factor in controllingthe electrical transport of NiCo2O4, with potentially large magnetoresistancefor spintronics application.

Absence of Metallic Behavior in Epitaxial NiCo2O4 Thin Films: Role of Microstructural Disorder