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The electrical and magnetic structures of Nisubstituted MnCo 2 O 4 have been studied theoretically and experimentally. With the increase of Ni concentration x, the calculated results show that the bond lengths of the Mn−O and Ni−O bonds along three directions (a, b, and c axis) tend to be consistent, and the lattice symmetry of Ni x Mn 1−x Co 2 O 4 changes from tetragonal to cubic symmetry. The inverse spinel Ni x Mn 1−x Co 2 O 4 was found to have a ferrimagnetic half-metallic ground state. The Ni dopant results in the flow of holes from the e g,β 1 level of Ni 3+ at the octahedral site to the e g,β 2 level of Co 2+ at the tetrahedral site. Therefore, the calculated conductivity type of Ni x Mn 1−x Co 2 O 4 changes from n-to p-type as the Ni concentration increases. Furthermore, Ni 3+ ion substitution in the MnCo 2 O 4 spinel changed the relative position of the e g,β 1 level of Ni 3+ at the octahedral site and the e g,β 2 level of Co 2+ at the tetrahedral site, which makes the activation energy E A of Ni x Mn 1−x Co 2 O 4 obtained from our experimental results decrease first and then increases with the x value increasing. Among all the doped samples, the Ni 0.25 Mn 0.75 Co 2 O 4 has the smallest activation energy and the highest electrical conductivity. The measured room-temperature resistivity of the system is reduced by 5 orders of magnitude due to Ni doping.

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Epitaxial FeCo2O4 films were grown by pulsed laser deposition on Al2O3 (0001) substrates at different growth temperatures in this work.

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