Electronic Structure and Magnetic Configuration of Ni-Substituted MnCo2O4 Spinel

The electrical and magnetic structures of Ni-substituted MnCo2O4 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 NixMn1–xCo2O4 changes from tetragonal to cubic symmetry. The inverse spinel NixMn1–xCo2O4 was found to have a ferrimagnetic half-metallic ground state. The Ni dopant results in the flow of holes from the eg,β1 level of Ni3+ at the octahedral site to the eg,β2 level of Co2+ at the tetrahedral site. Therefore, the calculated conductivity type of NixMn1–xCo2O4 changes from n- to p-type as the Ni concentration increases. Furthermore, Ni3+ ion substitution in the MnCo2O4 spinel changed the relative position of the eg,β1 level of Ni3+ at the octahedral site and the eg,β2 level of Co2+ at the tetrahedral site, which makes the activation energy EA of NixMn1–xCo2O4 obtained from our experimental results decrease first and then increases with the x value increasing. Among all the doped samples, the Ni0.25Mn0.75Co2O4 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.