Behavior of transition metal ions and ferromagnetism in oxidized NiCo2S4 caused by octahedron distortion

Abstract NiCo2S4 powders were synthesized using a hydrothermal method and oxidized in various environments. We have carried out the density functional theory (DFT) calculation of NiCo2S4 and its oxidation structure. Both theory and experiments show that unoxidized NiCo2S4 was found to be paramagnetic, but, the magnetic states of the oxidized NiCo2S4 powders were changed significantly. It is found by the DFT calculation that the octahedrons in the partially oxidized NiCo2S4 spinel lattice were distorted, leading to changes in the orbital ordering of the Co3+ t2g level. In particular, one electron of the dyz orbital jumps into the d ( x 2 − y 2 ) orbital of the eg level, and the nonmagnetic low spin Co3+ transforms to an intermediate spin state. The magnetic measurements show the ferromagnetism in the partially oxidized NiCo2S4 powders, which originated from the exchange interaction between intermediate spin Co3+ and Ni ions mediating through intervening O ions. When the NiCo2S4 powders were heavily oxidized, Co3+ ions were gradually reduced to Co2+. Due to a strong covalent effect, Co2+ makes little contribution to the magnetic properties of the sample, but antiferromagnetic exchange interactions occur between Ni3+ ions in the heavily oxidized sample. All the NiCo2S4 powders with different degrees of oxidation were found to retain metallic conductivity.