Magnetic properties of NiMn2−xCoxO4 spinel oxides

Magnetic properties of NiMn2−xCoxO4 spinel oxides have been investigated. The paramagnetic moment shows a direct correlation with the nominal cation concentration. A first transition, from paramagnetism to ferrimagnetism, occurs at high temperature, going from Tc = 125 K (NiMn1.8Co0.2O4) up to 210 K. A second transition is observed at lower temperature. It takes place at about 60 K and increases with Co content up to 160 K. Under an external field, both transitions merge into a single one, with a characteristic temperature Tmax rapidly decreasing with increasing fields. Magnetization loops show the presence of both antiferromagnetic-type field dependence and irreversible behaviors typical of soft-ferromagnets. Preliminary investigations of R(T, H) point towards novel magneto-transport phenomena.