Magnetic Behavior of a High Entropy Oxide

High entropy oxides are a new group of materials that have attracted attention because of interesting properties and potential applications. Previous workers have shown that equimolar mixtures of five binary oxides undergo a reversible transformation from single fcc phase material when sintered at high temperature (1000 ^oC) to multiphase material when sintered at 700 ^oC and back to single phase material when fired for a second time at 1000 ^oC. Using XRD, we have confirmed the entropy-driven reversible structural transition in equimolar mixtures of MgO, CuO, ZnO, NiO and CoO. However, reversibility was not found for magnetic properties. The field-cooled/ zero-field-cooled response was measured for a sample sintered at 700 ^oC which showed paramagnetic behaviour. When this sample was sintered at 1000 ^oC - when it was single phase fcc - the sample was antiferromagnetic with transition temperature around 110 K. But when the sample was forced to become multiphase by sintering again at 700 ^oC the overall magnetic behaviour was still antiferromagnetic.