Annealing-dependent structural and magnetic properties of nickel oxide (NiO) nanoparticles in a silica matrix

We show that annealing at high temperatures has a significant effect on the structural and magnetic properties of NiO/SiO2 nanostructures synthesized by a sol–gel combustion method. Samples underwent heat treatments at 500 °C, 800 °C, 950 °C and 1100 °C. As compared to the 500 °C sample, the 800 °C sample showed the following magnetic properties: much higher irreversibility temperature, a significantly broadened zero-field-cooled (ZFC) magnetization maximum, a decrease of the ZFC magnetization, an increase of the coercivity, and weaker inter-particle interactions. These changes can be attributed to agglomeration of nanoparticles in part of the sample. We believe that this agglomeration can be explained by the removal of thin regions of silica that separate nanoparticles in close proximity during the annealing process at 800 °C. Magnetic measurements for the 1100 sample reveal both an abrupt increase in size of the NiO nanoparticles, which is confirmed by TEM and XRPD measurements, and an increase in inter-particle interaction strength.