Magnetic interactions in high-energy ball-milled NiZnFe2O4/SiO2 composites

Composites Ni0.5Zn0.5Fe2O4/SiO2 are obtained after high-energy ball milling precursor oxides, in stoichiometric proportions, for 200 h at room temperature and further isothermal annealing for 1 h at 1273 K, under air and argon atmosphere, respectively. After 200 h grinding, a complex microstructure develops with small hematite crystals mixed with SiO2 and remanent NiO and ZnO particles, and very small NiZn ferrite clusters, reaching a mean size of ∼9nm. The high temperature treatments remove the hematite grains from the powder and promote the growth of NiZn ferrite grains to reach mean sizes nearly ∼20nm. For treatments in oxidizing atmospheres, the major phases are SiO2 and NiZn ferrite, while for annealing in Ar a new phase appears, fayalite, which is paramagnetic at room temperature. The M-H loops are all well described by the sum of a ferromagnetic and a superparamagnetic-like contribution. The observed properties are interpreted considering the different magnetic phases obtained, their crystal sizes and their mutual interactions.