Structure and magnetic properties of NiZn ferrite/SiO2 nanocomposites synthesized by ball milling

Magnetic NiZnFe2O4/SiO2 nanocomposites are synthesized by ball-milling a mixture of crystalline α-Fe2O3, NiO, ZnO and SiO2 powders. Crystallographic phases appearing during milling, their particle sizes and lattice parameters are determined from X-ray diffraction (XRD) measurements and the morphology of the as-milled powders observed by scanning electron microscopy (SEM). Room temperature hysteresis properties are characterized by vibrating sample magnetometry. The milling process, up to 260 h, promotes the progressive amorphization of the powders and the formation of different phases, such as NiZn-ferrite, α-Fe and Fe2SiO4. For milling times smaller than 80 h, the complete transformation of the precursor oxides into NiZn-ferrite is only achieved after annealing the as-milled powders for 1 h in air at 1273 K. This heat treatment favors the formation of NiZn-ferrite in detriment of the precursor oxides. On the other hand, annealing in air the powder milled 260 h, essentially amorphous, results in the formation of both hematite and NiZn-ferrite in the amorphous silica. When the powders milled for 260 h are heat treated in argon atmosphere, a biphase composite is obtained, with NiZn-ferrite crystallites of about 65 nm dispersed in an amorphous silica matrix. This last powder presents the highest values of saturation magnetization (29.87 Am2/kg) and coercivity (25.7 kA/m), being the latter two orders of magnitude larger than that of bulk NiZn-ferrite.