SrFe12O19/CoFe2O4 magnetic composites: Nanoparticle size and effect of annealing temperature on magnetic properties

Abstract

Nanostructured magnetic composites of hexagonal SrFe12O19 and cubic CoFe2O4 powders were prepared by ball milling technique varying the mean size of CoFe2O4 grains and using 80/20 mass ratio. Subsequently, the composites were divided into two series of distinct annealing conditions: i) at 1000 °C and ii) at temperature at which the corresponding CoFe2O4 phase was synthesized. X-ray powder diffraction was carried out for phase identification and structural analysis. Transmission electron microscopy and/or scanning electron microscopy were applied for morphological monitoring. Energy dispersive X -ray analysis allowed elemental analysis. Magnetic properties were studied at room temperature by magnetization versus the applied external magnetic field, direct current demagnetization and isothermal remanence magnetization measurements. A relation was established between the structural parameters of the composite and the magnetic properties, putting emphasis on the exchange coupling effect. Although the stored energy product for all the composites studied is lower than that of hexagonal SrFe12O19, a visible gain of remanence magnetization of selected composites is observed. In the first series of composites, the one made from the CoFe2O4 grains of the largest mean size exhibits the highest energy product, (BH)max = 9.5 kJ/m3. In the second series of composites the one thermally treated at 900 °C is magnetically the hardest, Hc = 3920 Oe.