Nanocomposite Permanent Magnets Based on SrFe12O19-Fe3O4 Hard-Soft Ferrites

Nanocomposites consisting of strontium ferrite and magnetite are prepared to investigate the effect of exchange coupling between the hard and soft magnetic phases. Hexagonal strontium ferrite, SrFe12O19, synthesized using a solid-state route involving the sintering of precursors at 1000 °C gives a coercivity value as high as 3.73 kOe. An increase in the sintering temperature results in an increase in particle size and a decrease in coercivity. The soft ferrite phase Fe3O4 synthesized by a reverse co-precipitation method shows a saturation magnetization as high as 84 emu/g. Simple homogenous mixing of soft and hard components resulted in an exchange-coupled magnetic nanocomposite. With an increase in the soft magnetic content, the magnetization of the composite increases while the coercivity decreases. On sintering the nanocomposites at 1000 °C, coercivity remains intact even for an increasing soft magnetic content indicating an exchange decoupling between the soft and hard phases. This is attributed to the phase transformation of Fe3O4 to α-Fe2O3 at elevated temperatures.