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, SrFe 12 O 19 , 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 Fe 3 O 4 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 Fe 3 O 4 to α-Fe 2 O 3 at elevated temperatures.