Synthesis of nanocomposites comprising iron and barium hexaferrites

Composites of nanometre-sized α-iron and barium hexaferrite phases, respectively, have been synthesized by the ceramic processing route. Pure barium hexaferrite (BaO·6Fe2O3) was first of all prepared by calcinations of the precursor oxides at a maximum temperature of 1200 deg. C for 4 h. By subjecting the resulting powder having particle size of the order of 1 μm to a reduction treatment in the temperature range 500-650 deg. C for a period varying from 10 to 15 min it was possible to obtain a composite consisting of nanosized barium hexaferrite and α-Fe. At reduction temperature of 650 deg. C for a period greater than 15 min all the ferrite phase was converted to α-Fe and Ba--the particle sizes being 59.4 and 43.6 nm, respectively. These conclusions are based on X-ray diffraction and Mossbauer studies of different samples. During reduction H+ ions are introduced into the hexaferrite crystallite. It is believed that due to a tensile stress the crystals are broken up into smaller dimensions and the reduction brings about the growth of nanosized α-Fe and barium, respectively, around the hexaferrite particles. Magnetic measurements show coercivity values for the reduced samples in the range 120-440 Oe and saturation magnetization varying from 158 to 53.7 emu/g. These values have been ascribed to the formation and growth of α-Fe particles as the reduction treatment is increased. By heating the nanocomposites at a temperature of 1000 deg. C for 1 h in ordinary atmosphere it was found that they were reconverted to the barium hexaferrite phase with a particle size ∼182.3 nm. The reaction described in this study is thus reversible