Synthesis of nanocomposites comprising iron and barium hexaferrites

Composites of nanometre-sized x-iron and barium hexaferrite phases, respectively, have been synthesized by the ceramic processing route. Pure barium hexaferrite (BaO (.) 6Fe(2)O(3)) was first of all prepared by calcinations of the precursor oxides at a maximum temperature of 1200 C for 4h. By subjecting the resulting powder having particle size of the order of I pm to a reduction treatment in the temperature range 500-650 C for a period varying from 10 to 15 min it was possible to obtain a composite consisting of nanosized barium hexaferrite and x-Fe. At reduction temperature of 650 C for a period greater than 15 min all the ferrite phase was converted to x-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 x-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 x-Fe particles as the reduction treatment is increased. By heating the nanocomposites at a temperature of 1000 C for 1 h in ordinary atmosphere it was found that they were reconverted to the barium hexaferrite phase with a particle size similar to 182.3 nm. The reaction described in this study is thus reversible. (C) 2003 Elsevier B.V. All rights reserved.