Nanoparticles of Barium Hexaferrite by Gel to Crystallite Conversion and their Magnetic Properties

Abstract

A novel method for the preparation of nanoparticles of barium hexaferrite is realized by the gel-to-crystallite (G-C) conversion method. Here, gels of Fe(OH)(3). xH(2)O, 70 < x < 110, were reacted with Ba(OH)(2). 8H(2)O in ethanol/water medium at 80-95 degreesC yielding the precursor, barium iron (III) oxy hydroxide hydrate which is x-ray amorphous but crystalline by electron diffraction (ED). Thermal analyses showed dehydroxylation of the precursor around 600 degreesC to barium hexaferrite which exhibits ED with spotty ring patterns. Samples heat-treated at 650 degreesC are X-ray crystalline with average particle size of 17 nm, which on recrystallization at 780-930 degreesC gives monocrystallites with spot patterns by ED. By varying the wet chemical conditions, precursors of variable Fe2O3/BaO ratios could be prepared which on heat-treatment yield monophasic hexaferrite of Fe2O3/BaO ratio ranging from 4.51 to 6. In hyperbarium compositions, annealing at 1350 degreesC leads to ordering of excess barium in anti-BR sites within Ba-O layers of beta -alumina type unit cells. Nanoparticles of barium hexaferrite with superparamagnetic as revealed by Mossbauer spectra, the temperature vs. magnetization plots and the absence of hysteresis in B-H curves. With increasing temperature of heat treatment, the area under the B-H loop increases continuously, with the magnetization increasing from 2 to 52 emu/g. The conversion from superparamagnetic to ferrimagnetic state is continuous because of the out-diffusion of cation vacancies, created to charge compensate hydroxyl ions, which, in turn, affects Fe3+-O2--Fe3+ superexchange interactions, with the addition of surface and size factors.