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

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·xH2O, 70 < x < 110, were reacted with Ba(OH)2·8H2O in ethanol/water medium at 80–95°C yielding the precursor, barium iron (III) oxy hydroxide hydrate which is x-ray amorphous but crystalline by electron diffraction (ED). Thermal analyses showed dehydro-xylation of the precursor around 600°C to barium hexaferrite which exhibits ED with spotty ring patterns. Samples heat-treated at 650°C are X-ray crystalline with average particle size of 17 nm, which on recrystallization at 780–930°C 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°C leads to ordering of excess barium in anti-BR sites within Ba—O layers of β-alumina type unit cells. Nanoparticles of barium hexaferrite with superparamagnetic as revealed by Mössbauer 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.