Nanoparticles of barium hexaferrite by gel to crystallite conversion and their magnetic properties

A novel method for the preparation of nanoparticles of barium hexaferrite i s 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 et hanol/water medium at 80-95 degreesC yielding the precursor, barium iron (I II) oxy hydroxide hydrate which is x-ray amorphous but crystalline by elect ron diffraction (ED). Thermal analyses showed dehydroxylation of the precur sor 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 we t chemical conditions, precursors of variable Fe2O3/BaO ratios could be pre pared which on heat-treatment yield monophasic hexaferrite of Fe2O3/BaO rat io ranging from 4.51 to 6. In hyperbarium compositions, annealing at 1350 d egreesC leads to ordering of excess barium in anti-BR sites within Ba-O lay ers of beta -alumina type unit cells. Nanoparticles of barium hexaferrite w ith superparamagnetic as revealed by Mossbauer spectra, the temperature vs. magnetization plots and the absence of hysteresis in B-H curves. With incr easing temperature of heat treatment, the area under the B-H loop increases continuously, with the magnetization increasing from 2 to 52 emu/g. The co nversion from superparamagnetic to ferrimagnetic state is continuous becaus e of the out-diffusion of cation vacancies, created to charge compensate hy droxyl ions, which, in turn, affects Fe3+-O2--Fe3+ superexchange interactio ns, with the addition of surface and size factors.