Wet chemical synthesis of multicomponent hexaferrites by gel-to-crystallite conversion and their magnetic properties

The hexaferrites of different compositions from the BaO-NiO-Fe2O3 ternary phase diagram were prepared by a wet chemical process of gel-to-crystallite conversion. Herein, the coprecipitated reactive gels of hydrated nFe(OH)(3) . p-Ni(OH)(2) . xH(2)O; 70<x<110 with n = 12-36 and p = 0-2 were reacted with mBa(OH)(2) . 8H(2)O (m = 0-4) in aqueous ethanol medium at 80degreesC yielding the precursors of the general formula BamFenNipOq(OH)(r) . sH(2)O (q = 13-35; r = 6-50; s = 5-30) which are crystalline by electron diffraction. Dehydroxylation of the precursor is accomplished by continuous weight loss as evidenced from thermogravimetry and is complete at similar to650degreesC by isothermal conditions. Differential thermal analysis showed exothermic peaks arising from the recrystallization followed by grain growth of the particles at similar to750degreesC. Direct phase evolution of hexaferrites are evidenced from X-ray diffraction studies. Monophasic nanoparticles of hexaferrites were obtained on heat-treating the precursor at 850degreesC. HREM of these crystallites show defect-free, regular, coherent growth of structural blocks. The evolution of para- to ferrimagnetic phase on heat-treatment is continuous and is through the superparamagnetic state. This is associated with a continuous increase in magnetization with heat-treatment temperature. Samples heat-treated below 850degreesC show diffuse magnetic transition near T-c, which results from the intergrowths observed in the lattices. Hysteresis loop for various compositions showed significant differences from hard ferrite characteristics for M-phase (sigma(s) = 52 emu/g; H-c = 4520 Oe) to soft ferrite (sigma(s) = 25.5 emu/g; H-c = 205 Oe) for Y-phase. Crystallographic domains of complex stacking sequences such as YMYM4, M2SM3S, MMYY, etc. are observed when the hexaferrites are heat-treated above 1350degreesC. (C) 2003 Elsevier Science B.V. All rights reserved.