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

Sudakar, C and Subbanna, GN and Kutty, TRN (2003) Wet chemical synthesis of multicomponent hexaferrites by gel-to-crystallite conversion and their magnetic properties. In: Journal of Magnetism and Magnetic Materials, 263 (3). pp. 253-268.

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The hexaferrites of different compositions from the $BaO-NiO-Fe_2O_3$ ternary phase diagram were prepared by a wet chemical process of gel-to-crystallite conversion. Herein, the coprecipitated reactive gels of hydrated $nFe(OH)_3\cdotpNi(OH)_2\cdotxH2O$; 70<x<110 with n=12–36 and p=0–2 were reacted with $mBa(OH)_2\cdot8H_2O$ (m=0–4) in aqueous ethanol medium at 80°C yielding the precursors of the general formula $Ba_mFe_nNi_pO_q(OH)_r\cdot_sH_2O$ (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 $\sim$650°C by isothermal conditions. Differential thermal analysis showed exothermic peaks arising from the recrystallization followed by grain growth of the particles at $\sim$750°C. Direct phase evolution of hexaferrites are evidenced from X-ray diffraction studies. Monophasic nanoparticles of hexaferrites were obtained on heat-treating the precursor at 850°C. 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 850°C 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, $M_2SM_3S$, MMYY, etc. are observed when the hexaferrites are heat-treated above 1350°C.

Item Type: Journal Article
Publication: Journal of Magnetism and Magnetic Materials
Publisher: Elsevier
Additional Information: Copyright of this article belongs to Elsevier.
Keywords: Gel-to-crystallite conversion;Hexaferrite;Multicomponent oxides;Nanoparticles
Department/Centre: Division of Chemical Sciences > Materials Research Centre
Date Deposited: 02 Jun 2006
Last Modified: 19 Sep 2010 04:28
URI: http://eprints.iisc.ac.in/id/eprint/7316

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