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Structural and magnetic characteristics of cobalt ferrite-coated nano-fibrous $\gamma-Fe_2O_3$

Sudakar, C and Kutty, TRN (2004) Structural and magnetic characteristics of cobalt ferrite-coated nano-fibrous $\gamma-Fe_2O_3$. In: Journal of Magnetism and Magnetic Materials, 279 (2-3). pp. 363-374.

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The presence of mesopores in $\gamma-Fe_2O_3$ is a limiting factor for its optimal magnetic properties. Therefore, high coercive $\gamma-Fe_2O_3$ nano-fibrous particles are obtained by coating cobalt ferrite on the surface. An aqueous solution cobalt nitrate+ammonium ferrous sulphate (in the ratio 1:2) precipitated at pH>12.5 in the presence of uniformly dispersed $\gamma-Fe_2O_3$ yields cobalt ferrite coating on the surface of the particles at $\sim90^oC.$ The nature of the coating, studied by X-ray diffraction and high-resolution electron microscopy (HREM), reveals the epitaxial growth of the $CoFe_2O_4$ layer on $\gamma-Fe_2O_3$ provided the extent of coating $(X_{Co})$ is maintained $\leq30 wt\%.$ The pores within the $\gamma-Fe_2O_3$ particles are coherently filled up as deciphered by the continuity in the HREM lattice fringes. The surface characteristics using X-ray photoelectron spectroscopy analysis further confirm the coherent coating of cobalt ferrite onto the $\gamma-Fe_2O_3$ surface. Studies of cobalt ferrite-coated samples by $M\H{o}$ossbauer spectra clearly reveal the hyperfine field characteristics of $\gamma-Fe_2O_3$ notwithstanding the $CoFe_2O_4$ coating at lower contents. As the thickness of the coating increases $(\sim30 wt\%),$ the spectrum consists of two overlapping sextets (one from the average hyperfine pattern of $\gamma-Fe_2O_3$ and A-site of $CoFe_2O_4$ and the second from the B-site in the bulk $CoFe_2O_4).$ The deposition of $CoFe_2O_4$-rich surface layer results in an increase in the coercive field (Hc = 585 Oe for XCo = 10 wt%; Hc = 1100 Oe for XCo = 30 wt%), which attributed to the formation of uniaxial anisotropy under the influence of the field created by the magnetization of the particles. Saturation magnetization $(\sigma_{s})$ also increases from 74.4 emu/g (uncoated) to $\sim78.6 emu/g$ (XCo = 30 wt%) with the extent of coating. In comparison, for Co-substituted nano-fibrous $\gamma-Fe_2O_3$ the presence of homogeneously distributed $Co^{2+}$ ions (10 at%) enhances the Hc (609 Oe) as a result of high magnetocrystalline anisotropy accompanied by shape anisotropy, whereas, $\sigma_{s}$ decreases $(\sim60 emu/g)$ due to the relaxation of spins on the surface atoms of slender particles with mesopores as revealed by $M\H{o}$ossbauer spectroscopy for $Co^{2+}$-substituted $\gamma-Fe_2O_3.$

Item Type: Journal Article
Additional Information: The copyright belongs to Elsevier.
Keywords: Maghemite;Epitaxial coating;M.ossbauer spectra;Nano-.bre;Magnetic properties
Department/Centre: Division of Chemical Sciences > Materials Research Centre
Depositing User: Anka Setty
Date Deposited: 25 Apr 2006
Last Modified: 19 Sep 2010 04:25
URI: http://eprints.iisc.ac.in/id/eprint/6429

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