Journal of Magnetism and Magnetic Materials 279 (2004) 363–374 Structural and magnetic characteristics of cobalt ferrite-coated nano-fibrous g-Fe 2 O 3 C. Sudakar, T.R.N. Kutty* Materials Research Centre, Indian Institute of Science, Bangalore 560012, India Received 5 December 2003; received in revised form 28 January 2004 Abstract The presence of mesopores in g-Fe 2 O 3 is a limiting factor for its optimal magnetic properties. Therefore, high coercive g-Fe 2 O 3 nano-fibrous particles are obtained by coating cobalt ferrite on the surface. An aqueous solution of cobalt nitrate+ammonium ferrous sulphate (in the ratio 1:2) precipitated at pH>12.5 in the presence of uniformly dispersed g-Fe 2 O 3 yields cobalt ferrite coating on the surface of the particles at B90  C. The nature of the coating, studied by X-ray diffraction and high-resolution electron microscopy (HREM), reveals the epitaxial growth of the CoFe 2 O 4 layer on g-Fe 2 O 3 provided the extent of coating (X Co ) is maintained p30 wt%. The pores within the g-Fe 2 O 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 g-Fe 2 O 3 surface. Studies of cobalt ferrite-coated samples by M. ossbauer spectra clearly reveal the hyperfine field characteristics of g-Fe 2 O 3 notwithstanding the CoFe 2 O 4 coating at lower contents. As the thickness of the coating increases (B30wt%), the spectrum consists of two overlapping sextets (one from the average hyperfine pattern of g-Fe 2 O 3 and A-site of CoFe 2 O 4 and the second from the B-site in the bulk CoFe 2 O 4 ). The deposition of CoFe 2 O 4 -rich surface layer results in an increase in the coercive field (H c ¼ 585Oe for X Co ¼ 10wt%; H c ¼ 1100Oe for X Co ¼ 30wt%), which is attributed to the formation of uniaxial anisotropy under the influence of the field created by the magnetization of the particles. Saturation magnetization (s s ) also increases from 74.4emu/g (uncoated) to B78.6emu/g (X Co ¼ 30wt%) with the extent of coating. In comparison, for Co-substituted nano-fibrous g-Fe 2 O 3 the presence of homogeneously distributed Co 2+ ions (10at%) enhances the H c (609Oe) as a result of high magnetocrystalline anisotropy accompanied by shape anisotropy, whereas, s s decreases (B60 emu/g) due to the relaxation of spins on the surface atoms of slender particles with mesopores as revealed by M . ossbauer spectroscopy for Co 2+ -substituted g-Fe 2 O 3 . r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Gg; 75.50.Tt; 81.20.n Keywords: Maghemite; Epitaxial coating; M. ossbauer spectra; Nano-fibre; Magnetic properties 1. Introduction The synthesis of maghemite (g-Fe 2 O 3 ) nano- particles with a high aspect ratio, controlled size and optimized magnetic properties has been of scientific and technological interest since long. ARTICLE IN PRESS *Corresponding author. Tel.: +91-080-2932784; fax: +91- 080-3600683. E-mail address: kutty@mrc.iisc.ernet.in (T.R.N. Kutty). 0304-8853/$-see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.02.005