Structural and magnetic characteristics of cobalt ferrite-coated nano-fibrous γ-Fe2O3

The presence of mesopores in γ-Fe2O3 is a limiting factor for its optimal magnetic properties. Therefore, high coercive γ-Fe2O3 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 γ-Fe2O3 yields cobalt ferrite coating on the surface of the particles at ∼90 C. The nature of the coating, studied by X-ray diffraction and high-resolution electron microscopy (HREM), reveals the epitaxial growth of the CoFe2O4 layer on γ-Fe2O3 provided the extent of coating (XCo) is maintained ≤ 30 wt%. The pores within the γ-Fe2O3 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 γ-Fe2O3 surface. Studies of cobalt ferrite-coated samples by Mössbauer spectra clearly reveal the hyperfine field characteristics of γ-Fe2O3 notwithstanding the CoFe2O4 coating at lower contents. As the thickness of the coating increases ( ∼30 wt%), the spectrum consists of two overlapping sextets (one from the average hyperfine pattern of γ-Fe2O3 and A-site of CoFe2O4 and the second from the B-site in the bulk CoFe2O4). The deposition of CoFe2O4-rich surface layer results in an increase in the coercive field (Hc = 5X50e for XCo = 10wt%; Hc = 1100 Oe for XCo = 30 wt%), which is attributed to the formation of uniaxial anisotropy under the influence of the field created by the magnetization of the particles. Saturation magnetization (σs) also increases from 74.4 emu/g (uncoated) to ∼78.6 emu/g (XCo = 30wt%) with the extent of coating. In comparison, for Co-substituted nano-fibrous γ-Fe2O3 the presence of homogeneously distributed Co2 ions (10 at%) enhances the Hc (609 Oe) as a result of high magnetocrystalline anisotropy accompanied by shape anisotropy, whereas, σs decreases (∼ 60 emu/g) due to the relaxation of spins on the surface atoms of slender particles with mesopores as revealed by Mössbauer spectroscopy for Co2+ -substituted γ-Fe2O3.