Partial substitution of cobalt by rare-earths (Gd or Sm) in cobalt ferrite: Effect on its microstructure and magnetic properties

Microstructure and magnetic properties of partially substituted cobalt ferrite, with small amounts of gadolinium or samarium, were investigated. Co1-xGdxFe2O4+delta (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+delta (x = 0.04, and 0.08) were obtained through the citrate precursors route. X-ray powder diffraction patterns, of samples treated at temperatures between 400 and 1000 degrees C, are consistent with a cubic spinel-type structure. Substituted ferrites, with x = 0.04 and 0.08 (600 degrees C/2 h), show the combination of maximum enhancement of crystallinity and minimum change of the lattice parameters. At this temperature and composition range the crystallite size decreases up to a 41%. Rietveld analyses of Co-1(-x)(Gd/Sm)(x)Fe2O4+delta (x = 0, 0.04, and 0.08), carried out considering that Gd-3(+) (or Sm-3(+)) occupies Co-2(+) positions, give rise to a good quality of refinement and cell parameters which are slightly smaller than those provided in the crystallographic database of CoFe2O4. Raman spectra of Gd-3(+) or Sm-3(+) substituted ferrites show the characteristic Raman modes expected in inverse spinel ferrites without additional signals. TEM micrographs reveal the presence of small agglomerates of nanoparticles. SAED patterns and HRTEM micrographs, are characteristic of polycrystalline powders while the interplanar distances well correspond to those of CoFe2O4. EDS analyses confirm the presence of Gd or Sm in substituted ferrites. Magnetic hysteresis loops of samples suggest a ferrimagnetic behavior and their squareness ratio values decrease up to 36% in Co1-xSmxFe2O4+delta (x = 0.04). Mossbauer studies reveal the presence of only Fe-3(+) ions, distributed in a proportion of 60 and 40% through octahedral and tetrahedral sites, respectively and a canted ferrimagnetic structure. Although the formation of the mixed spinel, rather than different substances (Sm or Gd oxide), fall in the sensitivity limits of the XRD technique all the results demonstrate that the replacement of Co-2(+) by Gd-3(+) or Sm-3(+) ions, in CoFe2O4, can tailor the crystal structure, microstructure, and magnetic properties of cobalt ferrite.