Magnetic properties of nano-crystalline Gd- or Pr-substituted CoFe2O4 synthesized by the citrate precursor technique

The magnetic properties of nano-crystalline CoMxFe2-xO4 (where M = Gd and Pr and x = 0, 0.1 and 0.2) powders prepared by a citrate precursor technique have been studied by using vibrating sample magnetometer (VSM). The crystallite sizes of the materials were varied by altering the synthetic conditions and are within the range of a minimum of 6.8 nm and a maximum of 87.5 nm. The materials were characterized by powder X-ray diffraction (XRD) and thermogravimetric (TG) measurements. TG study indicates the formation of the spinel ferrite phase at 220 C. The phase identification of the materials by XRD reveals the single-phase nature of the materials. The room temperature saturation magnetization of the ferrite materials decreases with the reduction of size. This has been attributed to the presence of superparamagnetic fractions in the materials and spin canting at the surface of nano-particles. Insertion of rare-earth atoms in the crystal lattice inhibits the grain growth of the materials in a systematic manner compared with that of the pure cobalt ferrite materials. The improved coercivity compared with those for the pure cobalt ferrites is attributed to the contribution from the single ion anisotropy of the rare-earth ions present in the crystal lattice and the surface effects resulting in alteration of magnetic structures on the surface of nano-particles. (C) 2002 Elsevier Science B.V. All rights reserved.