CoCr x Fe 2Àx O 4 (0 e x e 1.0) was synthesized by solÀgel auto combustion method using nitrates of respective elements and by keeping 1:3 ratio of metal nitrate to citrate. Representative sample was investigated by thermogravimeter/ differential thermal analyzer (TG/DTA); then, all samples were annealed at 500 °C for 4 h. The broad peaks in the X-ray diffraction patters (XRD) and evaluation of transmission electron microscopy (TEM) indicate a fine particle nature of the particles. Scanning electron microscopy (SEM) analysis and EDAX indicated that the samples were homogeneous and had the expected FeÀCoÀCr ratios. The lattice parameter, bulk density, and particle size are decreased, whereas the X-ray density, specific surface area, and porosity tend to increase with increasing Cr 3+ substitution. Cation distribution was estimated using XRD and by employing Bertaut method. Fourier transform infrared spectroscopy (FT-IR) is employed to determine the local symmetry in crystalline solids and to shed light on the ordering phenomenon. Saturation magnetization determined from vibrating sample magnetometer (VSM) decreases linearly with Cr 3+ concentration, suggesting that the superexchange interaction Fe(A)ÀOÀFe(B) link is stronger than that for the Fe(A)ÀOÀCr(B) link. Coercivity in the Cr-doped cobalt ferrites was larger than that in pure CoFe 2 O 4 compositions.
The effect of Ce4+ substitution in NiFe2O4, with a chemical formula Ni1-2xCexFe2O4 (0 ≤ x ≤ 0.25), ferrite prepared by a solid-state reaction is presented in this paper. Ce4+ ions enter the NiFe2O4 lattice by replacing Ni2+ and swell the lattice. This enlarges the lattice constant, which results in a moderate distortion of the lattice. The r.m.s. strain increases from 0.411 × 10−3 to 0.471 × 10−3 with increasing Ce4+ content. SEM images revealed that Ce4+ promotes grain growth in NiFe2O4. It was also revealed that x-ray density and porosity decreases, whereas a significant increase in the bulk density is observed with the Ce4+ content. Substitution of Ce4+ for Ni2+ caused a decrease in the saturation magnetization from 41.3 to 25.12 emu/g and a decrease in the Curie temperature of the nickel ferrite from 830 to 594 K, whereas the coercivity increased from 59.48 to 458.25 Oe.
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