Structural and ambient/sub-ambient temperature magnetic properties of Er-substituted cobalt-ferrites synthesized by sol-gel assisted auto-combustion method

摘要

Er-substituted cobalt-ferrites CoFe_(2-x)Er_xO_4 (0 ≤ x ≤ 0.04) were synthesized by sol-gel assisted auto-combustion method. The precursor powders were calcined at 673-873 K for 4h, subsequently pressed into pellets and sintered at 1273 K for 4h. X-ray diffraction (XRD) confirmed the presence of the spinel phase for all the compositions and, additional orthoferrite phase for higher compositions (x = 0.03 and 0.04). The XRD spectra and the Transmission Electron Microscopy micrographs indicate that the nanocrystalline particulates of the Er-substituted cobalt ferrites have crystallite size of ～120-200 nm. The magnetization curves show an increase in saturation magnetization (M_S) and coercivity (H_C) for Er-substituted cobalt-ferrites at sub-ambient temperatures. M_S for CoFe_2O_4, CoFe_(0.99)Er_(0.01)O_4, CoFe_(0.98)Er_(0.02)O_4, and CoFe_(0.97)Er_(0.03)O_4 peak at 89.7Am~2/kg, 89.3Am~2/kg, 88.8Am~2/kg, and 87.1Am~2/kg, respectively, at a sub-ambient temperature of ～150K. H_C substantially increases with decrease in temperature for all the compositions, while it peaks at x = 0.01- 0.02 at all temperatures. The combination of Er content-x ～ 0.02 and the temperature-～5 K provides the maximum H_C ～ 984 kA/m. Er-substituted cobalt-ferrites have higher cubic anisotropy constant, K_1, compared to pure cobalt-ferrite at ambient/sub-ambient temperatures. K_1 gradually increases for all compositions in the temperature decreasing from 300 to 100 K. While K_1 peaks at ～150K for pure cobalt-ferrite, it peaks at ～50K for CoFe_(0.99)Er_(0.01)O_4,, CoFe_(0.98)Er_(0.02)O_4, and CoFe_(0.96)Er_(0.04)O_4. The M_S (～88.7 Am~2/kg), at 5 K, for Er substituted cobalt-ferrite is close to the highest values reported for Sm and Gd substituted cobalt-ferrites. The M_S (～83.5 Am~2/kg) at 300 K for Er-substituted cobalt-ferrite is the highest among the lanthanide series element substituted cobalt-ferrites. The H_C (at 5 K) for Er substituted cobalt-ferrite is close to the highest values observed for La, Ce, Nd, Sm, and Gd substituted cobalt-ferrites.