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

Er-substituted cobalt-ferrites CoFe(2-x)ErxO(4) (0 <= x <= 0.04) were synthesized by sol-gel assisted auto-combustion method. The precursor powders were calcined at 673-873K for 4 h, subsequently pressed into pellets and sintered at 1273K for 4 h. 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 similar to 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 CoFe2O4, CoFe0.99Er0.01O4, CoFe0.98Er0.02O4, and CoFe0.97Er0.03O4 peak at 89.7Am(2)/kg, 89.3 Am-2 /kg, 88.8 Am-2/kg, and 87.1 Am-2/kg, respectively, at a sub-ambient temperature of similar to 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 similar to 0.02 and the temperature-similar to 5K provides the maximum H-C similar to 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 100K. While K-1 peaks at similar to 150K for pure cobalt-ferrite, it peaks at similar to 50K for CoFe0.99Er0.01O4, CoFe0.98Er0.02O4, and CoFe0.96Er0.04O4. The MS (similar to 88.7 Am 2 /kg), at 5K, for Er substituted cobalt-ferrite is close to the highest values reported for Sm and Gd substituted cobalt-ferrites. The MS (similar to 83.5 Am-2 /kg) at 300K for Er-substituted cobalt-ferrite is the highest among the lanthanide series element substituted cobalt-ferrites. The H-C (at 5K) for Er substituted cobalt-ferrite is close to the highest values observed for La, Ce, Nd, Sm, and Gd substituted cobalt-ferrites. (C) 2014 AIP Publishing LLC.