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

Er-substituted cobalt-ferrites CoFe_2−xEr_xO₄ (0 ≤ x ≤ 0.04) were synthesized by sol-gel assisted auto-combustion method. The precursor powders were calcined at 673–873 K for 4 h, subsequently pressed into pellets and sintered at 1273 K 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 ∼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, CoFe_0.99Er_0.01O₄, CoFe_0.98Er_0.02O₄, and CoFe_0.97Er_0.03O₄ peak at 89.7 Am²/kg, 89.3 Am²/kg, 88.8 Am²/kg, and 87.1 Am²/kg, respectively, at a sub-ambient temperature of ∼150 K. 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, K₁, compared to pure cobalt-ferrite at ambient/sub-ambient temperatures. K₁ gradually increases for all compositions in the temperature decreasing from 300 to 100 K. While K₁ peaks at ∼150 K for pure cobalt-ferrite, it peaks at ∼50 K for CoFe_0.99Er_0.01O₄, CoFe_0.98Er_0.02O₄, and CoFe_0.96Er_0.04O₄. The M_S (∼88.7 Am²/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 Am²/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.