Non-interacting Neél-Brown or interacting Vogel-Fulcher models in magnetic CoFe_2-xGd_xO₄ nanocrystals

CoFe_2-xGd_xO₄ (x=0-0.04 in a step of 0.01) ferrite nanocrystals were synthesized by a hydrothermal technique. The X-ray diffraction analysis indicated that single phase spinel ferrites were obtained. The XRD data were processed for Rietveld refinement of structure by Reflex program. The FE-SEM micrographs of the synthesized samples showed the presence of polyhedral-shaped grains. The gadolinium cations substitution greatly influenced the DC electrical resistivity of the synthesized nanocrystals. With an increase in gadolinium content, the electrical resistivity decreased from 1009.667 Ω-cm for x=0 to 6.397 Ω-cm for x=0.02. The results of magnetic hysteresis at a room temperature demonstrated that with an increase in gadolinium content, the coercive field increased from 664.6 Oe for x=0 to 2069 Oe for x=0.02. In addition, it was found that with substitutions of gadolinium cations, the values of saturation magnetization decreased from 104.17 emu/g for x=0 to 83.3 emu/g for x=0.02. It is also observed from the loop that with substitutions of gadolinium cations at higher contents in the composition (x>0.02), the coercive field decreased while the values of saturation magnetization increased. The variation of magnetization versus temperature for determining Curie temperature shows the increasing trend with Gd³⁺ substitution, indicating the strengthening in A-B interactions. Magnetic dynamics of the samples was also studied by measuring AC magnetic susceptibility versus temperature. The phenomenological Neél-Brown and Vogel-Fulcher models were used to distinguish between the interacting or non-interacting system.