Non-interacting Neél–Brown or interacting Vogel–Fulcher models in magnetic CoFe2−xGdxO4 nanocrystals

• CoFe2−xGdxO4 nanocrystals were synthesized by a hydrothermal technique.
• The electrical resistivity was decrease with an increase in Gd content.
• Magnetic interactions were found between nanoparticles.

CoFe2−xGdxO4 (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.