Coexistence of superparamagnetism and spin-glass like behavior in zinc-substituted cobalt ferrite nanoparticles

We have performed a systematic study on the effect of Zn substitution on the structural and magnetic properties of crystalline zinc-substituted cobalt ferrite nanoparticles, Zn x Co 1 - x Fe 2 O 4 with x = 0, 0.25, 0.5, 0.75, and 1, prepared by hydrothermal method. The structural and magnetic properties of these nanoparticles were investigated by XRD, TEM, FTIR, and VSM. All the ferrite nanoparticles were prepared with sizes smaller than 20 nm, thus lying within the range of single-domain regime. The results of Rietveld refinement revealed that all prepared nanoparticles were cubic and single phase, and the increase in Zn concentration resulted in an increase in the lattice constant, x-ray density, and the average bond length on tetrahedral sites. The TEM measurements showed that the nanoparticles were monodisperse and spherical in shape. All FTIR spectra of the prepared ferrites showed two dominant absorption bands, thus confirming the formation of single-phase spinel structure with two sub-lattices: tetrahedral (A-site) and octahedral (B-site). The room temperature M versus H magnetization measurements revealed that the ferrite nanoparticles were ferromagnetic for x = 0 and superparamagnetic for x ≥ 0.25 . At 10 K, all ferrite nanoparticles showed ferrimagnetic behavior that is weakened by Zn substitution. The saturation magnetization and the first anisotropy constant were observed to decrease with increasing Zn concentration. The zero field cooled and field cooled magnetization data revealed that both superparamagnetic and spin-glass like states may coexist together depending on amount of Zn concentration and temperature.