Effect of Cr and Al substitution cations on the structural and magnetic properties of Ni0.6Zn0.4Fe2−xCrx/2Alx/2O4 nanoparticles synthesized using the sol–gel auto-combustion method

• The hysteresis curves of the Ni0.6Zn0.4Fe2−xCrx/2Alx/2O4 nanoparticles exhibited the reduction of saturation magnetization with an increase in substitution value.
• The good agreement between the susceptibility data and the Vogel–Fulcher model confirms the existence of strong interactions between nanoparticles.
• The values of blocking temperature have inverse relation with particle size.

The Ni0.6Zn0.4Fe2−xCrx/2Alx/2O4 (x=0–0.5) nanoparticles were prepared by employing the sol–gel auto-combustion method. The effect of aluminum and chromium on the structural and superparamagnetic properties of prepared samples was investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy, vibrating sample magnetometry (VSM), ac magnetic susceptibility and magnetization temperature curve recording in ZFC mode analysis. The XRD analysis of all synthesized samples confirmed the formation of single-phase cubic spinel structure. The results of FTIR analysis indicated that the functional groups of Ni–Zn spinel ferrite were formed during the auto-combustion sol–gel process. Moreover, FE-SEM and TEM micrographs demonstrated that nanoparticles with narrow size distribution were obtained. According to VSM results by increasing substitution contents, saturation magnetization decreased. Magnetic dynamics of the samples was studied by measuring ac magnetic susceptibility versus temperature at different frequencies. The phenomenological Néel–Brown and Vogel–Fulcher models were employed to distinguish between the interacting or noninteracting system. Results exhibited that there is a strong interaction between nanoparticles. A frequency–dependence peak was observed in ac magnetic susceptibility versus temperature for nanoparticles which is well fitted by the Vogel–Fulcher model. It is confirmed that there is strong magnetic interaction among prepared nanoparticles.