Effects of Al Substitution on Structural and Magnetic Behavior of CoFe₂O₄ Ferrite Nanomaterials.

A sol-gel autocombustion method was used to synthesize Al ion-substituted cobalt ferrite CoAlFeO₄ ( = 0⁻1.5). According to X-ray diffraction analysis (XRD), cobalt ferrite was in a single cubic phase after being calcined at 1000 °C for 3 h. Moreover, the lattice constant decreased with increase in aluminum substituents. When the sample was analyzed by Scanning Electron Microscopy (SEM), we found that uniformly sized, well-crystallized grains were distributed in the sample. Furthermore, we confirmed that Al ion-substituted cobalt ferrite underwent a transition from ferrimagnetic to superparamagnetic behavior; the superparamagnetic behavior was completely correlated with the increase in Al ion concentration at room temperature. All these findings were observed in Mössbauer spectra. For the cobalt ferrite CoAlFeO₄, the coercivity and saturation magnetization decrease with an increase in aluminum content. When the annealing temperature of CoAlFeO₄ was steadily increased, the coercivity and saturation magnetization initially increased and then decreased.