Influence of Fe Substitution by Al on the Structural, Microstructural, and Magnetic Properties of Co0.2Ni0.3Zn0.5Fe2O4 Nanoparticles

A series of aluminum-substituted ferrite nanoparticles, Co0.2Ni0.3Zn0.5Fe2-xAlxO4 (0 ≤ x ≤ 1) was synthesized using the polyol method. The structural analysis confirmed the formation of pure cubic ferrispinels. The unit cell parameter decreased monotonically with increasing Al3+ content obeying Vegard's law. Besides, it was found that Al3+ ions prefer to enter octahedral sites. The produced particles consisted of roughly spherical ultrasmall nanocrystals with an average particle size of ∼3-6 nm. The nanoparticles are superparamagnetic with magnetic features (blocking temperature, saturation magnetization and Curie temperature) showing a decrease with the increase in Al3+ content. The magnetic tendency correlates with the weakening of the superexchange interactions in the spinel sublattices caused by substituting the non-magnetic Al3+ ions for Fe3+ ones. While, the coercivity and effective magnetic anisotropy increased with the increase of Al3+ concentration, suggesting the development of high surface anisotropy that largely compensates for the decrease of intrinsic magnetic anisotropy caused by the magnetic dilution.