Low-Temperature Synthesis, Magnetic and Microwave Electiomagnetic Properties of Substoichiometric Spinel Cobalt Ferrite Octahedra

Substoichiometric spinet cobalt ferrite octahedra were prepared by using a versatile coprecipitation/air oxidation method in a low-temperature aqueous system without any surfactants and directing agents. The microstructure, magnetic and microwave electromagnetic properties of the resultant products as functions of the nominal molar ratio [Co2+]/[Fe2+] were investigated. The results show that the [Co2+]/[Fe2+] ratio has a strong influence on the kinetic reaction rate of the coprecipitation/oxidation reaction of Co2+ and Fe2+ and consequently determines the chemical composition, crystal structure and morphology of the resultant products. Substoichiometric spinel cobalt ferrite octahedra of about 150 nm were available at a [Co2+]/[Fe2+] ratio of 1:4, due to the control over the nucleation and selective crystallographic direction growth. Too high [Co2+]/[Fe2+] values significantly accelerate the kinetic rate of copercipiation/oxidation reaction, which results in irregular CoxFe3-xO4 nanoparticles and a decreasing level of Co2+ incorporation in the crystal lattice, The inverse spinet Co0.59Fe2.41O4 octahedra have the largest saturation magnetization, coercivity, complex permittivity and permeability among all the resultant products. This phenomenon, together with the strong resonance behavior of the imaginary permittivity and imaginary permeability for Co0.59Fe2.41O4 octahedra at frequency above 16 GHz, is explained in terms of a correlation with the chemical composition, the Co2+ distribution over tetrahedral and octahedral sites and the incorporation level of Co2+ into the ferrite lattice.