CoFe₂O₄ and CoFe₂O₄‐SiO₂ Nanoparticle Thin Films with Perpendicular Magnetic Anisotropy for Magnetic and Magneto‐Optical Applications

This paper presents an efficient colloidal approach to process CoFe₂O₄ and SiO₂ nanoparticles into thin films for magnetic and magneto‐optical applications. Thin films of varying CoFe₂O₄‐to‐SiO₂ ratios (from 0 to 90 wt%) are obtained by sequential spin coating‐calcination cycles from the corresponding nanoparticle dispersions. Scanning electron microscopy analysis reveals a crack free and nanoparticulate structure of the sintered films with thicknesses of 480–1200 nm. Results from the optical characterization indicate a direct band gap ranging from 2.6 to 3.9 eV depending on the SiO₂ content. Similarly, the refractive indices and absorption coefficients are tunable upon SiO₂ incorporation. In‐plane measurements of the magnetic properties of the CoFe₂O₄ films reveal a superparamagnetic behavior with both Co²⁺ and Fe³⁺ contributing to the magnetism. Polar Kerr measurements show the presence of a spontaneous magnetization in the CoFe₂O₄ and CoFe₂O₄‐SiO₂ (with SiO₂ < 50 wt%) films, pointing to magnetic anisotropy perpendicular to the substrate. The origin of this effect is attributed to the constrained sintering conditions of the nano­particulate film and the negative magnetostriction of CoFe₂O₄.