CoFe 2 O 4 and CoFe 2 O 4 ‐SiO 2 Nanoparticle Thin Films with Perpendicular Magnetic Anisotropy for Magnetic and Magneto‐Optical Applications

This paper presents an efficient colloidal approach to process CoFe 2 O 4 and SiO 2 nanoparticles into thin films for magnetic and magneto‐optical applications. Thin films of varying CoFe 2 O 4 ‐to‐SiO 2 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 SiO 2 content. Similarly, the refractive indices and absorption coefficients are tunable upon SiO 2 incorporation. In‐plane measurements of the magnetic properties of the CoFe 2 O 4 films reveal a superparamagnetic behavior with both Co 2+ and Fe 3+ contributing to the magnetism. Polar Kerr measurements show the presence of a spontaneous magnetization in the CoFe 2 O 4 and CoFe 2 O 4 ‐SiO 2 (with SiO 2 < 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 CoFe 2 O 4 .