Surface spin effects in La-doped CoFe₂O₄ nanoparticles prepared by microemulsion route

A comparative study of pure CoFe₂O₄ nanoparticles and La-doped CoFe₂O₄ nanoparticles, prepared by microemulsion route has been performed. The samples were characterized using x-ray diffraction and transmission electron microscopy in order to obtain average particle size. The doping of small amount of La³⁺ ions (up to 3 molar %) causes significant reduction of the particle size using the identical preparation route. The samples were investigated by magnetization measurements, which revealed the coercivity values strongly dependent on particle size, but not significantly on level of La³⁺ doping. Detailed in-field Mössbauer spectroscopy studies were performed in order to determine spin canting angles and cation distribution within the spinel network. The non-negligible canting angles up to 40° in the La-doped samples were observed. The presence of the spin surface effects was also supported by magnetic measurement as the magnetization did not saturate even in considerably high magnetic fields (7 T). Moreover, significantly reduced values of the saturation magnetization were obtained. The observed features originated by the surface spin disorder in nanosized particles are explained in the frame of the core-shell model.