Influence of La3+substitution on the structure, morphology and magnetic properties of CoLaxFe2_xO4@SiO2 nanocomposites

Structural tuning of CoFe2O4 nanoparticles by doping with rare earth (La3+) ions stands out as a novel technique to tune their physical properties and to provide new nanocomposites for various applications. CoLaxFe2_xO4 (x = 0.0; 0.3; 0.6; 0.9; 1.2; 1.5) nanoparticles embedded in SiO2 (40 wt%) matrix were synthesized by sol-gel method and annealed at 400, 800 and 1200 degrees C. The effect of Fe3+ substitution by La3+ on the structure, morphology and magnetic properties of nano-structured Co-La ferrite embedded in the SiO2 matrix was investigated by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) surface area, atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and magnetic measurements. The annealing at low temperatures led to poorly crystalline ferrites, while the annealing at high temperatures directed highly crystalline ferrites accompanied by secondary phases. The crystallite size, dislocation density and porosity increased, while the lattice parameter, unit cell volume and hopping lengths decreased with the increase in La3+ content. By substituting Fe3+ ion with La3+ ion, the specific surface area decreased from 244 m2/g to 111 m2/g at 400 degrees C, reaching values below 0.5 m2/g at 800 and 1200 degrees C. The AFM revealed a spherical-shaped crystalline core covered by an amorphous SiO2 layer. The TEM images revealed spherical shape nanoparticles of size increasing with La3+ content and annealing temperature. The average particle size measured by AFM ranged between 14 nm and 72 nm, which is in good agreement with the XRD. SEM indicated that the ferrite nanoparticles were nearly spherical and had varying agglomeration degrees. The saturation magnetization, remanent magnetization, coercivity, squareness, magnetic moment per formula unit and anisotropy constant decreased with the increase in La3+ content and increased with the increase in crystallite size. By incorporating La3+ in the ferrite system, the gradual change from hard (CoFe2O4 @SiO2) to soft (CoLa1.5Fe0.5O4 @SiO2) magnetic ferrite occured. The magnetic properties of low La3+ content ferrites recommend them for a wide range of applications.