Mechano-synthesis, characterization, and magnetic properties of nanoparticles of cobalt ferrite, CoFe2O4

Cobalt ferrite, CoFe2O4, nanoparticles have been obtained from pre-prepared layered double hydroxide carbonate, LDH-CO3, by mechanical milling. X-ray powder diffraction shows the only product of the milling for 5 h of the LDH-CO3 is cobalt. ferrite nanoparticles. Longer-term milling induces particle growth characterized by sharpening of the Bragg peaks a-lid an increase of the blocking temperature, while prolonged milling results in the formation of some cobalt metal. Mossbauer spectra and temperature dependence of the magnetization of the 5-h milled sample suggest that it consists of nanoparticles of size less than 10 nm with blocking temperature of 200 K, in good agreement with microscopy showing an average size, of 6 nm. The magnetic properties exhibit a strong dependence on the particle size as a result of an unusual cation distribution and of surface effect. The-saturation magnetization at 5 K and the squareness of the hysteresis loops increase with the average particle size. The difference between zero-field-cooled and field-cooled hysteresis loops is correlated with the presence of randomness of the moments at the surface.