Magnetism of sol-gel fabricated CoFe_2O_4/SiO_2 nanocomposites

摘要

Details of synthesis and characterization of sol-gel-produced CoFe_2O_4 nanoparticles embedded in the amorphous SiO_2 matrix are presented together with results of an extended magnetization study of these materials. The particle size was found to increase from 6 to 15 nm by varying the temperature of a subsequent annealing from 800 to 1100℃. All samples exhibited superparamagnetic behavior with values of the blocking temperature T_B increasing with the particle size. At temperatures above T_B the magnetization curves follow the expected Langevin scaling of M vs H/T, which is consistent with the formation of the superparamagnetic state. For T < T_B, the coercivity field H_c was found to be proportional to T~(1/2) and the frequency-dependent ac susceptibility was found to obey the Neel-Arrhenius law. Both observations are compatible with a model of noninteracting randomly oriented single-domain particles.