Thermoremanence and zero-field-cooled/field-cooled magnetization study of Co_x(SiO₂)_1-x granular films

A systematic study of Co(SiO₂) granular films by means of transmission electron microscopy (TEM), dc and ac initial magnetic susceptibility, and thermoremanent magnetization (TRM) is presented. The experimental results are compared with simulations of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length λ, which results in a larger average "magnetic" size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent "particle clusters" of volume λ³, involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature 〈T_B〉 and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.