  The field dependence of the peak temperature of the zero-field-cooled (ZFC)
magnetization curve of a magnetic nanoparticle system was studied using a
diluted magnetic fluid composed of FePt nanoparticles. It is found that the
peak temperature increases with increasing applied field below 3 kOe; it then
decreases with further increasing the applied field. The non-monotonic field
dependence of the peak temperature in magnetic particle systems is attributed
to the anisotropic energy barrier distribution of the particles, and to the
slowly decreasing magnetization (or to the non-Curies law dependence of
magnetization) above the blocking temperature. The non-Curies law dependence of
the magnetization is caused by large magnetic anisotropy and Zeeman energy of
particles in high magnetic fields. Numerical simulation results, based on basic
thermodynamics, and pure thermal relaxation and energy barrier distribution
extracted from the low-field, experimental ZFC data show a good agreement with
experimental results.
