Shifts in blocking temperature spectra for magnetite powders as a function of grain size and applied magnetic field

Partial thermoremanent magnetization (PTRM) spectra have been obtained for a variety of sized magnetite powders dispersed in an inert matrix, the PTRMs having been acquired in a wide range of magnetic fields. Peaks in the spectra, indicative of the range of blocking temperatures in the samples, shift downward from the vicinity of the Curie temperature as the field strength is increased, in qualitative accord with theory. Extremely small (100 nm) colloidal powders show a strong dependence of the shape of the PTRM spectrum on concentration within the inert matrix, with the blocking temperature spectrum rising dramatically with increased clustering of the powder particles. The rule of additivity of PTRMs is roughly satisfied by the samples with some significant deviations.