REVERSAL MECHANISMS AND THE TEMPERATURE-DEPENDENCE OF SIGMA-R/SIGMA-S,HC, AND HK IN PARTICLE ASSEMBLIES

Hysteresis loops of randomly oriented assemblies of acicular particles were measured at temperatures from 4 to 296 K in maximum fields of 24 kOe. The compositions were γ-Fe2O3, γ-Fe 2O3 with 7% Fe2+ (precursor for Co surface treatment), Co surface-treated precursor with 4, 8, and 16 wt. % Co, and Fe. Mean HK 's were determined from first quadrant data and from susceptibility at remanence. The precursor and the Co surface-treated particles had a similar temperature dependence for HC, σR/ σS, and HK. This suggests that the effective Co 2+ ions replace Fe2+ cations on the surface. The utility of the different mean HK determinations for indicating the H K distribution width is discussed, as well as the sensitivity to superparamagnetic fractions of the HK determined from susceptibility.