INFLUENCE OF THE DOPING ON THE TIME-DEPENDENT MAGNETIC-BEHAVIOR OF FINE PARTICLES

Time dependent magnetic phenomena are analysed by measuring the change s of magnetization with time in the presence of a constant field on fi ne particles containing various percentage of dopants. The samples ana lysed are: iron oxides (Co-doped), CrO2 (Fe/Sb- and Ir-doped) and Ba f errite (Co/Ti-doped) particles. The doping causes a large increase of the coefficient S which characterizes the magnetic viscosity, and an i ncrease of the thermal fluctuation field H-f. The ''activation volume' ' V-ac, which is considered as the region of greatest magnetic instabi lity in the particle and the ''switching volume'' V-c decrease with do ping. These effects are principally attributed to the increase of the anisotropy directions distribution inside the particles and to the evo lution of the magnetization reversal mode in the modified particles.