Zero field viscosity in magnetic thin films and tapes

We have measured the relaxation of the remanent magnetization in a number of granular thin films in which delta-M measurements suggest the presence of exchange coupling. The relaxation measurements were made at zero field after application of a saturation field followed by a reverse field. The films had varying compositions (CoPtCrB, Co/CN, CoPt/SiO₂) and had thermal stability factors, KV/k_BT, ranging from 29 to 670. The most general behavior of the zero field relaxation, observed for the smaller thermal stability factors, was an initial increase in the remanent magnetization followed by a decrease at longer times. The maximum initial zero field viscosity is strongly correlated with the inverse thermal stability factor, k_BT/KV, and generally increases with the area under delta-M curve, normalized by the remanent coercivity. A small zero field relaxation was also observed in barium ferrite and CrO₂ tapes