Zero field viscosity in magnetic thin films and tapes

We have measured the relaxation of the remanent magnetization in a number o f granular thin films in which delta-M measurements suggest the presence of exchange coupling. The relaxation measurements were made at zero field aft er application of a saturation field followed by a reverse field. The films had varying compositions (CoPtCrB, Co/CN, COPt/SiO2) and had thermal stabi lity factors, KV/k(B)T, ranging from 29 to 670. The most general behavior o f the zero field relaxation, observed for the smaller thermal stability fac tors, was an initial increase in the remanent magnetization followed by a d ecrease at longer times. The maximum initial zero field viscosity is strong ly correlated with the inverse thermal stability factor, k(B)T/KV, and gene rally increases with the area under delta-M curve, normalized by the remane nt coercivity. A small zero field relaxation was also observed in barium fe rrite and CrO2 tapes.