Magnetization distribution in nearly isotropic, ultra-soft magnetic CoB amorphous thin films

An investigation was made of thin CoB films that belong to a subgroup of magnetically soft amorphous thin films (called ultrasoft films) that exhibit a number of anomalous responses, including reversible transitions in their bias-dependent transverse susceptibilities; low remanence; a rotatable anisotropy; nearly isotropic, in-plane susceptibilities; and highly damped, AC small-signal susceptibility spectra. The films were prepared by sequential RF cosputtering of cobalt and boron with an RF substrate bias of -60 V and postdeposition annealing at 275 degrees C for 30 min with an in-plane, rotating 1.8-kOe field. It is shown that the remanent state is qualitatively compatible with the presence of an ensemble of nanoscale spin vortices that minimize the total magnetostatic and exchange coupling energies in these heterogeneous films. The anomalous overdamped initial susceptibility spectra and reversible-bias susceptibilities observed in the ultrasoft films appear to be associated with the magnetic response of a microvortex magnetization distribution. The microvortex state is also consistent with the small remanence and rotatable anisotropy observed in the ultrasoft films. The anomalously large damping losses are consistent with a response requiring the participation of short-wavelength spin waves.>