Magnetization reversal ofCo∕Ptmultilayers: Microscopic origin of high-field magnetic irreversibility

We investigate the magnetization reversal in ${[\mathrm{Co}\left(4\mathrm{\AA }\right)∕\mathrm{Pt}\left(7\mathrm{\AA }\right)]}_{50}$ multilayers with perpendicular magnetic anisotropy both macroscopically by the first-order reversal curve (FORC) technique and microscopically by transmission x-ray microscopy (TXRM) and resonant x-ray small angle scattering (SAS). In particular, we focus on the nucleation and saturation processes. The onset of magnetization reversal is dominated by irreversible processes corresponding to the avalanchelike propagation of one-dimensional stripe domains that originate from earlier nucleated zero-dimensional bubble domains. In a second stage we observe mainly reversible behavior where the overall domain topology is preserved. Finally another irreversible process brings the sample to negative saturation, corresponding to the contraction and annihilation of domains. Interestingly, even well beyond the apparent major-loop saturation field, the FORC diagram exhibits pronounced irreversible switching and thus provides a direct measure of the true (and significantly higher) saturation field. TXRM and SAS measurements reveal on a microscopic level that some residual bubble domains with negligible moments still persist for fields well above the apparent saturation field. These residual domains, if unannihilated, significantly alter the subsequent magnetization reversal.