Kerr Imaging of Co/Pt multilayers with perpendicular anisotropy

Co/Pt multilayer films are promising materials for applications such as in high density magnetic recording media. For very thin Co layers these structures exhibit perpendicular magnetization. The exchange coupling between adjacent Co layers is ferromagnetic and the coupling strength oscillates as the nonmagnetic Pt layer thickness (t$_{Pt})$ ranges from 3 to 75\r{ }A. In order to understand the magnetization reversal process in these structures, we report on magneto-optical images of a series of [Co(4 {\AA})/Pt(t$_{Pt})$]$_{N}$ multilayers obtained with Kerr microscopy as a function of t$_{Pt}$ and layer repetition N. The images reveal the evolution of the magnetic reversal process that strongly depends on t$_{Pt}$ and therefore on the interlayer coupling. For Co/Pt multilayers with small t$_{Pt}$, e.g., 11 {\AA}, when Co layers are strongly coupled, the whole multilayer switches as a single ferromagnet. As Co layers are farther separated and decoupled, e.g., at t$_{Pt}$ =41 {\AA}, Co layers switch independently and layer by layer switching was observed by Kerr imaging. The response of these distinct magnetic phases to external magnetic fields and their relationship to details of the hysteresis loops will be discussed.