Magnetic Domain Structure in Ultrathin Films

The first magnetic force microscope (MFM) images of a series of epitaxial magnetic thin films is presented. The films studied, Ni $/$Cu $/$Si(001) capped by 2 nm of Cu, exhibit perpendicular magnetization over an exceptionally broad Ni thickness range $2<h<14$ nm. The Ni domain structure shows a sharp transition to a finer length scale above a finite critical thickness of order 9 nm. The average force measured by the MFM tip reflects this refinement in domain structure. Micromagnetic theory, combined with our measurements of Kᵉᶠᶠ(h), provides the first quantitative description for these general but previously unexplained phenomena.