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 $2<h<\mathrm{}14\mathrm{}$ 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 $K^{\mathrm{eff}}\left(h\right)$, provides the first quantitative description for these general but previously unexplained phenomena.