Shifted magnetization curves in ultrathin Co films on stepped Cu(100)

Ultrathin ferromagnetic films of Co, grown on a stepped Cu(100) substrate, exhibit shifted magnetization curves. We calculate the shift field of a film consisting of N planes. Our model includes a four-fold bulk anisotropy k/sub 4/, a uniaxial anisotropy k/sub u/ and a magnetic field applied perpendicularly to the easy magnetization axis. Even if k/sub u/ is limited to the outermost plane, a finite shift field h/sub s/ exists, which, for k/sub u//spl Lt/k/sub 4/, is proportional to k/sub u//N. This result explains some experimental data.