Step-induced anisotropy

For the cobalt/copper model system we have determined structural, elastic, magnetic, chemical, and confinement properties. We have found that step anisotropies oscillate with film thickness, that magnetic anisotropies are strongly influenced by minute amounts of adsorbates by decoration of step sites, and that quantum confinement leads to anisotropy oscillations. These results indicate that microstructures having a controlled lateral extent and yet unexplored magnetic characteristics can be produced. Such microstructures may well be of interest for such magnetic devices as spin-valve heads.

By growing epitaxial films on substrates with a well-defined step arrangement it is possible to study the effects of controlled "defects" such as atoms at step sites on structural and magnetic properties. For the Co/Cu(001) system shown in Figure 1, the steps in the substrate are replicated in the epitaxial film, shown schematically (top) and experimentally by STM images (bottom). The average step distance on the Cu substrate shown is 180 nm, and the thickness of the Co film grown on top is 8.9 monolayers (ML). The imaged area is 400 nm × 80 nm.

Magnetic hysteresis loops are completely different for a field applied parallel to the steps and perpendicular to the steps. In Figure 2, an easy axis hysteresis loop is observed parallel to the step edges (left), whereas a more complicated split hysteresis loop is characteristic for field perpendicular to the steps (right). The shift of the single hysteresis from the origin is proportional to the uniaxial anisotropy induced by the steps and is the most accurate method to determine anisotropy fields.

Measuring shifted hysteresis loops during growth of the Co film, Figure 3, allows us to investigate the evolution of magnetic anisotropy with increasing film thickness, and in particular to observe an oscillatory anisotropy arising from the repeated sequence of flat and rough morphology (complete and incomplete monolayers) during film growth. The entire sequence of hysteresis loops taken during growth allows one to observe the anisotropy oscillations, see below.

 References

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[2]	W. Weber, A. Bischof, R. Allenspach, Ch. Würsch, C.H. Back, D. Pescia, Phys. Rev. Lett. 76, 3424 (1996).
[3]	W. Weber, R. Allenspach, A. Bischof, Appl. Phys. Lett. 70, 520 (1997). J. Fassbender, A. Bischof, R. Allenspach, U. May, M. Lange, U. Rüdiger, G. Güntherodt, Surf. Sci. 488, 99 (2001).