Fourfold anisotropy and structural behavior of epitaxial hcp Co/GaAs(001) thin films

Thin Cr-capped Co films (thickness 50--150 \AA{}) have been grown epitaxially on GaAs(001) single-crystal substrates by molecular-beam epitaxy. In contrast to other investigations, transmission electron diffraction reveals that the epitaxial Co films have a hexagonal-close-packed (hcp) structure with the epitaxial relationships (1\ifmmode\bar\else\textasciimacron\fi{}21\ifmmode\bar\else\textasciimacron\fi{}0)[0001]Co\ensuremath{\parallel}(001)[110]GaAs and (1\ifmmode\bar\else\textasciimacron\fi{}21\ifmmode\bar\else\textasciimacron\fi{}0)[0001]Co\ensuremath{\parallel}(001)[1\ifmmode\bar\else\textasciimacron\fi{}10]GaAs, i.e., the c axis of each crystallite is assigned to either the in-plane [110] or the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction of the GaAs(001) substrate. In-plane magneto-optical Kerr-effect hysteresis loops and Brillouin light-scattering measurements show that the thinnest epitaxial hcp Co film (50 \AA{}) has a dominant fourfold anisotropy with easy axes along the in-plane 〈100〉 directions and that a uniaxial anisotropy becomes dominant with increasing film thickness. We are able to account for the magnetic anisotropy properties of these epitaxial Co films in terms of their oriented hcp microstructure.