Growth of ultrathin epitaxial FexCo1−x alloy films on Si(0 0 1): stabilization of metastable bcc Co

Structural information on ultrathin films of FexCo1−x alloys grown epitaxially by coevaporation on Si(0 0 1) at room temperature is obtained by X-ray photoelectron diffraction, low-energy electron diffraction, inelastic medium-energy electron diffraction and transmission electron microscopy. The body-centered-cubic structure is found to be stabilized for alloy thin films up to very high concentration of Co beyond the thermodynamically stable bcc range (0.25⩽x⩽1). For alloy films with thickness up to 10 monolayers, the bcc structure with (0 0 1)[1 0 0]FexCo1−x//(0 0 1)[1 0 0]Si is actually observed over the entire range of alloy concentrations that includes elemental bcc Co. All films are ferromagnetic with in-plane magnetization. They show clear in-plane fourfold magnetic anisotropy for all alloy concentrations and the relevant cubic anisotropy constant shows a strong composition and thickness dependence.