Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 $\times$ 1 surface in various sequences and during annealing of the formed structures to a temperature of 400$^\circ$C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe $3p$ and Co $3p$ electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe$_3$Si, Co–Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co–Si solid solution, Co, Fe–Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130$^\circ$C (for the Co/Fe/Si system) and $\sim$ 200$^\circ$C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe$_x$Co$_{1-x}$Si, Fe$_x$Co$_{2-x}$Si).