Characterization of Ultrathin Fe--Co Layer Grown on Amorphous Co--Fe--B by In situ Reflective High-Energy Electron Diffraction

The textured MgO(001) tunnel barrier grown on CoFeB is a fundamental building block for spintronic devices such as magnetic tunnel junctions. Although the insertion of an ultrathin Fe--Co layer between an MgO layer and a bottom CoFeB layer is a common technique for improving the magnetoresistance effect, the characteristics of this technique remain unclear. We systematically investigated the as-grown structure of Fe--Co by reflective high-energy electron diffraction and found that a highly textured MgO(001) is formed only on an amorphous Fe--Co surface. The diffusion of B atoms from underlying CoFeB into Fe--Co is what makes the as-grown Fe--Co layer amorphous.