Epitaxial growth of Fe and MgO layers on GaAs (001): Microstructure and magnetic property

Microstructure and the corresponding magnetic property of epitaxial MgO and subsequent Fe layers in situ grown on GaAs(001) substrates are investigated as a function of the epitaxial layer thicknesses. It is found that the MgO layers retain a 1.6 nm thick Mg-rich amorphous layer at the initial stage of the MgO growths regardless of its total thickness. Systematic x-ray diffraction analysis reveals that the MgO layers are under in-plane compressive strain which is partially relaxed as the total MgO thickness increases from 4 to 20 nm. The misfit strain within the MgO layers results in three-dimensional Fe islands forming at lower thickness with two different zone axis of [010] or [11¯0]. Furthermore, the islands coalesce as the nominal thickness increases, resulting in serpentine-shaped Fe islands. Finally, it completely covers the underlying MgO layers, forming an epitaxial Fe layer at the nominal thickness higher than 10 nm. The three-dimensional Fe islands at the initial stage show superparamagnetism, which becomes ferromagnetic as the Fe forms a two-dimensional layer.