Influence of MgO overlayers on the electronic states of Fe(001) thin films grown on GaAs(001)

The valence electronic states at the model tunneling junction interface MgO/Fe(001) system were systematically studied by comparing the spin-resolved photoemission spectroscopy of clean Fe(001) and MgO covered MgO/Fe(001) surfaces using linearly p-polarized light. For the clean Fe(001) film on GaAs(001), five distinct features including bulk and surface-related transitions are found. The bulk and surface-state transitions are well-accounted for by the direct transition model based on the calculated energy band structure of bcc bulk Fe(001). The previously observed minority feature at a binding energy E-B=-1.3 eV is reinterpreted as a surface roughness associated transition. Upon the MgO adsorption on Fe(001), the surface-state transitions at E-B=-0.3 eV below Fermi energy E-F appearing in both the majority and minority spin spectra at low photon energy (18 eV to 35 eV) were quenched. This is also the first direct experimental evidence of a minority spin surface state located just below the Fermi energy as predicted previously. The bulk states at the MgO/Fe(001) interface exhibit a layer-dependent modification, i.e., the bulk states in the deeper Fe layer(s) remain unaffected, while the states of Delta(down arrow)(5) band symmetry in the Fe layer(s) closest to the MgO/Fe(001) interface are strongly modified, in contrast to the states of Delta(up arrow)(1) symmetry. As a consequence of this interface effect, the partial spin polarization at the Fermi level changes sign from negative to positive values as seen in the spin asymmetry spectra at photon energies of 40 eV and 60 eV. The origin of this spin- and symmetry-dependent modification observed at MgO/Fe(001) interfaces is discussed.