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 $\mathrm{MgO}∕\mathrm{Fe}(001)$ system were systematically studied by comparing the spin-resolved photoemission spectroscopy of clean $\mathrm{Fe}(001)$ and $\mathrm{MgO}$ covered $\mathrm{MgO}∕\mathrm{Fe}(001)$ surfaces using linearly $p$-polarized light. For the clean $\mathrm{Fe}(001)$ film on $\mathrm{GaAs}(001)$, 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 $\mathrm{Fe}(001)$. The previously observed minority feature at a binding energy ${E}_{B}=\ensuremath{-}1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is reinterpreted as a surface roughness associated transition. Upon the $\mathrm{MgO}$ adsorption on $\mathrm{Fe}(001)$, the surface-state transitions at ${E}_{B}=\ensuremath{-}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below Fermi energy ${E}_{F}$ appearing in both the majority and minority spin spectra at low photon energy ($18\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to $35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) 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 $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface exhibit a layer-dependent modification, i.e., the bulk states in the deeper Fe layer(s) remain unaffected, while the states of ${\ensuremath{\Delta}}_{5}^{\ensuremath{\downarrow}}$ band symmetry in the Fe layer(s) closest to the $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface are strongly modified, in contrast to the states of ${\ensuremath{\Delta}}_{1}^{\ensuremath{\uparrow}}$ 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 $40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $60\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The origin of this spin- and symmetry-dependent modification observed at $\mathrm{MgO}∕\mathrm{Fe}(001)$ interfaces is discussed.