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

The spin polarization of the valence band electronic states of strained bcc $\mathrm{Co}(001)$ and $\mathrm{MgO}∕\mathrm{Co}(001)$ thin films grown onto a bcc $\mathrm{Fe}(001)$ seed layer on $\mathrm{GaAs}(001)$ are investigated by employing spin-resolved photoemission spectroscopy. The experimental results are compared with the calculated energy band structure of bcc and bct $\mathrm{Co}(001)$, and discussed in the framework of the interband transition model, which allows one to ascribe the observed spectral features to bands of given spin and spatial symmetry. In contrast to the positive spin polarization observed at the $\mathrm{MgO}∕\mathrm{Fe}(001)$ interface, a large negative spin polarization of the electronic states at the Fermi level is observed for the $\mathrm{MgO}∕\mathrm{Co}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ system. Such a large negative spin polarization is attributed to a change in the energy band structure at the bct Co/bcc $\mathrm{Fe}(001)$ interface.