Detection of the valence band in buried Co2MnSi–MgO tunnel junctions by means of photoemission spectroscopy.

This work reports on the detection of the valence band of buried Heusler compounds by means of hard x-ray photoemission spectroscopy. The measurements have been performed on the so-called “half” tunnel junctions that are thin films of Co₂MnSi underneath MgO. Starting from the substrate, the structure of the samples is MgO(buffer)–Co₂MnSi–MgO(t_MgO)–AlO_x with a thickness t_MgO of the upper MgO layer of 2 and 20 nm. The valence band x-ray photoemission spectra have been excited by hard x rays of about 6 keV energy. The valence band spectra have been used to estimate the mean free path of the electrons through the MgO layer to be 17 nm at kinetic energies of about 6 keV. In particular, it is shown that the buried Co₂MnSi films exhibit the same valence density of states as in bulk samples. [ABSTRACT FROM AUTHOR]