Heusler compounds are promising candidates for spintronics applications due to their predicted minority spin gap at the Fermi energy. For the functionality of tunneling magnetoresistance (TMR) devices the interface spin polarization is of utmost importance. However, even though large TMR ratios have been obtained with MgO barriers at low temperatures [1], the predicted bulk half metallicity has not been observed at the clean surface so far. In this contribution we present spin resolved low energy photoemission data as well as results from LEED and Auger spectroscopy of  fully epitaxial Co₂MnSi films deposited on MgO. The spectra suggest the presence of a Mn-Si terminated surface after sputter-cleaning and annealing to 600°C. The presence of minority surface states leads to a strong suppression and even inversion of the surface spin polarization at the Fermi energy, in good agreement with our theoretical calculations. We find that these states vanish upon deposition of a 2 nm MgO tunnelling barrier, yielding a maximum spin polarization of 30% after annealing to 500°C.

[1] S. Tsunegi, Y. Sakuraba, M. Oogane, K. Takanashi, and Y. Ando, Appl. Phys. Lett. 93, 112506 (2008)

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1. Investigations of metallo-phthalocyanines grown on ferromagnetic Co((001) thin films.
2. Tailoring the spin properties of metal/organic interfaces by alkali metal doping