  Using a combined approach of spin-resolved photoemission spectroscopy, band
structure and photoemission calculations we investigate the influence of bulk
defects and surface states on the spin polarization of Co2Mn(alpha)Si thin
films with bulk L21 order. We find that for Mn-poor alloys the spin
polarization at EF is negative due to the presence of Co_Mn antisite and
minority surface state contributions. In Mn-rich alloys, the suppression of
Co(Mn) antisites leads to a positive spin polarization at the Fermi energy, and
the influence of minority surface states on the photoelectron spin polarization
is reduced.
