  Spin-filtered time-of-flight photoelectron momentum microscopy reveals a
systematic variation of the band structure within a series of highly
spin-polarized ferromagnetic Heusler compounds with increasing number of
valence electrons (Co2MnGa, Co2MnSi and Co2Fe0.4Mn0.6Si). The positions of the
Fermi energy for minority and majority electrons deviate strongly from a simple
band-filling model. Photoexcitation at h$\nu$=6.05 eV (4th harmonic of a
Ti:sapphire laser) gives access to the spin-polarization texture P(EB,kx,ky) of
the bulk bands in a (kx,ky)-range with diameter 1.4{\AA}$^{-1}$ and energies
from the Fermi energy EF to a binding energy of EB=2 eV. The minority bands of
Co2MnGa cross the Fermi level, inhibiting half-metallicity; the crossing points
allow a precise adjustment of experimental and theoretical majority and
minority bands, requiring shifts in opposite directions. The top of the
minority band lies only 0.15 eV above EF, i.e. Co2MnGa is much closer to being
half-metallic than predicted by calculations. For half-metallic Co2MnSi and
Co2Fe0.4Mn0.6Si clear minority band gaps are visible, the topmost occupied
minority bands lie 0.5 and 0.35 eV below EF, in reasonable agreement with
theory; the exchange splitting is significantly smaller than in theory. The
comparison of all three compounds uncovers the surprising fact that with
increasing number of valence electrons the frontier majority bands (close to
EF) exhibit an increasing deficiency in filling, in comparison with the
prediction of a DFT calculation. The same trend is visible in comparison with a
DMFT calculation. For s-polarized excitation both half-metallic compounds
exhibit nearly complete positive spin polarization close to EF, consistent with
previous work in literature.
