Band structure tuning of Heusler compounds revisited: Spin- and momentum-resolved electronic structure analysis of compounds with different band filling
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$ν$=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 1.4Å$^{-1}$ 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.