  Spin injection from Co70Fe30 and Fe contacts into bulk GaAs(001) epilayers is
studied experimentally. Using nonlocal measurements, the spin polarization of
the differential conductance is determined as a function of the bias voltage
applied across the injection interface. The spectra reveal an interface-related
minority-spin peak at forward bias and a majority-spin peak at reverse bias,
and are very similar, but shifted in energy, for Co70Fe30 and for Fe contacts.
An increase of the spin-injection efficiency and a shift of the spectrum
correlate with the Ga-to-As ratio at the interface between CoFe and GaAs.
