  The minority-spin Fe/MgO interface states are at the Fermi level in density
functional theory (DFT), but experimental evidence and GW calculations place
them slightly higher in energy. This small shift can strongly influence
tunneling magnetoresistance (TMR) in junctions with a thin MgO barrier and its
dependence on the concentration of Co in the electrodes. Here, an empirical
potential correction to DFT is introduced to shift the interface states up to
match the tunnel spectroscopy data. With this shift, TMR in Fe/MgO/Fe junctions
exceeds 800% and 3000% at 3 and 4 monolayers (ML) of MgO, respectively. We
further consider the effect of alloying of the Fe electrodes with up to 30% Co
or 10% V, treating them in the coherent potential approximation (CPA). Alloying
with Co broadens the interface states and brings a large incoherent
minority-spin spectral weight to the Fermi level. Alloying with V brings the
minority-spin resonant states close to the Fermi level. However, in both cases
the minority-spin spectral weight at the Fermi level resides primarily at the
periphery of the Brillouin zone, which is favorable for spin filtering. Using
convolutions of $\mathbf{k}_\parallel$-resolved barrier densities of states
calculated in CPA, it is found that TMR is strongly reduced by alloying with Co
or V but still remains above 500% at 4 ML of MgO up to 30% of Co or 5% V. At 5
ML, the TMR increases above 1000% in all systems considered. On the other hand,
while TMR declines sharply with increasing bias up to 0.2 eV in the MTJ with
pure Fe leads, it remains almost constant up to 0.5 eV if leads are alloyed
with Co.
