  Spin-filter tunneling is a promising way to generate highly spin-polarized
current, a key component for spintronics applications. In this work we explore
the tunneling conductance across the spin-filter material CoFe2O4 interfaced
with Au electrodes, a geometry which provides nearly perfect lattice matching
at the CoFe2O4/Au(001) interface. Using density functional theory calculations
we demonstrate that interface states play a decisive role in controlling the
transport spin polarization in this tunnel junction. For a realistic CoFe2O4
barrier thickness, we predict a tunneling spin polarization of about -60%. We
show that this value is lower than what is expected based solely on
considerations of the spin-polarized band structure of CoFe2O4, and therefore
that these interface states can play a detrimental role. We argue this is a
rather general feature of ferrimagnetic ferrites and could make an important
impact on spin-filter tunneling applications.
