Influence of oxidation on the spin-filtering properties of CoFe 2 O 4 and the resultant spin polarization

We report the direct measurement of spin polarization in epitaxial ${\text{CoFe}}_{2}{\text{O}}_{4}$ tunnel barriers using the Meservey-Tedrow technique. By observing an asymmetry in the Al quasiparticle density of states in $\text{Pt}(111)/{\text{CoFe}}_{2}{\text{O}}_{4}(111)/\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}(111)/\text{Al}$ tunnel junctions, we prove the existence of spin filtering in our ${\text{CoFe}}_{2}{\text{O}}_{4}$ tunnel barriers. We further analyze the effect of oxidation conditions during film growth on the polarization of the tunneling current, revealing an important role played by oxygen vacancies in the spin-filter efficiency of this material.