Spin polarization $\left(P\right)$ of the electron tunnel current is observed for a ferromagnetic $\mathrm{EuO}$ tunnel barrier between nonferromagnetic electrodes. Spin filtering due to different barrier heights for the spin-up and spin-down electrons gives rise to $P$, which was measured using a superconducting Al electrode as the spin detector. A large internal exchange field is exerted by the ${\mathrm{Eu}}^{2+}$ ions on the $\mathrm{Al}$ quasiparticle density of states, evidenced by enhanced Zeeman splitting even at zero applied field. Using a ferromagnetic barrier to filter spins has potential as a means for injecting a polarized current into semiconductors. Our $\mathrm{EuO}$ films show structure, magnetic moment, and Curie temperature matching bulk $\mathrm{EuO}$.

• Received 19 April 2004

DOI:https://doi.org/10.1103/PhysRevB.69.241203