Magnetic tunnel junctions with MgO-EuO composite tunnel barriers

The chalcogenide compound EuO is best known as a highly efficient spin-filter tunnel barrier material. Using the molecular beam epitaxy method, we combine polycrystalline EuO with epitaxial MgO and construct magnetic tunnel junctions with such hybrid tunnel barriers. Tunnel magnetoresistance of over 40$%$ was achieved in junctions with oxygen-rich EuO. For lower oxygen concentration, magnetoresistance decreases dramatically and eventually vanishes, indicating that spin filtering is weakened when the transport is mainly mediated by excess conduction channels through defect sites.