Spin-filtering in thin magnetic insulator barriers: EuO

The magnetic insulator Europium Oxide (EuO) has been recognized as a promising material for the generation of highly spin-polarized currents. Its spin-filtering property is due to exchange splitting of the conductance band below the ferromagnetic transition temperature Tc. Especially challenging is to achieve sizable magnetic ordering and exchange splitting in thin (< 6 nm) EuO films, which is the thickness regime of spin-filter tunnel barriers. In this work, we studied the magnetic and transport properties of 1-6 nm EuO films to explore their spin filtering efficiency. We found the EuO thickness being one of the primary factors scaling the Curie temperature Tc and the onset of the metal-insulator transition. We show, that the reduced magnetic ordering at interfaces due to structural and chemical intermixing becomes particularly relevant in the low thickness regime. Current-voltage measurements of EuO-based tunnel junctions showed a strongly bias-dependent, two-step Fowler-Nordheim-type tunneling characteristics. Moreover, we could determine an exchange splitting of φ=0.5 eV of the conduction band (CB) of 4nm EuO barriers by fully electrical means. The results show a direct correlation of the temperature-dependent change of the CB height with the EuO magnetic state.