Spin-filter effect of ferromagnetic europium sulfide tunnel barriers

We have observed electron-spin polarization in the tunnel current in metal-EuS-metal junctions. The value of polarization can be as high as 85%. This polarization is attributed to the exchange splitting of the EuS conduction band as a result of the ferromagnetic transition occurring in EuS at about 15 K. This conduction-band splitting lowers the tunnel barrier height for spin-up electrons and raises it for spin-down electrons, giving rise to spin polarization in the tunnel current. Tunnel barrier lowering is also manifested in the observed decrease of junction resistance with temperature below 15 K. The exchange interaction between quasiparticles in Al and the magnetic ${\mathrm{Eu}}^{2+}$ ions in EuS greatly enhances the Zeeman splitting of the superconducting quasiparticle density of states; in fact, some junctions show Zeeman splitting corresponding to an internal field of ${\mathit{B}}_{\mathit{i}}^{\mathrm{*}}$=3 T before any external magnetic field is applied.