Spin-polarized tunneling study of thin superconducting films in contact with magnetic insulators

Spin-polarized tunneling on thin aluminum films in contact with the oxides of several rare earth oxides is used to measure the Zeeman splitting of the superconducting density of states. The splitting is found to be 2μ(B+B*) where B is the magnetic field applied parallel to the film surface and B* is of the order of μM, the magnetization of the rare earth oxide. A possible explanation for the enhancement B* involves a coupling to either the magnetization or exchange field of the magnetic insulators via tunneling of quasiparticles into the insulator from the Al film. The magnitude of B* increases approximately inversely proportional to the film thickness of the Al-supporting such a proximity effect type model. The critical field of the bilayer is compared to that of a bare Al film as a function of the angle between the magnetic field and film surface.