Magnetic proximity effect at a superconductor–rare-earth oxide interface

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Abstract
Using spin-polarized electron tunneling, we have observed an enhancement of an applied magnetic field in a thin superconducting Al film in contact with an oxide of a rare-earth metal. The Zeeman splitting of the superconducting density of states of this Al film is 2μ(B+B*), where B is the applied field and B* is of the order of the magnetization of the rare-earth oxide. The field is applied parallel to the plane of the film. Oxides of Eu, Gd, Tb, Dy, and Er have produced this effect. The enhancement can be quite large: an Al film in contact with Gd2O3 in an applied field of 1.5 T behaves as though it were in a field of over 4 T. Both S-I-S tunneling and critical field measurements provide quantitative confirmation of the enhancement. A proximity model by de Gennes accounts qualitatively for some but not all of the observations, and quantitative aspects of the experiment are not understood. This proximity effect may provide a method of studying the surface magnetism of insulating magnetic materials in an applied magnetic field.