We use spin-resolved electron tunneling to study the exchange field in the Al component of $\mathrm{EuS}/\mathrm{Al}$ bilayers, in both the superconducting and normal-state phases of the Al. Contrary to expectation, we show that the exchange field $H_{\mathrm{ex}}$ is a nonlinear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore, the magnitude $H_{\mathrm{ex}}$ is unaffected by the superconducting phase. In addition, $H_{\mathrm{ex}}$ decreases significantly with increasing temperature in the temperature range of 0.1—1 K. We discuss these results in the context of recent theories of generalized spin-dependent boundary conditions at a superconductor-ferromagnet interface.

• Received 25 February 2011

DOI:https://doi.org/10.1103/PhysRevLett.106.247001