Spin-polarized tunneling study of s-f exchange in superconductors

Spin-polarized tunneling into the side of an Al film covered with a submonolayer of Gd reveals the presence of the localized Ruderman-Kittel-Kasuya-Yosida spin polarization in the normal state and its absence in the superconducting state. This result is attributed to the fact that the long-range part of the spin susceptibility in the superconductor vanishes at low temperature. The exchange constant is derived from the Zeeman splitting of the superconductor density of states, and its value agrees with that obtained from the Abrikosov-Gor'kov theory.