Transmission electron-spin resonance in copper-niobium sandwiches

Transmission electron-spin resonance of copper foil coated with a thin film of niobium is observed over a temperature range spanning the superconducting transition of the film. The effect of the niobium is to broaden the copper conduction-electron-spin resonance by an amount independent of temperature just above the transition and decreasing rapidly with temperature below the transition. The decrease is interpreted as a consequence of the inability of unpaired electrons in the copper to penetrate the copper-niobium interface unless they have sufficient thermal excitation energy to surmount the superconducting energy gap in the niobium. A conclusion from this interpretation is that the extra relaxation provided by the niobium is associated with rapid spin relaxation within the niobium, not with relaxation at the interface. Estimates are made of the interfacial transmission coefficient for an electron at the Fermi level and the spin-relaxation time in niobium.