date:ECOLE SUPER PHYS & CHIM IND VILLE PARIS,F-75231 PARIS 05,FRANCE; MOSCOW MV LOMONOSOV STATE UNIV,MOSCOW 117234,RUSSIA
We have measured the EPR linewidth of a free radical layer as a function of height (0.28 to 10-mu-m) above the surface of an YBa2Cu3O7 superconducting ceramic. We find characteristic lengthscales of the order of 20-40-mu-m for the field distribution at the surface. This result rules out the Abrikosov vortex lattice as the main source of surface EPR broadening, which we assign to extrinsic sources such as the grain structure. In contrast, the width of the free radical line, when deposited on the surface of a Bi2Sr2CaCu2O8 single crystal, exhibits a significant broadening only below the temperature where irreversibility sets in.