On the possibility of antiferromagnetic vortex cores in superconducting YBa2Cu3O7-delta

Neutron scattering studies of optimally doped single crystal YBa2Cu3O7-delt a under applied magnetic field were used to assess the existence of antifer romagnetic (AF) correlations in vortices. In the superconducting (SC) state , a very weak signal at the (1/2 1/2 0) reciprocal lattice point was found. It is argued that this signal can be associated with AF correlations in vo rtex cores. An upper-limit average magnetic moment per vortex in each layer is estimated. Above T-c, weak paramagnetic scattering was observed, which we hypothesize is due to scattering from nearest-neighbor copper spins that are coupled ferromagnetically by a hole on an oxygen site between them. (C ) 2000 Elsevier Science B.V. All rights reserved.