Neutron spin depolarization by vortices in YBa2Cu307

The neutron spin depolarization technique has been used to investigate the mixed state in a single crystal of YBa2Cu3O7. The polarization of the transmitted beam has been measured as a function of temperature and field in both H parallel to c and H perpendicular to c configurations. When the sample is cooled in a constant external field, no depolarization is observed for fields up to 0.4 T. However a variation of the field from a zero field cooled state causes a strong depolarization of the neutrons even for moderate fields. The beam can recover an increasing polarization if the temperature is then raised above a characteristic temperature