Entropy induced fragmentation of the locked vortex state in YBa2Cu3Oy single crystals

Using a high angular resolution vector magnetometer we have studied the components of the magnetic moment parallel and transverse to the CuO-planes. The full shielding of the transverse field component originating from the infinite tilt modulus of the vortex locked state, is observed for the lowest fields only. We relate the suppression of the transverse shielding to entropy induced fragmentation of the locked stare by thermally activated kinks. We introduce a simple model that accounts for the reduced shielding as well as describing the angular behaviour of the critical current.