MAGNETIZATION OF SUBMICRON SUPERCONDUCTING YBCO PARTICLES - ISOLATED SINGLE VORTICES VERSUS CRITICAL-STATE MODEL

Strong suppression of the ratio of the thermoremanent (M(TR)) and the field-cooled (M(FC)) magnetizations M(TR)/M(FC), exceeding two orders of magnitude, when decreasing the size of superconducting YBCO particl es from micron to submicron scale has been observed. A high saturation field (H-s > 8 kOe) of M(TR)(H) in submicron particles excludes the a pplicability of critical state models for interpretation of the result s. A model of trapping and relaxation of isolated single vortices in p articles having size D less than or similar to lambda (lambda is the m agnetic field penetration depth) is introduced showing that interactio n between a vortex and its mirror image results in a fast relaxation o f M(TR) with size-dependent relaxation time tau proportional to D-2. T he reduction of the remanence and its temperature and field dependenci es in submicron particles are explained by the fast relaxation resulti ng in vortex-free particles and threshold fields of vortex trapping.