Effects of sample size on magnetic properties of Bi ₂Sr ₂CaCu ₂O _8+δ

The magnetic moment and relaxation rates in Bi2 Sr2 Ca Cu2 O8+δ crystals exhibit remarkable dependence on the sample size in the vicinity of the vortex order-disorder phase transition line. Specifically, we find that: (1) the magnitude of the normalized magnetic moment increases as the sample size decreases, (2) the relaxation rate increases with the sample size, and (3) the difference between the relaxation rates in samples of different size, decreases with temperature and field. These phenomena are attributed to dynamic coexistence of quasi-ordered and metastable disordered vortex states in the samples, in the temperature and field range near the vortex phase transition.