Vortex avalanches in the vortex lattice phase of Bi2Sr2CaCu2O8+y

Local magnetization and local permeability in the mixed state of Bi2Sr2CaCu 2O8+y single crystals have been measured by using a micro-flail-probe. When temperature is decreased in the vortex lattice phase, local magnetic induc tion shows step-like decrease, indicating that vortices are expelled from t he sample discontinuously. This behavior suddenly disappears at the so-call ed depinning temperature. In addition to this, broad band low-frequency noi se in the local permeability is observed in the region where the discontinu ous vortex expulsion occurs. The noise spectrum shows a power-law behavior. Magnetic field dependence of the local permeability suggests that these ph enomena are related to the bulk pinning. The discontinuous vortex expulsion and the power-law behavior of the local permeability noise are consistentl y explained by the vortex avalanches in the vortex lattice phase. (C) 2000 Elsevier Science B.V. All rights reserved.