Origin of the irreversible microwave absorption versus the state of vortex matter in Bi2Sr2CaCu2Ox single crystals

We have investigated properties of hysteretic microwave absorption (MWA) by the Bi2Sr2CaCu2Ox single crystals in the temperature range from 10 K to T-c. It was found that the shape of the MWA hysteresis loop and its dependence on the magnetic field modulation amplitude undergo essential changes with temperature. The analysis has shown that near 50 K, the origin of the signal detected by electron spin resonance spectrometer with magnetic field modulation changes. It is due to crossover to another pinning type. At T < 50 K where the bulk pinning is the most effective pinning mechanism, the signal registered by a spectrometer is the magnetically modulated MWA. At high temperatures, 50 K < T < T-c, when vortices are pinned by the surface or the edge barrier, the signal is the derivative of the absorption power with respect to field dP/dH. The shape of the MWA hysteresis loop and its modulation enable one to determine unambiguously the pinning type (bulk or surface) controlling the irreversible properties of a superconductor. (C) 2002 Elsevier Science B.V. All rights reserved.