NMR spin-echo measurements of reversible and irreversible motion of a driven vortex lattice

We report measurements of flux-line motion that is driven by a periodic roc king of the external magnetic field through a small angle in the type-II su perconductor NbTi alloy filaments. The effect of the motion is to reduce th e height of the Nb-93 NMR spin-echo signal. By investigating a broad range of parameters that characterize the rocking motion and the NMR pulse sequen ce, the scaling behavior discussed earlier [W. G. Clark, F. Lefloch, and W. H. Wong, Phys. Rev. B 52, 7488 (1995)] is found to persist into a regime w here irreversible flux-line motion causes an additional reduction of the ec ho height. The earlier model is extended phenomenologically to include irre versible displacements of the axis about which each vortex rocks. A second scaling relation for this mechanism that is seen experimentally is containe d in the extended model. The results suggest that the irreversible part of the motion is driven by the rf pulse that forms the spin echo. [S0163-1829( 99)08501-X].