LOCK-IN OSCILLATIONS IN MAGNETIC HYSTERESIS CURVES OF YBA2CU3O7-X SINGLE-CRYSTALS

We report the first direct experimental evidence of intrinsic pinning in ysteresis loops of untwinned YBa2Cu3O7-x single crystals, for magne tic fields perpendicular to the c axis. We have observed osciliations in the M-H loops that are well explained when one considers the layere d structure of this compound. The oscillations are found to be periodi c in H(a)-1/2 and are induced by a succession of transitions between s tates for which the vortex lattice is commensurate with the crystal la ttice periodicity in the direction perpendicular to the CuO2 planes. T he following anisotropy parameters were extracted from the experimenta l results: GAMMA(cb) = (m(c)/m(b))1/2 = 6.3, GAMMA(ca) = (m(c)/m(a)1/2 = 5.6, and GAMMA(ab) = (m(a)/m(b))1/2 = 1.13.