DIRECT OBSERVATION OF LOCK-IN PINNING IN MAGNETIC HYSTERESIS LOOPS OFYBA2CU3O7-X SINGLE-CRYSTALS

We present the first direct experimental evidence of intrinsic pinning in hysteresis loops of untwinned YBa2Cu3O7-x single crystals, for app lied magnetic fields (H-a) perpendicular to the c-axis. Our magnetic h ysteresis measurements have revealed oscillations in the magnetisation which are periodic in H-a(-1/2). These oscillations are well explaine d when one considers the layered structure of this compound. The swept applied magnetic field causes a succession of transitions between sta tes for which the vortex lattice is commensurate with the crystal latt ice periodicity in the direction perpendicular to the CuO2 planes. The anisotropy parameters have been extracted from the periodicity of the oscillations: 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.