Vortex pinning in microindented YBa2Cu3O7-x single crystals

A high resolution magneto-optical technique with a magnetic indicator film was used to study the effect of a regular array of micrometer-sized mechanical indentations on magnetization dynamics at the surface of a twinned single crystal of YBa2Cu3O7-delta as a function of applied magnetic field, H-a (0 to +/- 68 mT) at temperatures in the range of 10-70 K. Direct visualization of the magnetic flux interaction with the individual indentations was observed for the first time. During magnetization at low H-a up to 15 mT at 60 K, the first row of indentations provides resistance to the flux front. In the indented area, flux is preferably pinned at the indentation sites. With increasing H-a, the vortex density at the indentations approaches the magnetic flux density in the surrounding material. During demagnetization down to H-a=0 mT, the outgoing flux is trapped by the indentations in areas where the density of the trapped flux is small; flux is also trapped along the (110) twin boundaries behind the indentations. For H-a=0 to +/- 10 mT, the average value of the magnetic induction gradient in the vicinity of an indentation is approximate to 0.4 mT/mu m at 20-60 K. The average induction gradient and, thus, the critical current density in the indented material is estimated to be 1.5-2 times greater than in the unindented material at temperatures from 40 to 70 K and H-a from 18 to 68 mT. (C) 2000 American Institute of Physics. [S0021- 8979(00)07715-X].