Flux line matching effects in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ thin films

We present internal friction (IF) measurements in magnetic fields up to 1.6 T of epitaxial ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ thin films with different thicknesses. Measurements at constant temperature and varying field exhibit peaks in the IF at particular values of the field for H perpendicular to the crystallographic c axis. Assuming a rectangular flux line arrangement and taking the anisotropy of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ into account the positions of the peaks coincide with fields where the distance in c direction of the flux line rows becomes an integral fraction of the sample thickness. This matching effect is valid for different film thicknesses and seems to be independnet of temperature.