Effect of sample shape on hysteresis loops ofYBa2Cu3O7−xsingle crystals

We have found strong experimental evidence of the dependence of the shape of the magnetic hysteresis loops on the thickness d of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals when the width over the thickness is much larger than 1. For crystals with similar widths, the full penetration field ${\mathrm{\mu }}_0$ ${\mathit{H}}_{\mathit{p}}$, defined experimentally as the merging point between the virgin magnetization curve and the external envelope, increases as d increases. Similarly, the magnetic-field variation needed to reverse completely the magnetization increases with d. Our data are found to support the theoretical work performed by Clem and Sanchez using the Bean model for thin disks. Reverse legs measured at temperatures up to 70 K and for different reversal fields ${\mathrm{\mu }}_0$ ${\mathit{H}}_{\mathit{m}}$ up to 10 T are described well by this theory.