REVERSIBLE MAGNETIZATION AND ANISOTROPY IN YBA2CU4O8

The reversible magnetization for H parallel to ab has been studied for grain-aligned YBa2Cu4O8 in order to determine whether the Hao-Clem th eory can properly describe the vortex state in this orientation, as we ll as the orientation for H parallel to c. Between temperatures of 70 and 76 K, the magnetization is reversible to better than 2% and the re sults scale very well to the universal Hao-Clem curve even though the supercurrents have a highly noncircular path as they circulate around a vortex parallel to the ab plane. The thermodynamic critical field is the same for H parallel to ab as for H parallel to c, as expected, an d the Ginzburg-Landau parameter kappa(ab) = 380 compared to kappa(c) = 70 is consistent with an anisotropy value gamma = 5.8 +/- 0.4.