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Magnetization measurements have been performed on samples composed of oriented YBa 2 Cu 3 O 7 single crystals 1.4 K, 68 K and 78 K in magnetic fields up to 38 T applied along the c -axis and within the ab -plane. The critical currents are deduced from the irreversible magnetization and discussed in terms of pinning forces. At low temperature (1.4 K), the experimental data with the field along the c -axis are compared with a theoretical model for the field dependence of the pinning force. At higher temperatures (68 K and 78 K), the pinning force can be described by a Kramer-like scaling law: F p = A [ B 0 ( T )] m b p (1− b ) q , where B 0 is the field where the irreversibility of the magnetization disappears and where b (= B / B 0 ) is the reduced field. For the exponent m , the usual value of 2.5 is found; approximate values for p and q are around 2 and 4, respectively.

The magnetization of high- T c superconducting REBa 2 Cu 3 O 7 compounds, where RE is Y or one of the rare-earth elements Eu, Gd, Dy, Ho, Er or Yb, has been measured in fields up to 29 T. Though the precise analysis of the data is hampered by the incomplete penetration of the magnetic field due to the shielding-current effect of the superconductivity, the magnetism associated with the rare-earth ions can reasonably be interpreted by crystal-field effects. All of the samples investigated were confirmed to be superconducting at 4.2 K in fields as high as 29 T.