ANISOTROPIC FLUX-PINNING AND UPPER CRITICAL FIELDS OF THE HEAVY-FERMION SUPERCONDUCTOR URU2SI2

Using the Hall-probe technique, the magnetization anisotropy of URu2Si 2 was studied as a function of magnetic field and temperature. The flu x-pinning force, F-p(T, H), is anisotropic and for H parallel to a-axi s turns out to be strongly enhanced at large field. The temperature de pendence of H-c2 reveals the unusual nature of this compound: an uncon ventional order parameter is present or interactions between magnetism and superconductivity occur.