Anisotropic upper critical field of LuNi2B2C

The upper critical field, H-c2, Of LuNi2B2C has been determined from the temperature dependence of the magnetization. A temperature dependent anisotropy within the basal plane which decreases from a value of 1.1 at 4.5 K to a value of 1.0 at T-c and an almost temperature independent out-of-plane anisotropy have been observed. Near T-c the upper critical field along all directions shows a strong upward curvature. These features of H-c2 can be explained quantitatively using a nonlocal extension of the Ginzburg-Landau equations. The Fermi velocity averages determined from these measurements and those obtained in band structure calculations are in good agreement.