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.