Anisotropic Upper Critical Field of LuN${\mathrm{i}}_2$${\mathrm{B}}_2$C

The upper critical field, $H_{c2\mathrm{}}$, of LuN${\mathrm{i}}_2$${\mathrm{B}}_2$C 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 $H_{c2\mathrm{}}$ 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.