We report ${}^{27}\mathrm{Al}$ Knight shift ( ${}_{}{}^{27}{}_{}{}^{}K$) measurement on a single-crystal ${\mathrm{UNi}}_2{\mathrm{Al}}_3$ that reveals a coexistence of superconductivity and a spin-density-wave (SDW) type of magnetic ordering ( $T_{\mathrm{SDW}}=4.5\mathrm{}\mathrm{K}$). The spin part of ${}_{}{}^{27}{}_{}{}^{}K$, ${}^{27}{K}_{\mathrm{s}}$, does not change down to 50 mK across the superconducting (SC) transition temperature $T_c\sim 0.9\mathrm{K}$. In contrast with the isostructural compound ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ ( $T_c\sim 2\mathrm{K}$), which was identified to be a spin-singlet $d$-wave superconductor, the behavior of ${}_{}{}^{27}{}_{}{}^{}K$ strongly supports that ${\mathrm{UNi}}_2{\mathrm{Al}}_3$, like ${\mathrm{UPt}}_3$ and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, belongs to a class of spin-triplet SC pairing state superconductors.

• Received 12 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.89.037002