Theory of interplay of nuclear magnetism and superconductivity in AuIn2

The recently reported [S. Rehmann, T. Herrmannsd{umlt o}rfer, and F. Pobel, Phys. Rev. Lett. {bold 78}, 1122 (1997)] coexistence of a nuclear magnetic order, with the critical temperature T{sub M}=35 {mu}K, and superconductivity, with the critical temperature T{sub S}=207 mK, in AuIn{sub 2} is studied theoretically. It is shown that superconducting (S) electrons and localized nuclear magnetic moments interact predominantly via the contact hyperfine interaction, giving rise to a spiral or domainlike magnetic order in the superconducting phase depending on the strength of magnetic anisotropy. In clean samples (l{gt}{xi}{sub 0}) of AuIn{sub 2} the oscillatory magnetic order should produce a line of nodes in the quasiparticle spectrum of S electrons. The critical field H{sub c}(T=0) in the coexistence phase is reduced by a factor of 2 with respect to its bare value. {copyright} {ital 1997} {ital The American Physical Society}