Structure of magnetic order in Pauli limited unconventional superconductors

Abstract

We analyse the magnetic structure of the antiferromagnetic order induced in Pauli limited d-wave superconductors by Zeeman coupling to magnetic field. We determine the phase diagram in the H-T plane, and find that the magnetic phase, which is stabilized at low temperatures and just below the upper critical field, can have two realizations depending primarily on the shape of the underlying Fermi surface. The double-Q magnetic ordering may persist over the entire coexistence range. Alternatively, there may exist a weak first order transition from a double-Q structure at lower fields to a single-Q modulation at higher fields. Together with the calculations of the NMR line-shape these results suggest the second scenario as a serious candidate for describing the superconducting state of CeCoIn5.