  The implications of the spin exciton mechanism are exposed in the context of
a Spin Density Wave (SDW) instability occurring inside the superconducting
phase of a layered heavy electron compound. In this model a magnetic field
serves as a tuning parameter bringing the system to the point where the
transverse spin correlations are enhanced due to dx2 -y2 -wave
superconductivity and induces an instability to a phase with coexisting
superconductivity and SDW order. The model considers electrons in a crystal
with antiferromagnetic interactions and provides restrictions on the Fermi
surface characteristics and on the ordering wave-vector (that can be
commensurate or incommensurate close to commensuration). The applications of
the model are addressed to the low-temperature/high-magnetic-field phase of
CeCoIn5 [M. Kenzelmann et al., Science 321, 1652 (2008), Phys. Rev. Lett. 104,
127001 (2010)].
