  We theoretically investigate k-space structures of $d_{x^2-y^2}$-wave
superconducting (SC) and spin-density-wave (SDW) orders in their coexistent
phase induced by a paramagnetic pair-breaking (PPB) effect in relation to the
high field and low temperature (HFLT) SC phase in CeCoIn$_5$. It is shown that,
in k-space, the SDW order develops near the gap nodes where the SC order is
suppressed by PPB, and the nesting condition for the SDW ordering is satisfied.
By comparing the results in the $d_{x^2-y^2}$-wave SC model and those in an
artificial model with no sign change of the gap function in k-space with each
other, it is shown that the $d_{x^2-y^2}$-wave SC and SDW orders are enhanced
altogether in k-space due to the sign change of the $d_{x^2-y^2}$-wave gap
function there, and that this mutual enhancement largely stabilizes the
coexistence of these orders in real space. It is also discussed that the field
dependence of a SDW moment can be affected by the k-space structure of these
orders, which is dependent on the curvature of the Fermi surface.
