  A novel spin density wave (SDW) instability mechanism enhanced by vortices
under fields is proposed to explain the high field and low temperature (HL)
phase in CeCoIn$_5$. In the vortex state the strong Pauli effect and the nodal
gap conspire to enhance the momentum resolved spectral weight exclusively along
the nodal direction over the normal value, providing a favorable nesting
condition for SDW with ${\bf Q}=(2k_F, 2k_F, 0.5)$ only under high field ($H$).
Observed mysteries of the field-induced SDW confined within $H_{c2}$ are
understood consistently, such facts that ${\bf Q}$ is directed to the nodal
direction independent of $H$, SDW diminishes under tilting field from the $ab$
plane, and the SDW transition line in $(H,T)$ has a positive slope.
