  A novel feature of the iron arsenides is the magnetoelastic coupling between
the long wavelength in-plane strains of the lattice and the collective spin
fluctuations of the electrons near the magnetic ordering wavevectors. Here, we
study its microscopic origin from an electronic model with nested Fermi pockets
and a nominal interaction. We find the couplings diverge with a power-law as
the system is tuned to perfect nesting. Furthermore, the theory reveals how
nematicity is boosted by nesting. These results are relevant for other systems
with nesting driven density wave transitions.
