  We present a technique for engineering quantum magnets via ultracold polar
molecules in optical lattices and explore exotic interplay between its spin
superfluidity and solidity. The molecular ground and first excited rotational
states are resonantly coupled by a linearly polarized microwave field. The
spin-up (spin-down) states are presented by molecular rotational states of
electric dipole moment along (against) the coupling field. By controlling the
angle between the lattice direction and the coupling field, the inter-site
interaction can be tuned from antiferromagnetic to ferromagnetic. Furthermore,
the dipole-dipole interaction induces an exotic interplay between spin
superfluidity and solidity, and spin supersolid phases may appear in mediate
regions.
