  A system of magnetic molecules coupled to microwave cavities ($LC$
resonators) undergoes the equilibrium superradiant phase transition. The
transition is experimentally observable. The effect of the coupling is first
illustrated by the vacuum-induced ferromagnetic order in a quantum Ising model
and then by the modification of the magnetic phase diagram of ${\rm Fe_8}$
dipolar crystals, exemplifying the cooperation between intrinsic and
photon-induced spin-spin interactions. Finally, a transmission experiment is
shown to resolve the transition, measuring the quantum electrodynamical control
of magnetism.
