  In this article, we consider fixed spin 1/2 particles interacting through the
quantized electromagnetic field in a constant magnetic field. We give some
asymptotic expansions for the ground state and the ground state energy of the
Hamiltonian operator $H(h)$ describing this system. The first terms of these
expansions enable to recover elementary formulas for the energy and the
magnetic field of the spins when considered as magnets. A first order radiative
correction is computed for the energy.
