  Feedback stabilization of an ensemble of non interacting half spins described
by Bloch equations is considered. This system may be seen as a prototype for
infinite dimensional systems with continuous spectrum. We propose an explicit
feedback law that stabilizes asymptotically the system around a uniform state
of spin +1/2 or -1/2. The proof of the convergence is done locally around the
equilibrium in the H^1 topology. This local convergence is shown to be a weak
asymptotic convergence for the H^1 topology and thus a strong convergence for
the C^0 topology. The proof relies on an adaptation of the LaSalle invariance
principle to infinite dimensional systems. Numerical simulations illustrate the
efficiency of these feedback laws, even for initial conditions far from the
equilibrium
