Stable, metastable and unstable states in the mean-field random-field Ising model at T = 0

We compute the probability distribution of the number of metastable states at a given applied field in the mean-field random-field Ising model at T = 0. Remarkably, there is a non-zero probability in the thermodynamic limit of observing metastable states on the so-called ‘unstable’ branch of the magnetization curve. This implies that the branch can be reached when the magnetization is controlled instead of the magnetic field, in contrast to the situation for the pure system.