Scaling of the equilibrium boundary of three-dimensional random-field Ising-model systems

The onset of nonequilibrium behavior in three-dimensional random-field Ising-model (RFIM) systems (Feₓ{Zn}_{1{{-}}x}F₂; x=0.46,0.72) was studied via the capacitance method, using zero-field-cooling, field-cooling, and reverse-field-cooling techniques, in fields H{≤}100 kOe. Equilibrium was found to occur only above a boundary Teq(H), which lies slightly above the sharp phase transition Tc(H). Like TN-Tc(H), TN-T_eq(H)αH^2/φ, after mean-field corrections; {φ}=1.40±{}0.05 is the random-field crossover exponent. Thus, the onset of nonequilibrium is tied to rf critical behavior. The scaling property of Teq(H) follows directly from Villain's criterion for the breakdown of linear response in RFIM systems.