Hysteresis, metastability, and time dependence in d = 2 and d = 3 random-field Ising systems

The hysteretic properties of random-field Ising model (RFIM) systems were studied. Using neutron scattering, a relatively narrow metastability boundary T/sub F/(H) in the d = 2 (d = d/sub l/) RFIM system Rb2Co/sub 0.85/Mg/sub 0.15/F4 was found to lie well below the rounded specific heat peak ''T/sub c/(H)'' of the (destroyed) phase transition. T/sub F/(H) scales as T/sub N/-T/sub F/(H)proportionalH/sup 2/phi/ with the RF crossover exponent phi = 1.74 +- 0.02. At T/sub F/(H), equilibrium is approached logarithmically with time. In the d = 3 (d>d/sub l/) RFIM system Fe/sub 0.6/Zn/sub 0.4/F2 AF order was found to be stable against changes of T and H below the observed sharp phase transition boundary T/sub c/(H), while the field-cooled domain state was not. Capacitance measurements in Fe/sub 0.68/Zn/sub 0.32/F2 established that equilibrium prevails for T > or = T/sub eq/(H) which is slightly above T/sub c/(H). Pronounced logarithmic time dependence is observed at T < or approx. =T/sub c/(H)