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

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Abstract
The hysteretic properties of random‐field Ising model (RFIM) systems were studied. Using neutron scattering, a relatively narrow metastability boundary TF(H) in the d=2 (d=dl) RFIM system Rb2Co0.85Mg0.15F4 was found to lie well below the rounded specific heat peak ‘‘Tc(H)’’ of the (destroyed) phase transition. TF(H) scales as TN−TF(H)∝H2/φ with the RF crossover exponent φ=1.74±0.02. At TF(H), equilibrium is approached logarithmically with time. In the d=3 (d>dl) RFIM system Fe0.6Zn0.4F2 AF order was found to be stable against changes of T and H below the observed sharp phase transition boundary Tc(H), while the field‐cooled domain state was not. Capacitance measurements in Fe0.68Zn0.32F2 established that equilibrium prevails for T≥Teq(H) which is slightly above Tc(H). Pronounced logarithmic time dependence is observed at T≲Tc(H).