Critical behavior and metastability of the magnetization in the random-field Ising system ${\mathrm{Fe}}_{0.7}$${\mathrm{Mg}}_{0.3}$${\mathrm{Cl}}_2$

Optical Faraday rotation was used to measure the uniform magnetization M vs T of the random-field Ising-model (RFIM) system ${\mathrm{Fe}}_{0.7}$ ${\mathrm{Mg}}_{0.3}$ ${\mathrm{Cl}}_2$ in fields 65≤H≤500 mT. In disagreement with previous specific-heat results, but in accordance with the ‘‘standard’’ RFIM system ${\mathrm{Fe}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Zn}}_{\mathrm{x}}$ ${\mathrm{F}}_2$, critical exponents φ=1.41±0.05 and α̃∼0 are obtained from (δM/δT${)}_H$ and (δM/δH${)}_T$. Dynamical rounding due to the finite measurement time scale, τ∼100 s, occurs below ‖t‖∼0.003. Excess magnetization ΔM is found in the field-cooled domain state in agreement with previous computer simulations. Scaling as ΔM∝$H^2$ and the essential time independence are compatible with ‘‘opaque’’ potential-barrier models.