FARADAY ROTATION OF THE RANDOM FIELD ISING MODEL SYSTEM Fe0.7Mg0.3Cl2

Faraday rotation and magnetic circular dichroism were used to determine the magnetic phase diagram of Fe0.7Mg0.3Cl2 in an axial magnetic field. The tricritical point appears well-defined (Hi,t∼0.6 T, Tt∼8.5 K) provided that the data are taken after zero-field cooling. Random field crossover is observed below Hi∼0.4 T with an exponent φ=1.41±0.05 along with logarithmic divergences of (∂M/∂T)H and (∂M/∂H)T⋅The crossover into random field tricritical behavior is discussed with regard to recent theory.