Computer simulations of a three-dimensional Ising ferromagnet with quenched disorder

Recent high-resolution x-ray and neutron-scattering experiments for various crystals with quenched defects of the random temperature type near magneti c and structural phase transitions have revealed two different length scale s. The coexistence of two scales was interpreted as a superposition effect of scattering from the disordered layer containing defects, i.e., the surfa ce layer (resulting in the larger correlation length) and from the bulk of the sample (resulting in the shorter one which corresponds to the usual cri tical fluctuation). Later, experimental evidence was found for the coexiste nce of both scales in the same volume fraction of the sample. In our invest igations the phase transition in an Ising ferromagnet containing edge dislo cation dipoles is considered. The results of our computer simulations also show that the two different length scales observed in real crystals appear in the same volume Fraction. [S0163-1829(99)04034-5].