Random-Field Effects in Two- and Three-Dimensional Ising Antiferromagnets

Neutron scattering experiments have been performed in the diluted antiferromagnets Rb₂{Co}0.7Mg0.3{F}₄$ and ${Co}0.3Zn0.7{F}₂$ in a uniform magnetic field. These systems are isomorphous to, respectively, two- and three-dimensional Ising ferromagnets in a site-random magnetic field. It is shown that small random magnetic fields destroy the long-range order at all temperatures, consistent with three as the lower marginal dimensionality; the structure factor in the disordered state is predominantly a squared Lorentzian.