Neutron scattering experiments have been performed in the diluted antiferromagnets ${\mathrm{Rb}}_2$${\mathrm{Co}}_{0.7}$${\mathrm{Mg}}_{0.3}$${\mathrm{F}}_4$ and ${\mathrm{Co}}_{0.3}$${\mathrm{Zn}}_{0.7}$${\mathrm{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.

• Received 7 December 1981

DOI:https://doi.org/10.1103/PhysRevLett.48.438