Neutron scattering from Rb2CocMg1-cF4: observation of two-dimensional magnetic long-range order

Abstract
The elastic magnetic scattering from a series of compounds exhibiting two-dimensional Ising-like antiferromagnetic behaviour, Rb2CocMg1-cF4 (c=1.00, 0.89, 0.85 and 0.70), has been examined below their Neel temperatures. The magnetic scattering from Rb2Co0.7Mg0.3F4 takes the form of almost smooth elastic Bragg rods along the c direction rather than points in reciprocal lattice space, and provides the best approximation yet found to an ideal two-dimensional long-range order. This two-dimensional magnetic long-range order is found to depend strongly on the heat treatment of the crystals and on the concentration of Mg ions, becoming less pronounced as the concentration decreases.