Random-field effects in the diluted two-dimensional Ising antiferromagnet Rb2Co/sub 0.7/Mg/sub 0.3/F4

We report a comprehensive neutron scattering study of the spin correlations in the diluted two-dimensional (2D) Ising antiferromagnet Rb2Co/sub 0.7/Mg/sub 0.3/F4 in an applied magnetic field. As predicted by Fishman and Aharony, an applied field in this system produces a random staggered magnetic field. Random fields are expected to have drastic effects on the cooperative behavior of magnets although the detailed behavior remains controversial. It is found that the applied magnetic field, and by inference the concomitant random staggered fields, destroy the 2D long-range order for all temperatures and fields. The structure factor S/sup z/z(Q) = summation/sub r/e/sup i/Qxr <S/sup z//sub r/xS/sup z/0T is well described as the sum of a Lorentzian plus a Lorentzian squared with the Lorentzian-squared term dominating at low temperatures. The integrated intensity of the Lorentzian-squared term exhibits the same temperature dependence as the Bragg intensity at zero field. The correlation length kappa and structure-factor peak intensity scrS/sup z/z(Q0) exhibit power-law dependences on the applied field H, kappaapprox.H/sup 1.6/, and scrS/sup z/z(Q0)approx.H/sup -3.2/ at low temperatures; the measured exponents agree reasonably with the values 2 and -4, respectively, deduced from theories with the lower marginal dimensionality d/sub l/ = 3. The effective exponents initially increase slightly with increasing temperature and then decrease dramatically as T→T/sub N/, taking on values of about 0.7 and -1.2, respectively, at T/sub N/ = 42.5 K. For small but nonzero applied fields the inverse correlation length may be factorized into a random-field part and a thermal part, kappa = kappa/sub RF/+kappa/sub T/, with kappa/sub T/approx.ezz/k/sub B/T where J/sup z/z is the Ising exchange constant