Random-field critical behavior of a d=3 Ising system: Neutron scattering studies of Fe0.6Zn0.4F2.

Abstract

The effects of random fields on the critical scattering of neutrons in a d = 3, diluted Ising antiferromagnet have been observed for the first time. The experiments were made on the same crystal of Fe/sub 0.6/Zn/sub 0.4/F/sub 2/ in which earlier birefringence (..delta..n) studies showed a sharp phase transition T/sub c/(H) to exist. Only the scattering in the region above T/sub c/(H) was studied, where hysteretic behavior is absent. In this equilibrium region, new random-field critical behavior is observed, indicative of an approach to a sharp, second-order phase transition at T/sub c/(H), unlike the d = 2 case where rounding of the phase transition occurs within the same region. Hence the lower critical dimensionality of the equilibrium random-field Ising model (RFIM) is bounded by 2< or =d/sub l/<3. The critical scattering is well described by S(q) = A/(kappa/sup 2/+q/sup 2/)+B/(kappa/sup 2/+q /sup 2/)/sup 2/, with A scaling as kappa/sup eta-bar/ and B as kappaeta-bar. A preliminary analysis yields nu-bar = 1.0 +- 0.15, gamma-bar = 1.75 +- 0.20, and eta-barapprox. =(1/4) for the random-field thermal correlation length (kappa/sup -1/), staggered susceptibility (chi/sub st/), and correlation function () critical exponents, respectively.