Spin fluctuations in random magnetic-nonmagnetic two-dimensional antiferromagnets. III. An Ising system

Abstract

This paper reports a study using neutron scattering techniques of the spin fluctuations in the two-dimensional diluted Ising system, Rb2Co, Mg&, F4. The explicit concentrations studied, c =0.55, 0.575, 0.585, and 0.595 are all close to the percolation concentration for nearestneighbor interactions cp =0.594, so that these experiments span the percolation threshold. The experiments were performed at.the Brookhaven high-flux beam reactor. A detailed study was made of the dynamics for c =0.575. The results showed four dispersionless bands corresponding to spin deviations on Co ions surrounded by one, two, three, or four Co ions. Calculations of the spectra using the equation-of-motion method for computer simulations gave a good account of the results. The few minor discrepancies probably arise from the effects of those crystal-field levels of the Co ion which have energies above the ground-state doublet. The c =0.595 crystal has a transition at 30 K to a state of two-dimensional order with negligible order in the third direction. The inverse correlation length in this system is zero to within the errors at 30 K. In the more dilute systems the inverse correlation length is found to be well described as an additive sum of a geometrical and a thermal part as observed previously in Rb2Mn, Mg~, F4. The thermal part is given by tK](T)] ' . where K](T) is the inverse correlation length of a one-dimensional chain of Co ions. This exponent is consistent with the percolation exponent vp =1.35 and the theoretically calculated crossover exponent, @=1. The intensity of the scattering is more difficult to analyze in the absence of a knowledge of the form of the scaling function, but the behavior is consistent with the multicritical picture with @=1.

Ising-cluster excitations in Rb2Co Mg1−F4 near percolation threshold

“…Muons are relaxed by fluctuating fields from the spin flips due to the Ising-cluster excitations, and the AC magnetic field applied for the susceptibility measurements induce the excitations contributing the imaginary susceptibility. This picture is also consistent with the result of diffuse neutron scattering [4]; short range ordering develops rapidly below two-dimensional short range ordering temperature T s close to T m as shown in Fig. 1.…”

Untitled

“…Most of the theoretical works suggest that the 2d Ising model with a quenched disorder has the same critical behaviour as the 2d pure Ising model (except for logarithmic corrections) [39,40,[48][49][50][51][52][53] (see also review [54]). This result is corroborated by M C-simulations on two-dimensional lattices [55][56][57][58][59] and experiments [60,61].…”

Two-dimensional percolation transition at finite temperature: Phase boundary for in-plane magnetism in films with two atomic layers of Fe on W(110)

“…(5a) is exact and then fit for γ p = 2.4±0.2, but this seems to be of questionable value. This same difficulty was encountered in the early studies of quasi-2D diluted antiferromagnets 5,6 , where dilution concentrations more accurate than those found by chemical analysis of the samples were required to arrive at a value of γ p to compare to theory. In that case, in order to make progress, the dilution concentrations were calculated using a method that assumed the theoretical critical exponent for the percolation correlation length, ν p , was exact.…”

“…Classic experiments on 2D percolation at finite temperature were performed with neutron scattering on 3D crystals of quasi-2D diluted antiferromagnets [5][6][7] . Measurements of the correlation length of the staggered magnetization near percolation were used to study the multicritical percolation point that can be approached either along the concentration axis at T = 0 or as a function of temperature very close to the percolation concentration.…”