NEUTRON-SCATTERING STUDY OF MAGNETIC-ORDERING IN THE DOUBLE-LAYER ANTI-FERROMAGNET K3MN2F7

Elastic and quasielastic scattering of neutrons from the double-layer Heisenberg antiferromagnet with weak anisotropy K3Mn2F7 has been employed to study the magnetic structure, the variation with temperature of the sublattice magnetization, the generalized susceptibility, and the spin-spin correlation length. At TN=58.3±0.2 K K3Mn2F7 exhibits a second-order phase transition to antiferromagnetic order with the spins aligned along the tetragonal axis. In the low-temperature region, the sublattice magnetization excellently follows a previous renormalized spin-wave result up to ∼TN2. Near TN, the critical exponent for the order parameter is β=0.154±0.006. As is the case in single-layer structures, neutron scattering from spin-spin correlations above TN occurs along ridges in reciprocal space, with the noticeable difference that in the double-layer structure these ridges are modulated in intensity due to interference of the paired layers separated by δ=4.18±0.07 0.003 and c=21.55±0.02. From the development of the ridge profile, the critical exponents for the generalized susceptibility χ(q→) and inverse correlation length are found to be γ=1.9±0.3 and ν=1.1±0.2, and from these, with ν(2-η)=γ, η=0.20±0.05 for the deviation from the Ornstein-Zernike χ(q→). The critical exponents are essentially equal to those in comparable single-layer systems, such as K2MnF4 and Rb2MnF4, while they do not differ substantially from the exact two-dimensional Ising results. In addition, in K3Mn2F7 spin-spin correlation near TN extends over essentially the same length as in single layers. The general conclusion is that the phase transition in Heisenberg double-layer systems is of strictly two-dimensional nature. At ∼1.5TN, however, evidence is found for a more rapid decrease of the correlation length than in single layers, suggesting transition to three-dimensional behavior. © 1979 The American Physical Society.