Critical Magnetic Scattering from Dysprosium Aluminum Garnet (DAG)

Abstract

We have measured the critical magnetic scattering and the temperature dependence of a Bragg magnetic peak in DAG from TN−T=0.14°K to T−TN=0.04°K. Specific heat and magnetic measurements indicate that DAG behaves like a three-dimensional Ising antiferromagnet with dipolar forces accounting for more than half the magnetic interaction energy.1 The neutron data have been compared with recent theoretical calculations for near-neighbor three-dimensional Ising models.2,3 The critical scattering was measured in the range 0<q<0.15 Å−1, 0.007<ΔT/TN<0.017 and was found to exhibit Lorentzian shape. The temperature dependence of the inverse range parameter κ was fitted by a weighted least-squares procedure to2logκa=νlog[(T−TN)/TN]+B0,giving (on a relative scale) TN=2.5179±0.0003°K and ν=0.60±0.02, which is lower than the Ising model prediction of ν=0.64. For B0∼0.33, as indicated by theoretical calculations, the correlation distance a was found to be ∼4.8 Å, consistent with the value 4.3 obtained from computation of the second moment of the experimentally determined interactions in DAG. The intensity of coherent magnetic scattering, I, was measured in the range 0.001<ΔT/TN<0.056 and was fitted by least squares to the expression I∼[(TN−T)/TN]2β[1−2b(TN−T)/TN].For b=0, we have found β=0.27±0.01 and TN=2.5179±0.0006°K, in agreement with TN obtained from the critical scattering. For b=0.83, suggested by Ising model calculations,4 the corresponding values are β=0.29±0.01 and TN=2.5182±0.0006, and the value of ν is decreased to 0.58, which is just within the estimated error. In either case the value of β falls significantly below the expected value of 0.312.