Critical properties of pure and randomly dilute dysprosium aluminum garnet

Critical properties of the Ising antiferromagnet, dysprosium aluminum garnet (DAG), have been studied by neutron scattering techniques in polycrystalline specimens of the pure compound and in samples containing 1 at. % and 5 at. % yttrium substituted for dysprosium. All samples were prepared by a precipitation technique to ensure homogeneously random yttrium substitution. Critical indices and amplitudes for pure DAG showed satisfactory agreement with theory, lending confidence in the powder method and suggesting that the anomalously low value of β obtained with single crystals may have been caused by extinction. In the case of the diluted samples, the value of β increased as expected on the basis of recent calculations of Newman and Riedel. However, the temperature dependence of the correlation range parameter exhibited an anomaly for which there is at present no explanation applicable to an Ising system: The range parameter remained essentially ‘‘infinite’’ between $T_N$ and an ‘‘upper critical temperature,’’ above which it exhibited a normal power-law decrease.