The discovery of a stable icosahedra phase alloy in the R8Mg42Zn50 family, where R= rare earth, has generated recent interest because of the opportunity for new investigations of magnetism involving localized 4f moments on a quasiperiodic lattice. A great deal of previous work has been devoted to investigations of magnetism in quasicrystalline alloys including the metastable Al-Mn icosahedral phase, and the stable Al-Pd-Mn and Al-Cu-Fe icosahedral alloys. These investigations have revealed either spin-glass like behavior at low temperature, or a diamagnetic response. In all cases, the magnetism in these 3d electrons systems appears to be weak, making it difficult to discern the effects of aperiodicity upon magnetic interactions between ions. The existence of a stable icosahedral phase containing rare-earth ions with sizable local moments, offers a unique opportunity to study this issue in detail.

More recent work by Charrier and co-workers, however, presented evidence from powder neutron diffraction measurements that there is a transition to an aniferromagnetic long-range ordered ground state, at least for R=Tb and Ho. The intensity of the sharp peaks in the ordering, was quite small and was accompanied by significant diffuse magnetic scattering. Most of the sharp magnetic peaks that appeared at low temperature (TN = 20K for Tb) could be indexed to an icosahedral lattice with a propagation vector of (1/4,0,0,0,0,0), quadrupling the 6D-hypercubic lattice along the five-fold directions. Interestingly, for R=Dy and Er the sharp diffraction peaks carried even less weight in comparison to the diffuse background. Further, magnetic susceptibility data from all of these samples failed to display any significant features in the vicinity of TN, as determined by the neutron measurements, beyond a deviation from its high-temperature Curie-Weiss behavior.

Recently, we have performed bulk magnetization and powder neutron diffraction measurements of crushed single grains and cast polycrystalline samples of Tb-Mg-Zn produced by P. Canfield in the Ames Laboratory. For the crushed single grains, only short-range magnetic ordering was observed at low temperature by neutron diffraction, with no evidence of the long-range magnetic ordering reported previously. The cast polycrystalline samples, prepared by rapid cooling followed by an extended anneal, exhibit primarily diffuse magnetic scattering at low temperature, but at least one relatively sharp diffraction peak was observed. Interestingly, the temperature dependence of the intensities of both the sharp and diffuse components agree well with the data of Charrier et al. Our results indicate that for single grain samples there is no long-range magnetic ordering and that, at best, the magnetic ordering in these quasicrystalline alloys is not very robust.

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Alan Goldman.