University of Minnesota
School of Physics & Astronomy

Condensed Matter Seminar

Wednesday, April 6th 2016
1:25 pm:
Speaker: Alan I. Goldman, Iowa State University and Ames Laboratory
Subject: Magnetic rare-earth-based quasicrystals

In the thirty years since the discovery of quasicrystals by Dan Shechtman there has been tremendous progress in our understanding of the structure of quasicrystals and aperiodic systems in general. Indeed, the question first asked by Per Bak soon after Shechtman’s discovery, “Where are the atoms?”, can largely be answered for at least one class of quasicrystals, the i-YbCd5.7 icosahedral phase. Progress in our understanding of the consequences of aperiodicity for physical phenomena such as the electronic, magnetic, and optical properties has lagged somewhat but we have recently seen a surge of activity and new results. On the magnetism front, the discovery of a new family of magnetic quasicrystals and their closely related crystalline approximants, has allowed for a direct comparison of the impact of aperiodicity on magnetic interactions in compounds that have similar local structures.

Examples of stable binary icosahedral quasicrystals are quite rare and, before the discovery of icosahedral quasicrystals in the i-R-Cd system (R = Gd to Tm, Y), there were no known examples that featured localized magnetic moments. Local-moment-bearing binary quasicrystals represent the compositionally simplest system for the study of magnetic interactions in aperiodic compounds and, therefore, the new R-Cd quasicrystal family will play a key role in these studies, offering non-magnetic, Y, Heisenberg-like, Gd, and non-Heisenberg (CEF split) Tb to Tm members, in addition to the structural and compositional simplicity of a binary phase. Furthermore, the existence of a corresponding set of cubic approximants, RCd6, to the icosahedral phase allows for direct comparisons between the low- temperature magnetic states of crystalline and quasicrystalline phases with fundamentally similar local structures. RCd6 may be described as a body-centered cubic packing of the same clusters of atoms as found in the newly discovered icosahedral phase. Using x-ray resonant magnetic scattering we have shown that the RCd6 approximants manifest long-range magnetic order at low temperatures, whereas the related icosahedral phase exhibits only spin-glass-like freezing at low temperatures. In order to understand the reason for the absence of long-range magnetic ordering in the quasicrystalline phase, we have recently completed a full structural refinement of the i-R-Cd system (R = Gd, Dy) series and are in the midst of both elastic and inelastic neutron scattering measurements of i-Tb-Cd. Our results, to date, will be described and discussed.

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