Quasi on the inside: Rare earth crystals
- Published: Jun 15, 2013
- Author: David Bradley
- Channels: Chemometrics & Informatics
Scientists at the Ames Laboratory in the USA have used an algorithm to fish for new rare-earth quasicrystals and have pulled out a new family of binary magnetic icosahedral quasicrystals containing several different rare earth elements and cadmium. The findings demonstrate that such materials have spin glass behaviour rather than magnetic behaviour.
At first, glance quasicrystals are just like other crystals, they are ordered but that order is not periodic. While well-known to mathematicians and argued over for years by crystallographers it was not until 1982 and the discovery by materials scientist Dan Shechtman who obtained odd diffraction patterns for the aluminium-manganese alloys he had made, that hints of symmetries not usually thought possible in crystals were seen. Shechtman worked for years with much opposition to get his work accepted by his peers; ultimately it was with the award of the 2011 Nobel Prize in Chemistry. Since the time his work was finally accepted, synthetic quasicrystal alloys of Al-Li-Cu, Al-Mn-Si, Al-Ni-Co, Al-Pd-Mn, Al-Cu-Fe and Al-Cu-V as well as of Cd-Yb, Ti-Zr-Ni, Zn-Mg-Ho, Zn-Mg-Sc, In-Ag-Yb and Pd-U-Si have been found.
Keen on quasi
Materials scientists are keen to find more quasicrystals, and finding ones with magnetic properties would allow them to make more facile structural comparisons between the nearest true magnetic crystal and its quasi-counterpart. One way to search for novel quasicrystal materials is to look for them around close crystalline phases that have similar atomic motifs as suspected quasicrystals, so-called crystalline approximants. Writing in the journal Nature Materials, the Ames lab team has now found the first known magnetic rare earth icosahedral binary quasicrystals, which they say completes the "matched set" of magnetic quasicrystals and their closely related periodic cousins.
"This discovery of binary magnetic quasicrystals provides us with a means of doing a cleaner comparison of structural and magnetic properties between a quasicrystal and its periodic approximant," explains Alan Goldman. This finding represents the next step following on from the discovery by Paul Canfield of Iowa State University who was one of the first researchers to be able grow single-grain, rare-earth quasicrystals for diffraction analysis. "For the last ten years, we have been moving beyond just the innate beauty of these quasicrystalline structures to find out what else is interesting about them," Goldman says. "Are the electrical properties any different? Are the magnetic qualities unusual?" he asks. Specifically, scientists were curious as to whether quasicrystals could be antiferromagnetic or even ferromagnetic as are many conventionally crystalline, periodic solids. The issue being that although disordered magnets or spin glasses with an amorphous structure are known, quasicrystals are uniquely ordered but not periodic, could such a system be endowed with magnetic properties.
Theoretically magnetic order is not precluded by the aperiodicity of quasicrystals, it has simply taken a long time for scientists to observe it experimentally, which perhaps has implications for understanding magnetism in complex environments. Previously, researchers in Japan had discovered magnetic order in a series of periodic approximants of rare-earth cadmium materials. The Ames team worked with collaborators in France, Japan, and elsewhere in the US to characterise by scattering those magnetic structures.
Next crystalline step
The next step was to investigate whether quasicrystals very close in character to these rare earth cadmium approximants might have hidden within their phase space at different temperature and composition easily accessible quasicrystals. The team aimed to synthesise the approximant for further study and at the same time push the material's phase diagram by cooling to as low a temperature as possible before solidification occurred thus forming a quasicrystal.
What emerged from this push was the approximant itself as well as faceted pentagonal dodecahedral crystals, or more to the point quasicrystals. X-ray scattering work confirmed the material's status. The team explains that the rare earth cadmium approximants have magnetic order but their quasicrystalline counterparts display spin glass behaviour, which is akin to magnetic behaviour seen in amorphous materials.
"There’s still work to be done; it’s my hope that there is lurking out there a quasicrystalline antiferromagnet, which means an ordered magnetic structure," Goldman says. "It hasn't been theoretically ruled out."
Nature Mater , 2013, online: "A family of binary magnetic icosahedral quasicrystals based on rare earth and cadmium"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.