Scientists at the U.S. Department of Energy's (DOE) Ames Laboratory have discovered a new family of rare-earth quasicrystals using an algorithm they developed to help pinpoint them. Quasicrystalline materials may be found close to crystalline phases that contain similar atomic motifs, called crystalline approximants. And just like fishing experts know that casting a line in the right habitat hooks the big catch, the scientists used their knowledge to hone in on just the right spot for new quasicrystal materials discovery.
Their research resulted in finding the only known magnetic rare earth icosahedral binary quasicrystals, now providing a "matched set" of magnetic quasicrystals and their closely related periodic cousins.
The discovery has been published online by the journal Nature Materials in an article, "A family of binary magnetic icosahedral quasicrystals based on rare earth and cadmium."
"There's been a lot of theoretical and experimental work on magnetic quasicrystals and mathematically there's no reason why magnetic ordering can't happen," said Goldman. "But experimentally it was never observed. Why? What does this teach us about magnetism in complex environments?"
A few years ago, a series of periodic approximants of rare-earth cadmium were discovered that did order magnetically by research colleagues in Japan. The Ames Laboratory scientists worked to characterize by scattering the magnetic structures in collaboration with other researchers from France, Japan, and the United States.
Goldman and Canfield suspected that there could be quasicrystals very close to these rare earth cadmium approximants, hidden in very limited regions of temperature and composition space in the phase diagram, and most easily attainable through the flux growth method Canfield has used to grow other quasicrystals. Together with Ames Lab scientists Sergey Bud'ko, Andreas Kreyssig, Kevin Dennis, Mehmet Ramazanoglu, Anton Jesche, and physics graduate student Tai Kong, Goldman and Canfield initiated a new search for magnetic quasicrystals.
Goldman asked Canfield to start by growing the approximant, but Canfield was shooting for both.
"My intent was not just to go to the approximant, but to cool this as far as I could before everything solidified; I was fishing for the binary quasicrystal," Canfield said. "It was an attempt to survey the system. I know there's an approximant in there, but is there another surprise?"
And sure enough, there was. Canfield had grown the approximant, but he also found the presence of faceted pentagonal dodecahedra, one of the signatures of quasicrystals. Goldman's x-ray scattering work confirmed the material as a quasicrystal.
In the rare earth cadmium approximants, there is magnetic order. In the quasicrystalline materials, however, the scientists found spin glass behavior, similar to the magnetic behavior in amorphous materials.
"What we have here is proof of principle. Yes, you can find quasicrystals near approximants; you just have to search the right way," said Canfield.
"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. It hasn't been theoretically ruled out," said Goldman. "What I do know is that quasicrystals continue to surprise me."
The research was supported by DOE's Office of Science.
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