Quasicrystals become magnetic – researchers create ferromagnetic quasicrystals for the first time – scinexx.de

Until now it was unclear whether quasicrystals could be ferromagnetic at all. But now researchers have succeeded for the first time in bringing these exotic solids into a magnetic order. In the experiment, icosahedral quasicrystals made of gold, gallium and gadolinium or terbium changed into a ferromagnetic state when they cooled down. This is the first evidence of ferromagnetism in a quasicrystal – and now opens up new possibilities for using these materials.

In contrast to crystals, quasicrystals are not always made up of the same, symmetrical basic units. Instead, its grid shows several alternating basic geometric shapes – something that has long been considered impossible in nature. In fact, the few natural quasicrystals all come from meteorites; another quasicrystal was created in 1945 during the “Trinity” atomic bomb test in the USA. Even if quasicrystals can now be created specifically in the laboratory, many of their physical properties are still unknown.

One of the open questions concerned the magnetic properties of the quasicrystals: Attempts seemed to indicate that the quasi-periodic structures evade a magnetic order: if they are exposed to a magnetic field and cooled, they become a spin glass – their specific structure prevents the spins of their atoms from doing so to align itself in an orderly manner, as is typical for ferromagnets. Does this mean that quasicrystals cannot be ferromagnetic per se?

Structure of the icosahedral quasicrystal and its subunits (b). Blue and white are rare earth atoms, gold is gold and gallium is red.© Tamura et al./ Journal of the American Chemical Society 2021, CC-by-sa 4.0

Gold, gallium and rare earth metals

In search of an answer, Ryuji Tamura from Tokyo University of Science and his colleagues have now created and studied a new form of quasicrystals. These are metal alloys made from gold, gallium and a rare earth metal – here gadolinium or terbium – with a five-fold symmetry. Such icosahedral quasicrystals according to the Au scheme65Ga20R15 – R stands for gadolinium or terbium – have never been synthesized before, according to the researchers.

The impetus for this special alloy was provided by related crystalline solids, for which ferromagnetism has recently been demonstrated. The team therefore wanted to find out whether this could possibly also apply to quasicrystals with this composition and a similar ratio of electrons to atoms.

Ferromagnetic when it cools down

And indeed: When Tamura and his colleagues cooled their quasicrystals and checked their magnetism, there was an abrupt change at 16 or 23 Kelvin, depending on the variant: Unlike earlier approaches, these quasi-crystalline alloys did not become spin glass below this threshold temperature, but ferromagnetic. “This is the first time we have demonstrated a magnetic order in quasicrystals that acts over long distances,” the team writes.

This was also confirmed in supplementary neutron scattering analyzes: “We are clearly observing magnetic Bragg reflections below the threshold temperatures,” report the physicists. “This confirms the ferromagnetic nature.” They see the inclusion of the rare earth metals gadolinium or terbium as the decisive factors, but also the electron / atom ratio, which is 1.7.

New opportunities

According to the researchers, the synthesis of the first ferromagnetic quasicrystals now opens up new possibilities for the targeted production of further, tailor-made variants of these exotic solids in the future. “Nobody knows what strange behavior the quasicrystals will show or how they can be used to advance the technology, but we have made an important first step in that direction,” says Tamura.

According to the physicist, further elucidating the properties of these ferromagnetic quasicrystals will in any case contribute to the advancement of science. (Journal of the American Chemical Society, 2021; doi: 10.1021/jacs.1c09954)

Quelle: Tohoku University


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