Since the discovery of quasicrystals (QCs), solids that mimic crystals in
their long-range order but lack periodicity, scientists have sought physical
properties related to their peculiar structure. Now, an international group
of researchers led by Tokyo University of Science, have reported a
long-range magnetic order in QCs with icosahedral symmetry that turns
ferromagnetism below certain temperatures.
In 1984, a routine examination of an aluminum-manganese alloy revealed a
curious anomaly that was previously thought to be crystallographically
impossible—a five-fold rotational symmetry. The discovery of a QC, later
awarded the Nobel Prize, showed that its ordering is "quasiperiodic,"
leading to some exotic symmetries absent in crystals. Ever since then, QCs
have been the subject of enormous scientific interest.
But their potential applications remain uncertain since no physical property
signifying their long-range quasiperiodic order, such as long-range magnetic
order, has been observed. Until now, that is.
In a new study published in the Journal of the American Chemical Society, a
global team of scientists led by Professor Ryuji Tamura of Tokyo University
of Science (TUS), Professor Taku J. Sato of Tohoku University, and Professor
Maxim Avdeev of the Australian Nuclear Science and Technology Organisation
and the University of Sydney, have reported the first-ever observation of
long-range ferromagnetic order in icosahedral quasicrystals (i QCs or QCs
with five-fold rotational symmetry). Asuka Ishikawa and Shintaro Suzuki,
members of the Tamura Laboratory at TUS, also made invaluable contributions
to the project.
"This successful synthesis of ferromagnetic i QCs is the culmination of more
than 10 years of research in our laboratory," says Tamura, "Nobody knows
what kind of peculiar behavior they will further reveal or how they can be
exploited for the advancement of technology, but now we have finally taken
the first step. Elucidating the properties of these ferromagnetic QCs will
contribute greatly to the development of science."
There are four major types of magnetic order: ferromagnetism,
antiferromagnetism, paramagnetism, and diamagnetism. The discovery of
antiferromagnetic and ferromagnetic transitions in approximant crystals
(APs) -crystals with a somewhat similar structure to the related QCs that
can be studied using conventional techniques—inspired the research group to
look for magnetically ordered i QCs For their research, the team prepared
alloys of gold (Au), gallium (Ga) and gadolinium (Gd) and gold, gallium, and
terbium (Tb). Using conventional X-ray diffraction, they observed the
formation of an icosahedral quasicrystal phase for both Au-Ga-Gd and
Au-Ga-Tb.
They then investigated the properties of the two i QCs using magnetic
susceptibility and specific heat measurements. They found that both alloys
showed a ferromagnetic phase transition at 23 K (Gd i QC) and 16 K (Tb i
QC), a signature of long-range magnetic order. To further validate these
results, they performed neutron diffraction experiments using ECHIDNA
(ANSTO, Australia) and ISSP-GPTAS (JRR-3, Japan), and looked at the neutron
diffraction patterns of the i QCs at different temperatures. They observed
prominent Bragg peaks below their respective transition temperatures,
confirming the ferromagnetic nature of the i QCs.
Attempts to synthesize magnetic i QCs until now have all ended in
"spin-glass-like freezing," characterized by a disordered magnetic state.
The discovery of long-range ferromagnetic order in this study has
consequences far beyond the landscape of the physical properties of
materials and opens doors to tailored magnetic materials. "The crystal
symmetry of ferromagnetic QCs is much higher than that of conventional
periodic crystals, which makes it possible to apply them as ultrasoft
magnetic materials," says Tamura.
It took decades to discover long-range magnetic order in i QCs. With further
research pioneering the way, it won't be as long before the world sees what
their discovery entails.
Reference:
Rmyuji Tamura et al, Experimental Observation of Long-Range Magnetic Order
in Icosahedral Quasicrystals, Journal of the American Chemical Society
(2021).
DOI: 10.1021/jacs.1c09954
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