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A
crystal diagram shows the triangle-shaped atomic structure
of nickel gallium sulfide, which may have an unusual
magnetic "liquid" state at low temperatures.
Red spheres represent nickel, green spheres are gallium,
and yellow are sulfur.
Image
credit: S. Nakatsuji et al., Science, 9/9/2005 |
GAITHERSBURG,
MD—A novel material that may demonstrate a
highly unusual "liquid" magnetic state at extremely
low temperatures has been discovered by a team of Japanese
and U.S. researchers, according to tomorrow's issue of Science.*
The material,
nickel gallium sulfide (NiGa2S4), was
synthesized by scientists at Kyoto University. Its properties
were studied by both the Japanese team and by researchers
from The Johns Hopkins University (JHU) and the University
of Maryland (UM) at the Commerce Department's National Institute
of Standards and Technology (NIST).
The scientists
studied the polycrystalline sample using both X-rays and neutrons
as probes to understand its structure and properties. The
neutron experiments were conducted at the NIST Center for
Neutron Research.
The
team found that the triangular arrangement of the material's
atoms appears to prevent alignment of magnetic "spins,"
the characteristic of electrons that produces magnetism. A
"liquid" magnetic state occurs when magnetic spins
fluctuate in a disorderedly, fluid-like arrangement that does
not produce an overall magnetic force. The state was first
proposed as theoretically possible about 30 years ago. A liquid
magnetic state may be related to the similarly fluid way that
electrons flow without resistance in superconducting materials.
According
to Collin Broholm, a professor in the Department of Physics
and Astronomy at The Johns Hopkins University in Baltimore,
"the current work shows that at an instant in time the
material looks like a magnetic liquid, but whether there are
fluctuations in time, as in a liquid, remains to be seen."
Each
electron can be thought of as a tiny bar magnet. The direction
of its "north" pole is its spin. "An ordered
pattern of spins generally uses less energy, says Broholm,
"but the triangular crystal structure prevents this from
happening in this material."
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| Multi-colored
arrows show the disordered array of magnetic spins of
electrons within nickel gallium sulfide. The data were
collected by precisely measuring the timing and change
in direction of neutrons as they were passed through the
material. Image
credit: S. Nakatsuji et al., Science, 9/9/2005
|
The team
conducted their neutron experiments with an instrument called
a "disk chopper spectrometer." The only one of its
kind in North America, the instrument sends bursts of neutrons
of the same wavelength through a sample. Then, more than 900
detectors arranged in a large semicircle determine exactly
where and when the neutrons emerge, providing information
key to mapping electron spins.
"The
energy range and resolution we can achieve with this instrument
is ideal for studying magnetic systems," adds Yiming
Qiu, a NIST guest researcher from UM.
The wavelength
of the slowed-down (cold) neutrons available at the NIST facility—less
than 1 nanometer (billionth of meter)— also allows the
researchers to study nanoscale magnetic properties too small
to be measured with other methods.
The project
was funded by Grants-in-Aid for Scientific Research from the
Japan Society for the Promotion of Science and for the 21st
Century Center of Excellence ‘‘Center for Diversity
and Universality in Physics’’ from MEXT of Japan,
and by the Inamori Foundation. Work at The Johns Hopkins University
was supported by the U.S. Department of Energy. Work at NIST
was supported in part by the National Science Foundation.
As a
non-regulatory agency, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
* S.
Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas, C. Broholm,
H. Tsunetsugu, Y. Qiu, Y. Maeno."Spin Disorder on a triangular
lattice." Science, Sept. 9, 2005.
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| Photo
of the disk chopper spectrometer instrument at the cold
neutron facility of the NIST Center for Neutron Research.
Image
credit: R. Cappellitti/NIST
|
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