Giant magnetocaloric materials could have large impact on the environment
ARGONNE, Ill. (June 18, 2007) — Materials that change temperature in
magnetic fields could lead to new refrigeration technologies that reduce the
use of greenhouse gases, thanks to new research at the U.S. Department of Energy's
Argonne National Laboratory and Ames National
Laboratory.
Scientists carrying out X-ray
experimentation at the Advanced Photon Source at
Argonne — the nation's most
powerful source of X-rays for research — are learning new information
about magnetocaloric materials that have potential for environmentally friendly
magnetic refrigeration systems.
Magnetic refrigeration is a clean technology that uses magnetic fields to
manipulate the degree of ordering (or entropy) of electronic or nuclear magnetic
dipoles in order to reduce a material's temperature and allow the material
to serve as a refrigerant. New materials for refrigeration based on gadolinium-germanium-silicon
alloys display a giant magnetocaloric effect due to unusual coupling between
the material's magnetism and chemical structure.
Understanding this coupling is essential to moving this technology from the
laboratory to the household. Magnetic
refrigeration does not rely on hydrofluorocarbons (HFCs) used in conventional refrigeration systems. HFCs are greenhouse gases
that contribute to global climate change when they escape into the atmosphere.
A collaboration between researchers from Argonne and Ames has now revealed
key atomic-level information about these new materials that makes clear the
role played by the nominally non-magnetic germanium-silicon ions in the giant
magnetocaloric effect. In an article published in the June 15 issue of Physical
Review Letters, the researchers describe how they used high-brilliance,
circularly-polarized X-ray beams at the Advanced Photon Source to probe the
magnetism of gadolinium and germanium ions as the material underwent its bond-breaking
magneto-structural transition. In addition to the expected strong magnetization
of gadolinium ions, the researchers found significant magnetization attached
to the germanium ions.
“This is surprising and important,” said Argonne physicist Daniel Haskel,
who led the research team. “Germanium was expected to be non-magnetic. Its
magnetization is induced by the hybridization, or mixing, of otherwise non-magnetic
germanium atomic orbitals with the magnetic gadolinium orbitals. This hybridization
dramatically changes at the germanium-silicon bond-breaking transition, causing
the destruction of magnetic ordering and leading to the giant magnetocaloric
effect of these materials.”
By combining the novel experimental results with detailed numerical calculations
of the electronic structure carried out at Ames Laboratory, the researchers
were able to conclude that the magnetized germanium orbitals act as “magnetic
bridges” in mediating the magnetic interactions across the distant gadolinium
ions.
The magnetocaloric effect – a change in temperature accompanying a change
in a material's magnetization – is largest near a material's intrinsic magnetic
ordering temperature. In the case of rare-earth gadolinium, this ordering occurs
near room temperature and results in a temperature increase of 3-4 K/per Tesla
when a magnetic field is applied, making gadolinium the current material of
choice for magnetic refrigeration near room temperature.
The prospects for a viable magnetic refrigeration technology recently became
brighter with the report of a giant magnetocaloric effect in gadolinium-silicon-germanium
alloys. The addition of non-magnetic silicon and germanium ions brings about
a giant entropy change when germanium-silicon chemical bonds connecting the
magnetism-carrying gadolinium ions are quickly formed or broken, respectively,
by the application or removal of a magnetic field. As an added bonus, the magnetic
ordering temperature can be tuned by changing the ratio or germanium to silicon.
"As a result of this work we now have a better understanding of the role
of nonmagnetic elements, such as germanium, in enhancing magnetic interactions
between the rare-earth metals in these materials,” said co-author and Ames
Laboratory senior scientist Vitalij Pecharsky. “This discovery is counterintuitive,
yet it opens up a range of exciting new opportunities towards the engineering
of novel magnetic materials with predictable properties."
Other authors in the paper are Y. Lee, B. Harmon, Y. Mudryk, and K. Gschneidner
of Ames and Z. Islam, J. Lang, and G. Srajer at Argonne.
Ames Laboratory, celebrating
its 60th anniversary in 2007, is operated for the Department
of Energy by Iowa State University.
The Lab conducts research into various areas of national concern, including
energy resources, the synthesis and study of new materials, high-speed computer
design, and environmental cleanup and restoration.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology.
The nation's first national laboratory, Argonne conducts leading-edge basic
and applied scientific research in virtually every scientific discipline. Argonne
researchers work closely with researchers from hundreds of companies, universities,
and federal, state and municipal agencies to help them solve their specific
problems, advance America 's scientific leadership and prepare the nation for
a better future. With employees from more than 60 nations, Argonne is managed
by UChicago
Argonne, LLC for
the U.S.
Department of Energy's Office
of Science.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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