Magnetism loses under pressure
ARGONNE, Ill. (Jan. 29, 2008)—Scientists have discovered that the magnetic
strength of magnetite—the most abundant magnetic mineral on Earth—declines
drastically when put under pressure.
Researchers from the Carnegie
Institution's
Geophysical Laboratory, together with colleagues at the Advanced
Photon Source at Argonne National Laboratory, have found that when magnetite is subjected
to pressures between 120,000 and 160,000 times atmospheric pressure its magnetic
strength declines by half. They discovered that the change is due to what
is called electron spin pairing.
Magnetism comes from unpaired electrons in magnetic materials. The strength
of a magnet is a result of the spin of unpaired electrons and how the spins
of different electrons are aligned with one another. This research showed that
the drop in magnetism was due to a decrease in the number of unpaired electrons.
“Magnetite is found in small quantities in certain bacteria, in brains of
some birds and insects, and even in humans,” commented Yang Ding, the study's
lead author with the Carnegie-led High-Pressure Synergetic Consortium. “Early
navigators used it to find the magnetic North Pole and birds use it for their
navigation. And now it is used in nanotechnology. There is intense scientific
interest in its properties. Understanding the behavior of magnetite is difficult
because the strong interaction among its electrons complicates its electronic
structure and magnetic properties.”
To study the mineral, the researchers developed and applied a novel technique,
called X-ray Magnetic Circular Dichroism (XMCD) at the Advanced Photon Source,
a high-energy synchrotron facility. The technique uses high-brilliance circularly
polarized X-rays to probe the magnetic state of magnetite as a diamond anvil
cell subjects a sample to many hundreds of thousands of atmospheres. The researchers
combined their experimental results with theoretical calculations by collaborators*
to pinpoint why the magnetic strength changes. The study, to be published in
February in Physical Review Letters, reveals the electron-spin configuration
in the iron sites of the mineral to be the origin of the phenomenon.
This discovery not only shows the profound effects of pressure on magnetism,
it also discloses, for the first time, that pressure induced a spin pairing
transition that results in changes in the electron mobility and structure.
“The discovery is important,” Ding said. “It advances our understanding of
the correlation of magnetism, electron transport, and structural stability
in materials with strong electron interactions, like magnetite.”
“It is not surprising to see that a new phenomenon has been trigged by pressure
in the oldest magnet. Pressure can directly change electron-electron interactions
by squeezing the spacing between them,” said Ho-kwang Mao, the director of
the High-Pressure Synergetic Consortium and the High-Pressure Collaborative
Access Team. “In the future, the integration of high pressure with novel synchrotron
techniques will no doubt lead to more new discoveries.”
Collaborators are at the Kirensky Institute of Physics (Russia). Other authors
in the paper are Daniel Haskel, Sergei G. Ovchinnikov, Jonathan C. Lang, Yuan-Chieh
Tseng, and Yuri S. Orlov.
The Carnegie Institution has been a pioneering force in basic scientific research since 1902. It is
a private, nonprofit organization with six research departments throughout
the U.S. Carnegie scientists are leaders in plant biology, developmental biology,
astronomy, materials science, global ecology, and Earth and planetary science.
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of Science.
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