Research predicts size-induced transition
to nanoscale half-metallicity
ARGONNE, Ill. (Nov. 2, 2007) — How big does a cluster of metal atoms
actually have to be before it starts acting like a metal: ductile, malleable
and a conductor?
The emergence of metallic attributes, usually referred to as the transition
to metallicity, is among the most intricate aspects of the size evolution of
properties of atomic clusters that are metals in bulk quantities. Researchers
at Argonne and other research centers worldwide are looking for answer to this
question, which is central for establishing limits of miniaturization in nanoelectronic
devices.
An even more intricate question is whether researchers can identify a nanoscale
analog of the bulk half-metallic state and the size-driven transition to that
state.
A recent study by Argonne theorists suggests that the answer to this question
is yes. Their work represents the first prediction of a nanoscale analog of
the bulk half-metallic state.
In distinction from normal metals, in which electrons with alpha and beta
spins carry the electrical current, half metals are elements or compounds with
spin-polarized conductivity. Electrical current in half metals translates into
spin transport, which lies at the foundation of spintronics technology. Scientists
in the field of spintronics study how to use “spin” or magnetic properties
of particles, such as electrons, to develop novel and better sensors, recording
devices, switches and quantum computers.
Even the common metallic state
becomes a complex phenomenon at the nanoscale,
Julius Jellinek of Argonne's Chemical Science and Engineering Division.
“Small or medium atomic clusters of metallic elements may lack all attributes
normally associated with the bulk metallic state,” he said. “These attributes
then grow in as clusters grow in size. The same should be true of the half-metallic
state, and our research shows that it is.”
The Argonne theorist collaborated with an experimental group at the Johns
Hopkins University led by Kit Bowen, Jr. The experiments have indicated
that, as a small, negatively charged manganese cluster grows, the gap between
the energies of its two most external electrons decreases and closes when
the cluster size reaches six atoms.
Computations and subsequent analysis
by Jellinek and his colleagues revealed that the closure of the gap between
the electron energy levels takes place in one spin manifold, but not the
other. This spin-polarized nature of the energy gap closure is what constitutes
the nanoscale analog of the bulk half-metallic state. Understanding the
finite-size analog of the bulk half-metallicity and the size-driven transition
to it is central for many areas of nanoscience and nanotechnology, in particular
nanospintronics.
The prediction of finite-size half-metallicity must still be tested experimentally
using future spin-polarized photoelectron spectroscopy measurements. “The finite-size
analog of half-metallicity may be more ubiquitous than the bulk half-metallic
state,” Jellinek said. “Nanoscale half-metallicity may emerge as a transient
state in the size-driven evolution of properties of systems even for elements
and substances that are not half-metals in bulk quantities.”
The study was published in the journal Physical
Review B and republished
by the Virtual
Journal of Nanoscale Science and Technology. Collaborators
on this research were Julius Jellinek, Paulo H. Acioli and Juan Garcia-Rodeja
from Argonne National Laboratory, and Weijun Zheng, Owen C. Thomas and Kit
Bowen, Jr. from the Johns Hopkins University.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology.
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and applied scientific research in virtually every scientific discipline. Argonne
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the U.S.
Department of Energy's Office
of Science.
By Jennifer deAngelis.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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