While the future of quantum computing offers
the potential for substantially greater data storage and faster
processing speeds, its advancement has been limited by the absence
of certain critically important materials—in particular, a
semiconductor that is magnetic at room temperature. Recent experiments
only hinted at the possibilities. Now, scientists at the Department
of Energy’s Pacific Northwest National Laboratory have created a
semiconductor material that has superior magnetic properties at
room temperature and that may propel research one step closer to
realizing the potential of quantum computing.
Using a special synthesis technique, PNNL scientists created a
thin-film semiconductor material made of titanium, oxygen and cobalt.
In collaboration with scientists at the IBM Almaden Research Center
in San Jose, Calif., they showed that the materials required for
quantum computing and the emerging area of spintronics likely can
be obtained.
"Although other scientists have created similar materials,
their films had considerably poorer magnetic properties," said
Scott Chambers, a chemist and PNNL senior chief scientist. "Our
material has superior magnetic strength—an improvement of nearly
a factor of five.
"Our synthesis technique, while difficult to perform, is more
controllable at the atomic level, and therefore yields better results.
The next step is to refine the growth process."
The PNNL work builds upon experiments conducted by scientists in
Japan who created the same material using laser ablation, an effective
but less controlled thin-film synthesis method. The strength of
laser ablation is that it allows researchers to cover a wide range
of growth conditions and film compositions rather quickly when used
in what is known as the combinatorial mode. In this way, several
materials can be screened relatively rapidly to determine promising
candidates. This kind of search by the Japanese group revealed that
the material the PNNL team has synthesized in a more controlled
fashion has significant potential for the applications at hand.
A description of the Japanese research was published in the Feb.
2, 2001, issue of Science.
The current generation of computers uses an electron’s charge to
store and process information, but this approach limits the ultimate
speed and storage density that can be achieved. Magnetic storage,
such as that found in a computer hard drive, relies on the magnetic
properties created by an electron’s spin. However, if an electron’s
spin can be harnessed within a semiconductor, the potential exists
to create entirely new ways of computing and signal processing that
will greatly increase speed and data storage densities. The exploitation
of an electron’s spin to carry information, rather than its charge,
often is referred to as spintronics.
Spintronics would provide the basic properties required for advanced
technologies, such as on-chip integration of magnetic storage and
electronic processing functions and quantum computing, which depends
on coherent spin states to transmit and store information. A material
is permanently magnetic if the majority of its electrons spin in
the same direction.
In order to be practical, spintronics will need to use semiconductors
that maintain their magnetic properties at room temperature. This
is a challenge because most magnetic semiconductors lose their magnetic
properties at temperatures well below room temperature, and would
require expensive and impractical refrigeration in order to work
in an actual computer.
Chambers and his team of scientists achieved these properties in
a crystalline oxide film known as anatase titanium dioxide that
is infused with a small amount of cobalt, a magnetic impurity. These
results will be presented in a poster presentation at the 2001
Spintronics Workshop in Washington, D.C., Aug. 9 to 11.
Chambers and his team created this magnetic semiconductor material
using a synthesis method called molecular beam epitaxy. In this
growth method, individual beams of atoms—in this case, titanium,
oxygen and cobalt—are generated in a highly controlled vacuum
environment and directed onto a crystalline surface of strontium
titanate where the atoms condense and form a crystalline film with
dimensions on the nanoscale. Chambers designed and built this equipment
in the early and mid-90s, and his particular system was the first
of its kind in the world when installed at the William
R. Wiley Environmental Molecular Sciences Laboratory, a DOE
user facility at PNNL.
After the material was created, a team of scientists at IBM,
led by research staff scientist Robin Farrow, validated the results
by characterizing the material’s magnetic properties. In the material
synthesized at PNNL and characterized at IBM, each cobalt atom’s
magnetic moment, which is a measure of the material’s magnetic strength,
is about five times larger than in the Japanese scientists’ material.
The research is in its first of three years and is funded by PNNL’s
Nanoscience and Nanotechnology
Initiative. While early results are promising, PNNL scientists
will continue their research to determine the material’s ideal growth
temperature, growth rate, composition and choice of substrate, and
then optimize the structural properties required to achieve the
desired magnetic properties.
Business inquiries on this research or other PNNL technologies
should be directed to 1-888-375-PNNL or e-mail: inquiry@pnl.gov.
Pacific Northwest National Laboratory is a DOE research facility
and delivers breakthrough science and technology in the areas of
environment, energy, health, fundamental sciences and national security.
Battelle, based in Columbus, Ohio, has operated the laboratory for
DOE since 1965.
