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Electron spins (arrows) on silicon atoms at the step edges of the Si(553)-Au surface. Credit: Naval Research Laboratory
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The
integration of single-spin magnetoelectronics into standard silicon
technology may soon be possible, if experiments confirm a new
theoretical prediction by physicists at the Naval Research Laboratory
(NRL) and the University of Wisconsin-Madison. The researchers predict
that a family of well-known silicon surfaces, stabilized by small
amounts of gold atoms, is intrinsically magnetic despite having no
magnetic elements. None of these surfaces has yet been investigated
experimentally for magnetism, but the new predictions are already
supported indirectly by existing data. The complete findings of the
study are published in the August 24, 2010, issue of the journal Nature
Communications.
Silicon
provides a unique entry point for combining magnetoelectronics based on
single spins with standard electronics technology. If a single-spin
device can be built on a silicon wafer, input and output electronics can
be directly integrated with the magnetic part of the device. This has
been an obstacle for current spintronics approaches. For example, spin
injection from a metal into silicon is very inefficient unless the
metal/semiconductor interface is carefully optimized.
These
latest results have the advantage that nature itself guides, by a
self-assembly process, the formation of long chains of polarized
electron spins with atomically precise structural order. "This
integration of structural and magnetic order is crucial for future
technologies based on single spins at the atomic level" said Dr. Steven
Erwin, a physicist at NRL and lead theorist on the project.
The
magnetic silicon surfaces, one of which is illustrated here, naturally
form steps which are stabilized by chains of gold atoms (yellow).
According to the team's calculations, some of the silicon atoms at the
step edges have unpaired electrons that are fully spin polarized and
probably magnetically ordered at sufficiently low temperatures.
The
atom chains on the Si(553)-Au surface were discovered in the group of
co-author Dr. Franz Himpsel at the University of Wisconsin-Madison.
Several other groups worldwide have been investigating such
"one-dimensional" silicon surfaces in recent years. As Himpsel noted,
"The idea of creating magnetism in a nonmagnetic material by
manipulating its structure has long intrigued scientists. The hope of
realizing this idea in silicon has been widely discussed for decades,
but so far none of these speculations has held up under scrutiny."
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This is a theoretical scanning tunneling microscopy image of the magnetic Si(553)-Au surface with coexisting threefold and twofold ripples. Credit: Naval Research Laboratory
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The
work of Erwin and Himpsel suggests several experiments, such as
spin-polarized scanning tunneling microscopy, to test their predictions
directly. But there is already indirect experimental evidence to support
the possibility of magnetism at silicon surfaces. Two research groups,
at Yonsei University in Korea and at Oak Ridge National Laboratory in
the US, have found that Si(553)-Au develops periodic "ripples" with two
different periodicities at low temperatures. One ripple occurs along the
silicon step edges with three times the normal periodicity, and the
other along the gold chains with two times the normal periodicity. The
prediction of Erwin and Himpsel, shown here, reproduces this pattern
perfectly. Moreover, this pattern only emerges when magnetism is allowed
in the calculation. When magnetism is "turned off" in the theory, the
ripples completely vanish. Thus the observation of threefold and twofold
ripples offers indirect – if preliminary – confirmation of magnetism.
Linear
chains of spin-polarized atoms provide atomically perfect templates for
the ultimate memory and logic, in which a single spin represents a bit.
One potential application is a "spin shift register" recently proposed
theoretically by Gerald D. Mahan, a theoretical physicist at
Pennsylvania State University. Another application is the storage of
information in single magnetic atoms. Erwin and Himpsel's work also
predicts that the magnitude, and even the sign, of the spin coupling can
be changed by doping electrons or holes into surface states. The
closely related Si(111)-Au surface can be electron-doped by adsorbates
(for example, silicon adatoms) on the surface. By varying this adsorbate
population one can perform band-structure engineering with
extraordinary precision. The possibility of tuning surface magnetism on
Si(553)-Au and its relatives using surface chemistry suggests a
fascinating new research direction. This work was supported by the
Office of Naval Research and by National Science Foundation awards.
Original article
