[tt] NRL generates, modulates, and electrically detects pure spin currents in silicon

[Eugen Leitl]

http://www.eurekalert.org/pub_releases/2007-12/nrl-ngm120307.php

Public release date: 3-Dec-2007

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Naval Research Laboratory

NRL generates, modulates, and electrically detects pure spin currents in
silicon

Scientists at the Naval Research Laboratory (NRL) have generated, modulated
and electrically detected a pure spin current in silicon, the semiconductor
used most widely in the electronic device industry. Magnetic contacts on the
surface of an n-type silicon layer enable generation of a spin current which
flows separately from a charge current. The spin orientation is electrically
detected as a voltage at a second magnetic contact. The relative
magnetizations of these contacts allow full control over the orientation of
the spin in the silicon channel. This was accomplished in a lateral transport
geometry using lithographic techniques compatible with existing device
geometries and fabrication methods. This demonstration by NRL scientists is a
key enabling step for developing devices which rely on electron spin rather
than electron charge, an emergent field known as “semiconductor spintronics.”
Progress in this field is expected to lead to devices which provide higher
performance with lower power consumption and heat dissipation. The complete
findings of this study, titled “Electrical injection and detection of
spin-polarized carriers in silicon in a lateral transport geometry,” are
published in the 19 November 2007 issue of Applied Physics Letters.

The electronics industry has relied largely on the control of charge flow,
and through size scaling (i.e. reducing the physical size of elements such as
transistors) has continuously increased the performance of existing
electronics. However, size scaling cannot continue indefinitely as atomic
length scales are reached, and new approaches must be developed. Basic
research efforts at NRL and elsewhere have shown that spin angular momentum,
another fundamental property of the electron, can be used to store and
process information in metal and semiconductor based devices.

The 2007 Nobel Prize in Physics was awarded for the discovery of giant
magnetorsistance, a phenomenon based upon spin-polarized electron currents in
metals. This research moved from discovery in 1988 to commercial products in
approximately 10 years, and is credited with the availability of low-cost,
high density hard disk drives which are widely found in consumer products
ranging from computers to video games and hand-held electronics. The spin
angular momentum of electrons can be used to store and process information in
semiconductor devices just as in metals. Indeed, the International Technology
Roadmap for Semiconductors (ITRS) has identified the use of the electron’s
spin as a new state variable that should be explored as an alternative to the
electron’s charge. The use of pure spin currents to process information is
regarded as the “holy grail” of semiconductor spintronics, as it frees one
from the constraints of capacitive time constants and resistive voltage drops
and heat buildup which accompany charge motion.

Much of the initial research success in this field was achieved in III-V
semiconductors with a direct band gap such as gallium arsenide, where
powerful optical spectroscopic techniques are relatively easy to apply and
enable detailed insight into the behavior of the spin system. Significant
strides have recently been made by NRL scientists to utilize spin transport
in silicon, an indirect gap material, as they demonstrated efficient
injection of spin-polarized electrons from a ferromagnetic metal contact
(Nature Physics 3, 542 (2007)). They have now taken an important step towards
the realization of a functional silicon spintronic device. In this very
recent work, NRL scientists first inject a spin polarized electrical current
from a ferromagnetic iron / aluminum oxide tunnel barrier contact into
silicon, which generates a pure spin current flowing in the opposite
direction (see figure). This spin current produces shifts in the
spin-dependent electrochemical potential, which can be electrically detected
outside of the charge path at a second magnetic contact as a voltage. The NRL
team showed that this voltage is sensitive to the relative orientation of the
spin in the silicon and the magnetization of the detecting contact . They
further showed that the orientation of the spin in the silicon could be
uniformly rotated by an applied magnetic field, a process referred to as
coherent precession, demonstrating that information could be successfully
imprinted into the spin system and read out as a voltage. The generation of
spin currents, coherent spin precession and electrical detection using
magnetic tunnel barrier contacts and a simple lateral device geometry
compatible with "back-end" silicon processing will greatly facilitate
development of silicon-based spintronic devices.

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The NRL research team includes Drs. Berend Jonker, Olaf van ‘t Erve, Aubrey
Hanbicki, Connie Li, Mike Hollub and Chaffra Awo-Affouda from the Materials
Science and Technology Division, and Phillip Thompson from the Electronics
Science and Technology Division. This work was supported by NRL core programs
and the Office of Naval Research.