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| A
simulation made with NIST micromagnetic software shows
the interaction of "spin waves" emitted by
two nano-oscillators that generate microwave signals.
The ability of these tiny spintronic devices to spontaneously
synchronize their emissions may lead to smaller, cheaper
wireless communications components.
Credit:
NIST
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BOULDER,
Colo.—Like the flashing of fireflies and ticking
of pendulum clocks, the signals emitted by multiple nanoscale
oscillators can naturally synchronize under certain conditions,
greatly amplifying their output power and stabilizing their
signal pattern, according to scientists at the Commerce Department’s
National Institute of Standards and Technology (NIST).
In the
Sept. 15 issue of Nature,* NIST scientists describe
“locking” the dynamic magnetic properties of two
nanoscale oscillators located 500 nanometers apart, boosting
the power of the microwave signals given off by the devices.
While an individual oscillator has signal power of just 10
nanowatts, the output from multiple devices increases as the
square of the number of devices involved. The NIST work suggests
that small arrays of 10 nano-oscillators could produce signals
of 1 microwatt or more, sufficient for practical use as reference
oscillators or directional microwave transmitters and receivers
in devices such as cell phones, radar systems and computer
chips.
“These
nanoscale oscillators could potentially replace much bulkier
and expensive components in microwave circuits,” says
Matthew Pufall, one of the NIST researchers. “This is
a significant advance in demonstrating the potential utility
of these devices.”
The NIST-designed
oscillators consist of a sandwich of two magnetic films separated
by a non-magnetic layer of copper. Passing an electrical current
through the device causes the direction of its magnetization
to switch back and forth rapidly, producing a microwave signal.
The circular devices are 50 nanometers in diameter, about
one-thousandth of the width of a human hair and hundreds of
times smaller than the typical microwave generators in commercial
use today. The devices are compatible with conventional semiconductor
technology, which is expected to make them inexpensive to
manufacture.
The type
of signal locking observed at NIST was first described by
the 17th-century Dutch scientist Christiaan Huygens, who found
that two pendulum clocks mounted on the same wall synchronized
their ticking, thanks to weak coupling of acoustic signals
through the wall. This phenomenon also occurs in many biological
systems, such as the synchronized flashing of fireflies, the
singing of certain crickets, circadian rhythms in which biological
cycles are locked to the sun, and heartbeat patterns linked
to breathing speed. There are also examples in the physical
sciences, such as the synchronization of the moon’s
rotation with respect to its orbit about the Earth.
Locking
is already exploited in many technologies, such as wireless
communications and certain types of antenna networks. For
instance, in many telecommunications schemes, a receiver oscillator
must lock to a signal transmitted by a sender.
The work
described in Nature is an advance in the field of
“spintronics,” which takes advantage of the fact
that the individual electrons in an electric current behave
like minuscule bar magnets, each having a “spin”
along a particular direction, analogous to a magnet's north
or south pole. Conventional electronics, by contrast, relies
on the electrons’ charge. Spintronics is already exploited
in read heads for computer hard-disk drives and may provide
new functionalities in a variety of other electronic devices.
When an electric
current passes through the NIST oscillators, the electrons
in the current align their spins to match the orientation
of the first magnetic layer in the device. When the now-aligned
electrons flow through the second magnetic layer, the spin
of the electrons is transferred to the film. The result is
that the magnetization of the film oscillates much like a
spinning top. The oscillation generates a microwave signal,
which can be tuned from less than 5 gigahertz (5 billion oscillations
a second) to more than 35 gigahertz by manipulating the current
or an external magnetic field. In contrast, most cell phones
transmit and receive signals at frequencies between 1 and
2 gigahertz.
Scientists long
have known that an oscillator can be forced to sympathetically
synchronize to an applied signal that is close to its own
frequency. That is, if small, periodic “nudges”
are applied to an oscillator, eventually it will synchronize
to those nudges. In the latest NIST experiments, certain combinations
of currents applied to both oscillators cause their respective
frequencies to approach each other and eventually lock together.
In a
related paper published Aug. 5 in Physical Review Letters,**
the NIST research group demonstrated that nano-oscillators
can be locked to an externally applied signal. This work also
showed how to vary the phase of the oscillation (the positions
of the peaks and troughs of the wave pattern), a technique
used in radar and directional transmissions. “This work
suggests the interesting possibility of using the oscillators
for ‘nano-wireless’ communications within or between
chips on a circuit board,” says William Rippard, a member
of the NIST group.
NIST scientists
are still studying exactly why locking occurs between nano-oscillators.
One possible mechanism is the emission of “spin waves,”
the magnetic analog of waves in the ocean. In magnetic systems
these waves are alternating variations in the direction of
the magnetization. The waves created by the two oscillators
may overlap and synchronize. Alternatively, each oscillator
can be thought of as a bar magnet spinning around its midpoint
or end over end. Attractive and repulsive forces between the
devices’ poles may cause them to spin in a complementary
pattern, thereby synchronizing the oscillations.
The spintronics
work at NIST was funded in part by the Defense Advanced Research
Projects Agency.
As an agency of
the U.S. Department of Commerce’s Technology Administration,
NIST develops and promotes measurement, standards and technology
to enhance productivity, facilitate trade and improve the
quality of life.
* S.F.
Kaka, M.R. Pufall, W.H. Rippard, T.J. Silva, and S.E. Russek.
2005. Mutual Phase-Locking of Microwave Spin Torque Nano-Oscillators.
Nature. Sept. 15.
** W.H.
Rippard, M.R. Pufall, S.F. Kaka, T.J. Silva, S.E. Russek,
and J.A. Katine. 2005. Injection Locking and Phase Control
of Spin Transfer Nano-Oscillators. Physical Review Letters.
Aug. 5.
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