Like Fireflies And Pendulum Clocks, Nano-oscillators Synchronize Their Behavior

In the Sept. 15 issue of Nature,* NISTscientists describe "locking" the dynamic magnetic properties of twonanoscale oscillators located 500 nanometers apart, boosting the powerof the microwave signals given off by the devices. While an individualoscillator has signal power of just 10 nanowatts, the output frommultiple devices increases as the square of the number of devicesinvolved. The NIST work suggests that small arrays of 10nano-oscillators could produce signals of 1 microwatt or more,sufficient for practical use as reference oscillators or directionalmicrowave transmitters and receivers in devices such as cell phones,radar systems and computer chips.

"These nanoscale oscillatorscould potentially replace much bulkier and expensive components inmicrowave circuits," says Matthew Pufall, one of the NIST researchers."This is a significant advance in demonstrating the potential utilityof these devices."

The NIST-designed oscillators consist of asandwich of two magnetic films separated by a non-magnetic layer ofcopper. Passing an electrical current through the device causes thedirection of its magnetization to switch back and forth rapidly,producing a microwave signal. The circular devices are 50 nanometers indiameter, about one-thousandth of the width of a human hair andhundreds of times smaller than the typical microwave generators incommercial use today. The devices are compatible with conventionalsemiconductor technology, which is expected to make them inexpensive tomanufacture.

The type of signal locking observed at NIST wasfirst described by the 17th-century Dutch scientist Christiaan Huygens,who found that two pendulum clocks mounted on the same wallsynchronized their ticking, thanks to weak coupling of acoustic signalsthrough the wall. This phenomenon also occurs in many biologicalsystems, such as the synchronized flashing of fireflies, the singing ofcertain crickets, circadian rhythms in which biological cycles arelocked to the sun, and heartbeat patterns linked to breathing speed.There are also examples in the physical sciences, such as thesynchronization of the moon's rotation with respect to its orbit aboutthe 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 oscillatormust lock to a signal transmitted by a sender.

The work describedin Nature is an advance in the field of "spintronics," which takesadvantage of the fact that the individual electrons in an electriccurrent behave like minuscule bar magnets, each having a "spin" along aparticular 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-diskdrives and may provide new functionalities in a variety of otherelectronic devices.

When an electric current passes through theNIST oscillators, the electrons in the current align their spins tomatch the orientation of the first magnetic layer in the device. Whenthe now-aligned electrons flow through the second magnetic layer, thespin of the electrons is transferred to the film. The result is thatthe magnetization of the film oscillates much like a spinning top. Theoscillation generates a microwave signal, which can be tuned from lessthan 5 gigahertz (5 billion oscillations a second) to more than 35gigahertz by manipulating the current or an external magnetic field. Incontrast, most cell phones transmit and receive signals at frequenciesbetween 1 and 2 gigahertz.

Scientists long have known that anoscillator can be forced to sympathetically synchronize to an appliedsignal that is close to its own frequency. That is, if small, periodic"nudges" are applied to an oscillator, eventually it will synchronizeto those nudges. In the latest NIST experiments, certain combinationsof currents applied to both oscillators cause their respectivefrequencies to approach each other and eventually lock together.

Ina related paper published Aug. 5 in Physical Review Letters,** the NISTresearch group demonstrated that nano-oscillators can be locked to anexternally applied signal. This work also showed how to vary the phaseof the oscillation (the positions of the peaks and troughs of the wavepattern), a technique used in radar and directional transmissions."This work suggests the interesting possibility of using theoscillators for 'nano-wireless' communications within or between chipson a circuit board," says William Rippard, a member of the NIST group.

NISTscientists are still studying exactly why locking occurs betweennano-oscillators. One possible mechanism is the emission of "spinwaves," the magnetic analog of waves in the ocean. In magnetic systemsthese waves are alternating variations in the direction of themagnetization. The waves created by the two oscillators may overlap andsynchronize. Alternatively, each oscillator can be thought of as a barmagnet spinning around its midpoint or end over end. Attractive andrepulsive forces between the devices' poles may cause them to spin in acomplementary pattern, thereby synchronizing the oscillations.

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The spintronics work at NIST was funded in part by the Defense Advanced Research Projects Agency.

Asan agency of the U.S. Department of Commerce's TechnologyAdministration, NIST develops and promotes measurement, standards andtechnology to enhance productivity, facilitate trade and improve thequality of life.

* S.F. Kaka, M.R. Pufall, W.H. Rippard, T.J.Silva, and S.E. Russek. 2005. Mutual Phase-Locking of Microwave SpinTorque 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.