Focus: Timing is Everything
Like spies on a secret mission, the different parts of an electronic system must carry precisely synchronized clocks to coordinate their tasks. Recently, researchers harnessed the spins of electrons to create oscillations which they said suggested a new type of electronic clock. Now a report in the 9 January PRL demonstrates an improved version with far higher quality oscillations, which experts say demonstrates the commercial potential of the device as a small, versatile clock for electronic chips.
Data from the processor of a personal computer must be sent to the memory at the expected time or it will be lost; cell phones and base stations must agree on the frequency they use. Quartz crystal oscillators–like those in modern watches–can provide regular “clock” pulses to keep these devices communicating. But a typical crystal is millimeters in size, huge by modern standards. And it’s an “add-on” to the computer chip, rather than an integrated part of it.
In 2003, a team from Cornell University in Ithaca, New York, generated high frequency oscillations in a way that might be put directly onto a computer chip . In a magnetic field, they forced electrons with spins aligned to one layer of magnetic material into a neighboring layer with different magnetic properties. To concentrate the electrons, the team pushed the current through a tiny cylinder containing the magnetic layers, which they called a “nanopillar.” The injected electrons caused the magnetization of the destination layer to wobble and emit microwaves, just as blowing into a whistle generates sound waves. But the microwaves emerged at many frequencies, like the jumble of sound frequencies from a cymbal crash. Only by carefully adjusting the conditions could the team observe a single frequency.
To create oscillations better suited to commercial electronics, Bill Rippard of the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and his colleagues designed their oscillator differently. The NIST researchers injected current from a 40-nanometer-wide contact on top of a large magnetic layer. This arrangement avoided the spin-degrading rough edges of the nanopillar but kept the electrons concentrated in a small region.
The new device emits a pure microwave “tone,” like the sound of a tuning fork. Rippard says recent results show as many as 18,000 oscillations before losing time, comparable to the quartz crystals in watches. This so-called quality factor, or “Q,” is much higher than the value of about 50 achieved by the Cornell team, although the NIST researchers had reached a Q of only 800 at the time they submitted their paper to PRL. By varying the magnetic field, they could adjust the oscillation frequency between about 5 and 40 gigahertz, a wider range than the previous device, and one that includes high-speed internet communications and collision-avoidance radar for cars.
This is the first publication showing that the structure “has technological potential,” says Nick Rizzo of Motorola, Inc, in Phoenix, Arizona. Motorola is collaborating with the NIST and Cornell groups in a project funded by the Defense Department’s Advanced Research Projects Agency (DARPA). Rizzo believes that processes being developed by Motorola and IBM might be adapted to graft the NIST-style “nano-oscillators” onto silicon chips.
- “Microwave oscillations of a nanomagnet driven by a spin-polarized current, ” S.I. Kiselev, J.C. Sankey, I.N. Krivorotov, N.C. Emley, R.J. Schoelkopf, R.A. Buhrman, and D.C. Ralph, Nature (London) 425, 380 (2003)