Nanomagnets tapped to generate microwave fields

HANCOCK, N.H. — A new form of electromagnetic interaction in which electron spin changes the magnetic direction of cobalt nanomagnets is being explored at Cornell University with an eye toward new types of memory and signal-processing devices.

Experiments have shown that the impact of spin-polarized electrons causes the nanomagnets to precess at high speed so that a direct current can produce microwave-frequency oscillations.

The effect might be useful in creating wireless communications between layers in an integrated circuit.

"We are at about 100 nanometers with the devices, which is in range of what silicon people are doing-it might be a next-generation memory technology," said Daniel Ralph, who leads the project.

In previous work, the Cornell group had to use features as small as 10 nm to produce the effect. A new architecture has made it possible to define the magnets with masks and conventional deposition processes.

Test devices are fabricated at the Cornell Nanofabrication center using electron-beam equipment and ion-milling masks to define features.

Ralph and his colleagues are developing the technology as a possible terabit nonvolatile-memory technology that could be driven by electric currents rather than switched with magnetic fields. And because the effect is so novel, there may be other, unforeseen applications for it. Previously, the only way to change the magnetic orientation of a magnet would have been with a magnetic field.

The new effect results from the transfer of electron spin to the magnet. Spin interactions are an active area of research involving a variety of "spintronic" devices. Rather than using the electron charge to maintain a voltage, as in conventional electronics, spintronic circuits use another fundamental property of the electron-its spin-as the signal.

In the latest architecture, the nanomagnets are created as thin cobalt layers in a gold pillar with a diameter of less than 100 nm. The pillar is sandwiched between copper films and surrounded by polyimide, a neutral filler. The effect is not dependent on that particular materials system: The research group has duplicated the effect with many types of magnetic materials and found consistent results across them.

In one configuration the group tried, two magnets are separated by a layer of copper, with one magnet thicker than the other. That system can be switched between two stable magnetic configurations with a spin-polarized current of electrons and could be used as a nonvolatile memory element. "We have been working on getting the right impedances and a low enough switching current to make it competitive with the type of things Motorola is doing with magnetic-driven nonvolatile RAM," Ralph said. "We recently were able to reduce the current by four orders of magnitude, which we think makes it competitive."

The truly novel aspect of the device emerges when a magnetic field is applied and the electrons transfer their spin to the nanomagnets. "Any magnet pushed out of its preferred direction will begin to precess, with its own natural frequency determined by factors such as the magnetic field," Ralph said. "In this case, the spin of electrons displaces the magnet from its preferred direction, causing a rapid oscillation."

Another unusual property of the spin-induced oscillation is its high degree of constancy. "The two things people are thinking about are a kind of clock oscillator-this is accurate to one part in 20,000-[and] chip-to-chip communications," Ralph said.

A group at the National Institute of Standards and Technology lab in Boulder, Colo., is working with the system, since it might have applications in measurement and standards.