“This project is part of continuing work in ‘spintronics’ - trying to make electrical devices that take advantage of the electron’s intrinsic spin, and not just its charge,” Dan Ralph of Cornell told nanotechweb.org. “The electron spin interacts strongly with magnetic elements in devices, and in the past there has been a great deal of work dealing with how the orientation in magnets affects the flow of spin-polarized electrons. Our paper is a step forward in understanding the inverse effect - can the flow of spin-polarized electrons affect the orientation of magnetic elements in a device?”
Ralph and colleagues used a multilayer consisting of 80 nm of copper, 40 nm of cobalt, 10 nm of copper, 3 nm of cobalt, 2 nm of copper and 30 nm of platinum on an oxidized silicon wafer. They milled the multilayer into a pillar with an elliptical cross-section measuring 130 x 70 nm. By transmitting or reflecting electrons from the thicker ‘fixed’ cobalt layer, the scientists produced a spin-polarized current that could apply a torque to the thinner ‘free’ cobalt layer. Changes in the magnetization of this free layer relative to the fixed layer altered the device resistance. As a result, under a d.c. current bias, magnetic dynamics resulted in a time-varying voltage with typical frequencies in the microwave range.
“It was discovered about four years ago that spin-polarized electrons can apply torques on magnets, but no-one had developed the experimental techniques to understand exactly how a magnet can move in response to this torque,” said Ralph. “We set out to develop some way to measure this.”
According to Ralph, the team found that the torques from spin-polarized electrons were strong enough to produce large-angle periodic oscillations of a magnet. The researchers also saw several different dynamical modes (or types of motions) for the magnet. “Comparisons of these observations to different theories will help to produce an improved understanding about the strength and angular dependence of the torques themselves,” said Ralph.
“Finally, we found that as the magnet oscillates, and consequently the resistance of the device also oscillates, the magnitude of the resistance changes is surprisingly large,” Ralph continued. “This is related to the fact that the oscillations cover large angles. Since a current is already flowing through the device to generate the initial torques, the oscillating resistance results in an oscillating voltage at microwave frequencies. The devices might therefore serve naturally as nanoscale sources of microwaves.”
Ralph believes that the main applications for the phenomenon will be new approaches to spintronic devices. “We can think about using currents to manipulate magnets, whereas most past work has used magnets to manipulate currents,” he said. “That means, in addition to making things like magnetic memories, one can consider devices with more intricate time dependence, like small microwave sources or oscillators.”
Now the researchers, who reported their work in Nature, plan to optimize the device to see if they can “bring the parameters necessary to produce dynamical effects into a range that makes applications practicable”.