Submitted by: Cornell University Press Office
Edited by: John R. Gyorki, Editor in Chief
Putting a spin on it: Physicists measure current-induced torque in nonvolatile magnetic memory devices.
Tomorrow’s nonvolatile computer memory devices that retain stored information even when not powered will profoundly change electronics. Cornell University researchers have discovered a new way of measuring and optimizing their performance.
Using a high-speed oscilloscope, researchers led by Dan Ralph, the Horace Wright Professor of Physics and Robert Buhrman, the J. E. Sweet Professor of Applied and Engineering Physics, have discovered a way to measure the magnitude of the current-induced torque that writes information in magnetic tunnel junction memories.
Magnetic tunnel junctions comprise a sandwich of two ferromagnets with an oxide insulator–measured in nanometers–between them. The electrical resistance for parallel orientations is different than that for random orientations of the magnetic electrodes. This allows the two states to create a nonvolatile memory that does not require electricity for storing information.
An example of nonvolatile memory is flash memory, but this type is a silicon device that can lose its capability to store information after repeated writing cycles. By comparison, magnetic memory does not have this limitation.
The intrinsic fact that magnetic fields are required to switch the magnetic states–that is, write the information–has hampered magnetic-memory development. The magnetic fields themselves limit the magnetic memory size and efficiency because the fields occupy some surrounding space and are relatively weak. By comparison to solid-state devices, magnetic devices need larger currents to generate sufficient field strength to switch the device.
The Cornell researchers are studying a new generation of magnetic devices that can write information without using magnetic fields. Instead, they use a mechanism called “spin torque,” which arises from the idea that electrons have a fundamental spin. When the electrons interact with the magnets in the tunnel junctions, they transfer some of their angular momentum. This can provide an extremely strong torque per unit of current and has been demonstrated to be at least 500 times more efficient than magnetic fields to write magnetic information.
To measure these spin torques, the researchers used an oscilloscope in a facility operated by Cornell’s Center for Nanoscale Systems. They applied torque to the magnetic tunnel junctions using an alternating current and measured the amplitude of the resulting oscillations that were generated by the varying resistance. Because the resistance depends on the relative orientation of the two magnets in the tunnel junction, the amplitude of the oscillations could be related directly to the amplitude of the magnetic motion, and hence, the magnitude of the torque.
The researchers hope such experiments will help industry make better nonvolatile memory devices by understanding exactly how to structure them, and select the best materials for the oxide insulators and the ferromagnets surrounding them.
The work was supported by the National Science Foundation, the Army Research Office, and the Office of Naval Research. It also included collaborators Chen Wang, graduate student and first author; graduate student Yong-Tao Cui; and Jordan A. Katine from Hitachi Global Storage Technologies.
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