Torque lends spin to memory storage

CORNELL (US) — Researchers have discovered a way to measure and optimize performance of computer memory that can retain stored information without power.

Using an accelerated oscilloscope, researchers at Cornell University have figured out how to quantify the strength of current-induced torques used to write information in memory devices called magnetic tunnel junctions.

The research is reported online in the journal Nature Physics.

Magnetic tunnel junctions are memory storage devices made of a sandwich of two ferromagnets with a nanometers-thick oxide insulator in between.

The electrical resistance of the device is different for parallel and nonparallel orientations of the magnetic electrodes, so that the two states create a nonvolatile memory element that doesn’t require electricity for storing information.

A current example of nonvolatile memory is flash memory, but that’s a silicon-based technology subject to wearing out after repeated writing cycles, unlike magnetic memory.

What has held back magnetic memory technology is that it has required magnetic fields to switch the magnetic states—that is, to write information, limiting size, and efficiency. Because they are long-ranged and relatively weak, magnetic memory needs large currents and thick wires to generate a large-enough field to switch the device.

Researchers are studying a new generation of magnetic devices that can write information using a mechanism called “spin torque,” instead of magnetic fields.

The spin torque technique comes from the idea that electrons have a fundamental spin (like a spinning top). When the electrons interact with the magnets in the tunnel junctions, they transfer some of their angular momentum providing a strong torque per unit current.

It has been demonstrated to be at least 500 times more efficient than using magnetic fields to write magnetic information, says Dan Ralph, professor of physics.

To measure these spin torques, Ralph and Robert Buhrman, professor of engineering, used an oscilloscope in a shared 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 resistance oscillations that resulted. Since the resistance depends on the relative orientation of the two magnets in the tunnel junction, the size of the resistance oscillations could be related directly to the amplitude of the magnetic motion, and hence to the size of the torque.

Such experiments are expected to help make better nonvolatile memory devices by understanding exactly how to structure them, and also, what materials would best be used as 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.

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