Spin torque used to write info on memory devices

Cornell University researchers have discovered, with the help of a very fast oscilloscope, how to quantify the strength of current-induced torques used in writing information on magnetic tunnel junctions. The results of the study, which was led by Horace White professor of Physics Dan Ralph and J.E. Sweet professor of Applied and Engineering Physics Robert Buhrman, can be viewed 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 these two states create a non-volatile memory element that does not require electricity for storing information. An example of non-volatile memory today is flash memory, but that is 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 always required magnetic fields to switch the magnetic states, that is, to write information. This limits the size and efficiency of magnetic fields because they are long-ranged and relatively weak. Hence, large currents and thick wires are needed to generate a large-enough field to switch the device.

The Cornell researchers are studying a new generation of magnetic devices that can write information without using magnetic fields. They are instead using 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 a very strong torque per unit current, and has been demonstrated to be at least 500 times more efficient than using magnetic fields to write magnetic information, Ralph said.

To measure these spin torques, the researchers 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.

The researchers hope such experiments will help the industry make better non-volatile memory devices by understanding exactly how to structure them and determining what materials should be used for the oxide insulators and ferromagnets surrounding them.

- Julien Happich
??EE Times