Tomorrow’s nonvolatile memory devices—computer memory that can retain stored
information even when not powered—will change electronics, and Cornell Univ.
researchers have discovered a new way of measuring and optimizing their
performance.
Using an oscilloscope, researchers led by Dan Ralph, the Horace White
Professor of Physics, and Robert Buhrman, the J.E. Sweet Professor of Applied
and Engineering Physics, have figured out how to quantify the strength of
current-induced torques used to write information in memory devices called
magnetic tunnel junctions. The results were published online in 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
nonvolatile memory element that doesn’t require electricity for storing
information. An example of nonvolatile 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 required
magnetic fields to switch the magnetic states—that is, to write information.
This limits their size and efficiency because magnetic fields are long-ranged
and relatively weak, so that 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. 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 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 industry 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.
