have demonstrated a new strategy for making energy- efficient, reliable nonvolatile
magnetic memory devices—which retain information without electric power.
Reported online in Science, the researchers use a physical
phenomenon called the spin Hall effect, that turns out to be useful for memory
applications because it can switch magnetic poles back and forth—the basic
mechanism needed to make magnet-based computer memory.
The Cornell researchers discovered that the spin Hall effect
in the metal tantalum can be twice as strong as in any material investigated
previously, and it can provide an efficient new way to manipulate magnetic
moments. The Cornell device could give the leading nonvolatile magnetic memory
technology, called the magnetic tunnel junction, a run for its money.
“The spin Hall effect is interesting because it’s a bit
of physics people haven’t paid all that much attention to using in
applications,” said Dan Ralph, the Horace White Professor of Physics,
member of the Kavli Institute at Cornell for Nanoscale Science and the paper’s
senior co-author with Robert A. Buhrman, the J.E. Sweet Professor of Engineering.
The spin Hall effect works like this: In a heavy metal like
tantalum, electrons with intrinsic spins pointing at different angles
(electrons, in quantum mechanics, spin like a top) are deflected sideways in
different directions. Consequently, a charge current produces a net-sideways
flow of spins. This spin current can be absorbed by an adjacent magnetic layer,
applying a torque to flip the magnetic orientation. The magnet stays in place
even when no current flows, making the memory nonvolatile.
Currently, the leading technology for developing nonvolatile
magnetic memory devices is the magnetic tunnel junction, which consists of two
magnetic layers sandwiching a thin barrier. When an electrical current passes
perpendicular to the layers of a magnetic tunnel junction, one magnetic layer
polarizes the electrons, acting as a filter to produce a spin-polarized
current. The next layer can absorb this spin current and receive a torque to
flip the magnet.
A disadvantage to magnetic tunnels junctions is that the
same current path is used for both reading and writing information, making it
difficult to pass enough current through the device to achieve magnetic
switching without occasionally damaging the barrier layer, Ralph said. The
Cornell researchers’ new design uses different pathways for the reading and
writing, which is slightly less space efficient, but with as good or better
results for switching efficiency and overall reliability.