  Rotating the magnetization of a shape anisotropic magnetostrictive nanomagnet
with voltage-generated stress/strain dissipates much less energy than most
other magnetization rotation schemes, but its application to writing bits in
non-volatile magnetic memory has been hindered by the fundamental inability of
stress/strain to rotate magnetization by full 180 degrees. Normally,
stress/strain can rotate the magnetization of a shape anisotropic elliptical
nanomagnet by only up to 90 degrees, resulting in incomplete magnetization
reversal. Recently, we predicted that applying uniaxial stress sequentially
along two different axes that are not collinear with the major or minor axis of
the elliptical nanomagnet will rotate the magnetization by full 180 degrees.
Here, we demonstrate this complete 180 degree rotation in elliptical
Co-nanomagnets (fabricated on a piezoelectric substrate) at room temperature.
The two stresses are generated by sequentially applying voltages to two pairs
of shorted electrodes placed on the substrate such that the line joining the
centers of the electrodes in one pair intersects the major axis of a nanomagnet
at ~+30 degrees and the line joining the centers of the electrodes in the other
pair intersects at ~ -30 degrees. A finite element analysis has been performed
to determine the stress distribution underneath the nanomagnets when one or
both pairs of electrodes are activated, and this has been approximately
incorporated into a micromagnetic simulation of magnetization dynamics to
confirm that the generated stress can produce the observed magnetization
rotations. This result portends an extremely energy-efficient non-volatile
"straintronic" memory technology predicated on writing bits in nanomagnets with
electrically generated stress.
