Collage of NIST “nano-eggs”—simulated magnetic patterns in NIST’s egg-shaped nanoscale magnets. Image: Talbott/NIST
Magnetics researchers at the National Institute of Standards
and Technology (NIST) colored lots of eggs recently. Bunnies and children might
find the eggs a bit small—in fact, too small to see without a microscope. But
these “eggcentric” nanomagnets have another practical use, suggesting
strategies for making future low-power computer memories.
For a study described in a new paper,* NIST researchers used
electron-beam lithography to make thousands of nickel-iron magnets, each about
200 nm in dia. Each magnet is ordinarily shaped like an ellipse, a slightly
flattened circle. Researchers also made some magnets in three different egg-like
shapes with an increasingly pointy end. It’s all part of NIST research on nano-scale
magnetic materials, devices, and measurement methods to support development of
future magnetic data storage systems.
It turns out that even small distortions in magnet shape can
lead to significant changes in magnetic properties. Researchers discovered this
by probing the magnets with a laser and analyzing what happens to the
“spins” of the electrons, a quantum property that’s responsible for
magnetic orientation. Changes in the spin orientation can propagate through the
magnet like waves at different frequencies. The more egg-like the magnet, the
more complex the wave patterns and their related frequencies. (Something
similar happens when you toss a pebble in an asymmetrically shaped pond.) The
shifts are most pronounced at the ends of the magnets.
To confirm localized magnetic effects and “color”
the eggs, scientists made simulations of various magnets using NIST’s
object-oriented micromagnetic framework (OOMMF).** Lighter colors indicate
stronger frequency signals.
The egg effects explain erratic behavior observed in large
arrays of nanomagnets, which may be imperfectly shaped by the lithography
process. Such distortions can affect switching in magnetic devices. The egg
study results may be useful in developing random-access memories (RAM) based on
interactions between electron spins and magnetized surfaces. Spin-RAM is one
approach to making future memories that could provide high-speed access to data
while reducing processor power needs by storing data permanently in
ever-smaller devices. Shaping magnets like eggs breaks up a symmetric frequency
pattern found in ellipse structures and thus offers an opportunity to customize
and control the switching process.
“For example, intentional patterning of egg-like
distortions into spinRAM memory elements may facilitate more reliable switching,”
says NIST physicist Tom Silva, an author of the new paper.
“Also, this study has provided the Easter Bunny with an
entirely new market for product development.”
*H.T. Nembach, J.M. Shaw, T.J. Silva, W.L. Johnson, S.A.
Kim, R.D. McMichael and P. Kabos. Effects of shape distortions and
imperfections on mode frequencies and collective linewidths in nanomagnets. Physical
Review B 83, 094427, March 28, 2011.
** See http://math.nist.gov/oommf/.