A tiny grid pattern has led materials scientists at the National
Institute of Standards and Technology (NIST) and the Institute of Solid
State Physics in Russia to an unexpected finding-the surprisingly strong and
long-range effects of certain electromagnetic nanostructures used in data storage.
Their recently reported findings* may add new scientific challenges to the design
and manufacture of future ultra-high density data storage devices.
The team was studying the behavior of nanoscale structures that sandwich thin
layers of materials with differing magnetic properties. In the past few decades
such structures have been the subjects of intense research because they can
have unusual and valuable magnetic properties. The data read heads on modern
high-density disk drives usually exploit a version of the giant magnetoresistance
(GMR) effect, which uses such layered structures for extremely sensitive magnetic
field detectors. Arrays of nanoscale sandwiches of a similar type might be used
in future data storage devices that would outdo even today’s astonishingly
capacious microdrives because in principle the structures could be made even
smaller than the minimum practical size for the magnetic islands that record
data on hard disk drives, according to NIST metallurgist Robert Shull.
The key trick is to cover a thin layer of a ferromagnetic material, in which
the magnetic direction of electrons, or “spins,” tend to order themselves
in the same direction, with an antiferromagnetic layer in which the spins tend
to orient in opposite directions. By itself, the ferromagnetic layer will tend
to magnetize in the direction of an externally imposed magnetic field—and
just as easily magnetize in the opposite direction if the external field is
reversed. For reasons that are still debated, the presence of the antiferromagnetic
layer changes this. It biases the ferromagnet in one preferred direction, essentially
pinning its field in that orientation. In a magnetoresistance read head, for
example, this pinned layer serves as a reference direction that the sensor uses
in detecting changing field directions on the disk that it is “reading.”.
Researchers have long understood this pinning effect to be a short-range phenomenon.
The influence of the antiferromagnetic layer is felt only a few tens of nanometers
down into the ferromagnetic layer—verticallly. But what about sideways?
To find out, the NIST/ISSP team started with a thin ferromagnetic film covering
a silicon wafer and then added on top a grid of antiferromagnetic strips about
10 nanometers thick and 10 micrometers wide, separated by gaps of about 100
micrometers. Using an instrument that provided real-time images of the magnetization
within grid the structure, the team watched the grid structure as they increased
and decreased the magnetic field surrounding it.
What they found surprised them.
As expected, the ferromagnetic material directly under the grid lines showed
the pinning effect, but, quite unexpectedly, so did the uncovered material in
regions between the grid lines far removed from the antiferromagnetic material.
“This pinning effect extends for maybe tens of nanometers down into the
ferromagnet right underneath,” explains Shull, “so you might expect
that there could be some residual effect maybe tens of nanometers away from
it to the sides. But you wouldn’t expect it to extend 10 micrometers away—that’s
10 thousand nanometers.” In fact, the effect extends to regions 50 micrometers
away from the closest antiferromagnetic strip, at least 1,000 times further
than was previously known to be possible.
The ramifications, says Shull, are that engineers planning to build dense arrays
of these structures onto a chip for high-performance memory or sensor devices
will find interesting new scientific issues for investigation in optimizing
how closely they can be packed without interfering with each other.
* Y.P. Kabanov, V.I. Nikitenko, O.A. Tikhomirov, W.F. Egelhoff, A.J. Shapiro
and R.D. Shull. Unexpectedly long-range influence on thin-film magnetization
reversal of a ferromagnet by a rectangular array of FeMn pinning films. Physical
Review B 79, 144435, 2009. DOI: 10.1103/PhysRevB.79.144435.