Measurement
results presented today by scientists at the Commerce Department’s
National Institute of Standards
and Technology (NIST) do not corroborate reports of dramatic
changes in electrical resistance attributed to ultra-small
magnetic sensors that exploit a quantum phenomenon regarded,
by some, as the next big thing in magnetic-data-storage technology.
The findings
were presented at the American Vacuum Society’s 50th
annual international symposium and exhibition in Baltimore,
Md.
Reports
since mid-2002 have credited the so-called ballistic magnetoresistance
(BMR) effect with increases in electrical resistance ranging
from a few hundred percent to a million percent, fueling preliminary
prospects for vastly improved read heads in hard-disk drives.
NIST
research chemist William Egelhoff Jr. said such “impressive”
changes in electrical resistance described in articles and
at conferences appear to stem from “artifacts,”
or factors unrelated to the quantum effect. For example, when
two nickel wires in a T-shaped arrangement were exposed to
a magnetic field, they contracted. Shortening of the wires
can stretch and severely distort the cluster of atoms that
formed the nanometer-scale contact between them, resulting
in large increases in resistance.
In contrast,
Egelhoff said, measurements on nanocontacts between wires
that do not shorten in the presence of a magnetic field, thereby
eliminating this artifact, failed to show sizable resistance
increases.
In fact,
one set of studies demonstrated that repeatedly exposing a
tiny contact, about 10 nanometers in diameter, to a magnetic
field results in the equivalent of molecular-scale tug of
war. As the two nickel wires shortened or lengthened by a
calculated 35 nanometers, the nanocontact between them would
break and then reform. If unrecognized, the cycle of breaking
and rebuilding the narrow bridge between the wires would yield
values that could be mistaken for an infinite BMR effect,
Egelhoff said.
The NIST
team has demonstrated several other types of artifacts that
can mimic electrical-resistance swings corresponding closely
to BMR-effect values reported in scientific journals and at
conferences.
To prevent unwanted
influences that can mask true experimental results, Egelhoff
and his colleagues developed a set of recommended procedures
for “performing measurements in an artifact-free manner.”
The NIST work is
continuing, with the aim of achieving a more complete understanding
of BMR. The phenomenon is predicted to occur when electrons,
the carriers of electric current, traverse through channels
so narrow that they must proceed in a straight line, all with
an unchanged spin, or magnetic orientation. However, theoretical
physics does not offer an explanation for why large changes
in electrical resistance would result.
NIST results to
date might account for why some researchers have not been
able to reproduce the high BMR-effect values reported by others.
“The BMR
effect is predicted by theory,” Egelhoff explains, “but
the ‘impressive’ results are much too large. At
this point, it’s inconclusive as to whether a real BMR
effect will be found and, if so, whether it will prove large
enough to be of much interest to the magnetic-sensor community.”
Magnetic sensors
serve as read heads, a key technology of the estimated $50
billion hard-disk-drive industry. As read heads decrease in
size and increase in sensitivity to magnetic signals, more
data can be squeezed onto smaller spaces on disks. Since the
late 1990s, data-storage capacity on magnetic disks has been
doubling almost annually.
Reports that nanometer-scale
metal sensors were tremendously more sensitive to magnetic
fields—due, seemingly, to the BMR effect—sparked
speculation that storage capacity might be increased by a
factor of a thousand or more.
Egelhoff and his
team have collaborated with several laboratories that reported
these initial results. The NIST team evaluated the experiments
and identified several potential factors unrelated to BMR
that could lead to results attributed to the quantum phenomenon.
Last December,
Egelhoff co-authored a paper, based on work carried out by
collaborators at two other institutions, that attributed a
400 percent change in electrical resistance to the BMR effect.
On the basis of follow-up studies, he now concludes that the
reported change was the consequence of contact deformation
and other artifacts that were not reckoned with during the
initial experiment.
As a non-regulatory
agency of the U.S. Department of Commerce’s Technology
Administration, NIST develops and promotes measurement, standards
and technology to enhance productivity, facilitate trade and
improve the quality of life. For more information on NIST,
visit www.nist.gov.
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