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GAITHERSBURG,
MD -- Nanocomposite materials seem to flout conventions
of physics. In the latest example of surprising behavior,
reported* by scientists at the National Institute
of Standards and Technology (NIST) and Brookhaven
National Laboratory, a class of nanostructured materials
that are key components of computer memories and other
important technologies undergo a previously unrecognized
shift in the rate at which magnetization changes at
low temperatures.
The team suggests that the
apparent anomaly described as an “upturn” in magnetization
may be due to the quantum mechanical process known
as Bose-Einstein condensation. They maintain that,
in nanostructured magnets, energy waves called magnons
coalesce into a common ground state and, in effect,
become one. This collective identity, the researchers
say, results in magnetic behavior seemingly at odds
with a long-standing theory.
The new finding could prompt
a reassessment of test methods used to predict technologically
important properties of "ferromagnetic"
materials. The results also could point the way to
marked improvements in the performance of microwave
devices. Magnets are integral to these devices, used
in a variety of communication and defense technologies.
Ferromagnets, including iron,
cobalt, nickel and many tailor-made materials, become
magnetic when exposed to an external magnetic field.
As the strength of the external field increases, the
materials become more magnetic, an atomic-level, temperature-influenced
process called magnetic saturation. When the external
field is removed, ferromagnets undergo an internal
restructuring and the acquired magnetization decays,
or fades, very slowly at a rate that increases with
temperature.
Determined through accelerated
testing methods, the temperature dependence of magnetic
saturation and the rate of magnetization decay are
key concerns in the design of permanent magnets, hard
disks and other magnetic data storage systems.
The curious “upturn” in magnetic
saturation is consistent with another magnetic anomaly
reported in 1987 by NIST materials scientist Lawrence
Bennett and colleagues. In an analysis of magnetic
decay in a nickel-copper alloy, the team found a then-inexplicable
peak in the decay rate within a range of low temperatures.
“Two very different experiments,
almost 20 years apart, gave us similar results,” explains
Bennett. “These phenomena appear to be confined entirely
to nanostructured materials.”
Bennett is a co-author of the
new report, along with Edward Della Torre, a NIST
materials researcher and engineering professor at
George Washington University, and Richard Watson,
a theorist at Brookhaven National Laboratory.
In ferromagnetic materials
immersed in a magnetic field, magnetization increases
as the temperature drops. Cooling permits electrons,
whirling like tops as they rotate about and among
atoms that make up the materials, to line up their
spins with the external field. As more heat energy
is lost, more electrons align their spins in a very
tidy arrangement. The strength of magnetization rises
as this long-range ordering extends inside the material.
In so-called single-crystal
ferromagnets, with their lattice-like atomic arrangement,
the alignment of spins proceeds almost systematically.
In fact, this seemingly straightforward relationship
between temperature and magnetization had been reduced
to a formula (known as Bloch’s temperature law) more
than seven decades ago.
The more structurally disordered
multilayered cobalt-platinum ferromagnet initially
evaluated by the researchers did not conform with
the textbooks, however. As the temperature was lowered,
the magnetization started increasing faster than expected,
beginning at 14 degrees above the coldest possible
temperature, called absolute zero. And the rate remained
unexpectedly high down to 2 degrees above absolute
zero.
The researchers attribute this
apparent law-defying behavior to the banding together
of variously dispersed magnons into a kind of quantum
confederation. The shared identity technically termed
a Bose Einstein condensate has a countervailing influence
on normally unruly magnons.
Magnons typically are isolated
wave patterns that are out of magnetic alignment with
the rest of a sample, an indication that spinning
electrons are breaking ranks. In effect, magnons could
be classified as “anti magnetic.” Bose-Einstein condensation
results in a collective behavior that appears to counter
this tendency among magnons, leading to the observed
upturn in magnetization.
Rather than rewriting a long-accepted
law of physics, this new understanding can be used
to extend Bloch’s law into the nanostructural regime,
explains Della Torre. After inserting a term that
accounts for energy change in a system, the team used
the law to predict the high rate of saturation magnetization
observed in several types of ferromagnetic nanocomposites.
“Now,” says Bennett, “the challenge
is to determine how the size, shape and other features
of nanostructured materials are related to the Bose-Einstein
condensation temperature.”
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.
CONTACT: Mark Bello, mark.bello@nist.gov
(301) 975-3776
*E. Della Torre, L.H. Bennett, and R.E. Watson,“Extension
of the Bloch T3/2 Law to Magnetic Nanostructrures:
Bose-Einstein Condensation,” Physical Review Letters,
April 15, 2005.
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