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.
*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.
