Chill!
Atoms in zincochromite, a "geometrically frustrated
magnet," resolve their frustration through group
spin control. Neighboring tetraheda (solids with four
triangular faces) contribute a side each to create
hexagonal (six-sided) spin clusters. A hexagon bunches
the spins of magnetic atoms-one at each corner-into
a single "spin director" (arrows). The composite
behavior achieves local magnetic order.
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When
"frustrated" by their arrangement, magnetic atoms
surrender their individuality, stop competing with their
neighbors and then practice a group version of spin controlacting
collectively to achieve local magnetic orderaccording
to scientists from the Commerce Department's National Institute
of Standards and Technology, Johns Hopkins University and
Rutgers University writing in the Aug. 22, 2002, issue of
the journal Nature.
The
unexpected composite behavior detected in experiments done
at the NIST Center for Neutron Research (NCNR) accounts
for the range of surprisingand, heretofore, unexplainableproperties
of so-called geometrically frustrated magnets, the subject
of intensifying research efforts that may lead to new types
of matter. The finding also may shed light on natural clustering
processes including the assembly of quarks and other minuscule
components into atoms, the folding of proteins and the clumping
of stars in galaxies, the scientists say.
These
and other important phenomenaincluding high-temperature
superconductivitysuggest that there are "higher-order
organizing principles that are intrinsic to nature,"
explains lead author Seung-Hun Lee, NCNR staff physicist.
The
team discovered that self-organized "spin clusters"
emerge out of competing interactions in a geometrically
frustrated magnet. Though involving interactions on a very
tiny scalemeasured in nanometers (billionths of a
meter)the team says its discovery may provide a new
model for exploring "emergent structure in complex
interacting
systems" on different levels. They singled out research
on protein folding as a potential beneficiary. In protein
folding, cells assemble units called amino acids into complex
three-dimensional shapes that dictate the function of the
resulting protein.
Lee
and colleagues set out to determine how atoms arrayed in
the latticelike geometry of frustrated magnets resolve
an apparent predicament: how to align their spins-the sources
of magnetismwhen faced with a bewildering number of
options.
As a
conventional magnet cools, atoms pair up with their neighbors
and line up their spins, so that they spin in parallel or
in opposition (antiparallel). At a temperature unique to
the type of material, the magnet undergoes a phase transition,
at which a highly symmetrical, long-range ordering of spins
is achieved. The material and each spin are said to be in
their ground state, a condition of equilibrium, or ultimate
stability.
For
illustration, this spin-ordering is accomplished easily
in materials with squares as a structural building block.
An atom can spin antiparallel to the spins of the atoms
in the two adjacent corners.
This
is not the case for a geometrically frustrated magnet, which
is assembled from triangular units. If atoms at two corners
spin antiparallel, the atom in the third is left with a
no-win situation. Whichever orientation it chooses, the
third atom will be out of sync with one of its two neighbors.
As a result, the entire system is "geometrically frustrated"
and all spins can fluctuate among a range of potential ground
states. Long-range order is not attainable, raising the
question as to how spins organize locally to cope with a
seemingly confusing array of alignment options.
At the
NCNR, researchers used neutrons, which are sensitive to
magnetic spins, to probe magnetic interactions in zincochromite,
a mineral whose crystal structure consists of tetrahedral
building blocks with four triangular faces. Beams of neutrons
can serve as a high-power magnetic microscope that reveals
the geometric arrangement of spins in a solid and how this
arrangement evolves as temperature changes. Patterns of
neutrons that scattered after they were beamed at zincochromite
samples revealed orderly groupings of spins.
The
researchers determined that, at low temperatures, the spins
organize into six-sided, or hexagonal, structures that repeat
throughout the material. Six neighboring tetrahedra contribute
one side each to the hexagon. In turn, six spins, one at
each corner, are arranged so that each one is antiparallel
to its two nearest neighborsa highly stable organization.
The
patterns of scattered neutrons also suggest that the six
hexagon spins act in concert, bunching all spins into one
and creating what Lee and his colleagues call a "spin
director." Each hexagon achieves local magnetic order
and its spin director is largely confined, interacting only
weakly with the spin directors of neighboring hexagons.
As a
result, the researchers say, geometrically frustrated magnets
are not, as suspected, a system of strongly interacting
spins, but rather a "protectorate of weakly interacting"
composite spins.
In addition
to Lee, collaborators include Collin Broholm of Johns Hopkins
University and the NCNR; William Ratcliff of Rutgers University;
Goran Gasparovic of Johns Hopkins; Qing Zhen Huang of the
NCNR; Tae Hee Kim of Rutgers; and Sang-Wook Cheong of Rutgers.
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