|
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.
|
When
"frustrated" by their arrangement, magnetic atoms surrender
their individuality, stop competing with their neighbors and
then practice a group version of spin control—acting
collectively to achieve local magnetic order—according 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 surprising—and, heretofore, unexplainable—properties
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 phenomena—including high-temperature
superconductivity—suggest 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 scale—measured 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
lattice—like geometry of frustrated magnets resolve an
apparent predicament: how to align their spins-the sources of
magnetism—when 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 neighbors—a 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.
As a
non-regulatory agency of the U.S. Department of Commerce's
Technology Administration, NIST develops and promotes
measurements, standards, and technology to enhance
productivity, facilitate trade and improve the quality of
life.
For more
information on NIST, visit http://www.nist.gov/.
