Just as cobalt blue’s lustrous hue attracts artists and decorators, the antiferromagnetic properties of the responsible compound—cobalt aluminate—are attracting neutron scientists at DOE’s Oak Ridge National Laboratory. Studies of magnetic interactions deep within the material’s atomic structure may provide clues toward the development of energy-efficient technologies. Light sconce image: B. Jefferson Bolender |
Neutron scattering studies of “cobalt
blue,” a compound prized by artists for its lustrous blue hue, are
revealing unique magnetic characteristics that could answer questions about
mysterious properties in other materials.
Experiments at the Spallation Neutron Source
(SNS) and High Flux Isotope Reactor (HFIR), both located at the Department of
Energy’s Oak Ridge National Laboratory (ORNL), indicate novel behaviors in the
antiferromagnetic material cobalt aluminum oxide,—CoAl2O4, or cobalt aluminate—which
researcher Gregory MacDougall of ORNL’s Neutron Scattering Sciences Division
describes as a “highly frustrated magnetic system.”
“Frustrated” in this context
refers to a condition where competing interactions between the magnetic spins
within the atomic structure prevent the establishment of a long-range ordered
state.
“Frustration is often associated with
exotic behavior in materials, including piezoelectricity, multiferrocity, and
high-temperature superconductivity, each of which is potentially important for
future energy-efficient technologies,” MacDougall says.
Antiferromagnetism is a type of magnetic
order commonly found in materials below a certain temperature where the
microscopic magnetic moments (often called “spins”) on neighboring
atoms align with their north and south poles oriented in opposite directions.
Long-range antiferromagnetic order is technologically important for magnetic
information storage.
The single-crystal experiments performed at
ORNL showed the magnetic properties of cobalt aluminate exhibited drastic
changes at the numbingly low temperature of 6.5 K. The experiments showed that
effects from competing interactions may be responsible for its intriguing but
poorly understood magnetic properties.
“Cobalt blue demonstrates behaviors
that have never before been appreciated in a frustrated magnet, but have been
seen in other materials,” MacDougall says.
“Typically, frustration in the lattice
from different energy scales and competing interactions drives the ordering
temperature down. What we’ve found is, instead of eliminating ordering
entirely, the long-range order is broken up into several small domains, in
which the motion of the domain walls is frozen into place,” MacDougall
says.
Sharp walls separate those smaller
atom-scale domains, set apart by the orientation of the atoms’ magnetic spin.
The result of freezing such walls into place is a glass-like behavior, normally
indicative of highly disordered structure.
In cobalt aluminate’s case, however, the
glass-like behavior is exhibited on a very clean, ordered crystal. “We
think this may explain unexpected glass-like behavior in other frustrated
systems,” MacDougall says.
The research, reported in Proceedings of the National Academy of
Sciences, is part of a larger program to study magnetic frustration—what
happens in magnetic systems when the geometry of the system or competing
interactions frustrate or suppress the interactions that normally drive order,
allowing novel behaviors to emerge.
“This is where you discover new physics,”
MacDougall says.
Cobalt aluminate is the compound responsible
for cobalt blue, a vivid pigment used in paintings, colored glass, and even to
color concrete.
“In the past seven or eight years
people have become interested in cobalt blue’s magnetic properties because it
turns out to be a prototypical system where competing interactions suppress
magnetic order, and it is predicted to have novel ground states,”
MacDougall says.
The experiments were performed on two of
HFIR’s Triple Axis Spectrometers and the SNS’s Cold Chopper Neutron
Spectrometer (CNCS), making use of both thermal and “chilled”,
low-energy neutrons to study the cobalt aluminate at low, near absolute-zero
temperatures. The single-crystal samples were fabricated in collaboration with ORNL’s
Correlated Electron Materials group.
MacDougall and colleagues used
the triple-axis spectrometers at HFIR to study the ordering pattern of the
cobalt blue lattice, which revealed the smaller domains forming at low
temperatures. With SNS’s CNCS, the researchers were able to study how
long-lengthscale perturbations in the magnetic ordered states, known as
“spin-waves”, moved through the system. The speed of those spin waves
in different directions is a sensitive measure of the strength of the interactions
between atoms in the cobalt blue system.
