Brookhaven physicists Stuart Wilkins (left) and John Hill at NSLS beamline X1A2, where their research was performed with a new soft X-ray scattering facility. Photo: Brookhaven National Laboratory
Researchers at the U.S. Department of Energy’s Brookhaven
National Laboratory have observed a new way that magnetic and electric
properties—which have a long history of ignoring and counteracting each other—can
coexist in a special class of metals. These materials, known as multiferroics,
could serve as the basis for the next generation of faster and energy-efficient
logic, memory, and sensing technology.
The researchers, who worked with colleagues at the Leibniz
Institute for Solid State and Materials Research in Germany,
published their findings in Physical Review Letters.
Ferromagnets are materials that display a permanent magnetic
moment, or magnetic direction, similar to how a compass needle always points
north. They assist in a variety of daily tasks, from sticking a reminder to the
fridge door to storing information on a computer’s hard drive. Ferroelectrics are materials that display a
permanent electric polarization—a set direction of charge—and respond to the
application of an electric field by switching this direction. They are commonly
used in applications like sonar, medical imaging, and sensors.
“In principle, the coupling of an ordered magnetic material with
an ordered electric material could lead to very useful devices,” says
Brookhaven physicist Stuart Wilkins, one of the paper’s authors. “For instance,
one could imagine a device in which information is written by application of an
electric field and read by detecting its magnetic state. This would make a
faster and much more energy-efficient data storage device than is available
But multiferroics—magnetic materials with north and south poles
that can be reversed with an electric field—are rare in nature.
Ferroelectricity and magnetism tend to be mutually exclusive and interact
weakly with each other when they coexist.
The crystal structure of YMn2O5, which is made of yttrium, manganese, and oxygen. The oxygen atoms are shown in red and the yttrium atoms are gray. The magnetic moments on the manganese are shown as arrows. Ferroelectric polarization occurs between the oxygen and manganese atoms. Image: Brookhaven National Laboratory
Most models used by physicists to describe this coupling are
based on the idea of distorting the atomic arrangement, or crystal lattice, of
a magnetic material, which can result in an electric polarization.
Now, scientists have found a new way that electric and magnetic
properties can be coupled in a material. The group used extremely bright beams
of X-rays at Brookhaven’s National Synchrotron Light Source (NSLS) to examine
the electronic structure of a particular metal oxide made of yttrium,
manganese, and oxygen. They determined that the magnetic-electric coupling is
caused by the outer cloud of electrons surrounding the atom.
“Previously, this mechanism had only been predicted
theoretically and its existence was hotly debated,” Wilkins says.
In this particular material, the manganese and oxygen electrons
mix atomic orbitals in a process that creates atomic bonds and keeps the
material together. The researchers’ measurements show that this process is
dependent upon the magnetic structure of the material, which in this case,
causes the material to become ferroelectric, i.e. have an electric
polarization. In other words, any change in the material’s magnetic structure
will result in a change in direction of its ferroelectric state. By definition,
that makes the material a multiferroic.
“What is especially exciting is that this result proves the
existence of a new coupling mechanism and provides a tool to study it,” Wilkins
The researchers used a new instrument at NSLS designed to answer
key questions about many intriguing classes of materials such as multiferroics
and high-temperature superconductors, which conduct electricity without
resistance. The instrument, developed by Wilkins and Brookhaven engineers D.
Scott Coburn, William Leonhardt, and William Schoenig, will ultimately be moved
to the National Synchrotron Light Source II (NSLS-II). NSLS-II will produce X-rays
10,000 times brighter than at NSLS, enabling studies of materials’ properties
at even higher resolution.