Novel properties of ferroelectric materials
discovered at the Department of Energy’s Oak Ridge National Laboratory are
moving scientists one step closer to realizing a new paradigm of electronic
memory storage.
A new study led by ORNL’s Peter Maksymovych
and published in Nano Letters,
revealed that contrary to previous assumptions, domain walls in ferroelectric
materials act as dynamic conductors instead of static ones.
Domain walls, the separation zones only a
few atoms wide between opposing states of polarization in ferroelectric
materials, are known to be conducting, but the origin of the conductivity has
remained unclear.
“Our measurements identified that subtle
and microscopically reversible distortions or kinks in the domain wall are at
the heart of the dynamic conductivity,” Maksymovych said. “The domain
wall in its equilibrium state is not a true conductor like a rigid piece of
copper wire. When you start to distort it by applying an electric field, it becomes
a much better conductor.”
Ferroelectrics, a class of materials that
respond to the application of an electric field by microscopically switching
their polarization, are already used in applications including sonar, medical
imaging, fuel injectors, and many types of sensors.
Now, researchers want to push the boundaries
of ferroelectrics by making use of the materials’ properties in areas such as
memory storage and nanoelectronics. Gaining a detailed understanding of
electrical conductance in domain walls is seen as a crucial step toward these
next-generation applications.
“This study shows for the first time
that the dynamics of these defects—the domain walls—are a much richer source of
memory functionality,” Maksymovych said. “It turns out you can dial
in the level of the conductivity in the domain wall, making it a tunable, metastable,
dynamic memory element.”
The domain wall’s tunable nature refers to
its delayed response to changes in conductivity, where shutting off an electric
field does not produce an immediate drop in conductance. Instead, the domain
wall “remembers” the last level of conductance for a given period of
time and then relaxes to its original state, a phenomenon known as memristance.
This type of behavior is unlike traditional electronics, which rely on silicon
transistors that act as on-off switches when electric fields are applied.
“Finding functionality intrinsic to
nanoscale systems that can be controlled in a novel way is not a path to
compete with silicon, but it suggests a viable alternative to silicon for a new
paradigm in electronics,” Maksymovych said.
The ORNL-led team focused on bismuth ferrite
samples, but researchers expect that the observed properties of domain walls
will hold true for similar materials.
“The resulting
memristive-like behavior is likely to be general to ferroelectric domain walls
in semiconducting ferroelectric and multiferroic materials,” said ORNL
co-author Sergei Kalinin.
