It's the Clark Kent of oxide compounds; and—on its
own—it is pretty boring. But slice europium titanate nanometers thin and
physically stretch it, and then it takes on super hero-like properties that
could revolutionize electronics, according to new Cornell research. (Nature,
Aug. 19, 2010.)
Researchers report that thin films of europium
titanate become both ferroelectric and ferromagnetic when stretched across a
substrate of dysprosium scandate, another type of oxide. The best
simultaneously ferroelectric, ferromagnetic material to date pales in
comparison by a factor of 1,000.
Simultaneous ferroelectricity and ferromagnetism
is rare in nature and coveted by electronics visionaries. A material with this
magical combination could form the basis for low-power, highly sensitive
magnetic memory, magnetic sensors, or highly tunable microwave devices.
The search for ferromagnetic ferroelectrics dates
back to 1966, when the first such compound—a nickel boracite—was discovered.
Since then, scientists have found a few additional ferromagnetic
ferroelectrics, but none stronger than the nickel compound—that is, until now.
"Previous researchers were searching directly
for a ferromagnetic ferroelectric—an extremely rare form of matter," said
Darrell Schlom, Cornell professor of materials science and engineering, and an
author on the paper.
"Our strategy is to use first-principles
theory to look among materials that are neither ferromagnetic nor
ferroelectric, of which there are many, and to identify candidates that, when
squeezed or stretched, will take on these properties," said Craig Fennie,
assistant professor of applied and engineering physics, and another author on
the paper.
This fresh strategy, demonstrated using the
europium titanate, opens the door to other ferromagnetic ferroelectrics that
may work at even higher temperatures using the same materials-by-design
strategy, the researchers said.
Other authors include David A. Muller, Cornell
professor of applied and engineering physics; and first author June Hyuk Lee, a
graduate student in Schlom's lab.
The researchers took an ultra-thin layer of the
oxide and "stretched" it by placing it on top of the disprosium
compound. The crystal structure of the europium titanate became strained
because of its tendency to align itself with the underlying arrangement of
atoms in the substrate.
Fennie's previous theoretical work had indicated
that a different kind of material strain—more akin to squishing by compression—would
also produce ferromagnetism and ferroelectricity. But the team discovered that
the stretched europium compound displayed electrical properties 1,000 times
better than the best-known ferroelectric/ferromagnetic material thus far,
translating to thicker, higher-quality films.
This new approach to ferromagnetic ferroelectrics could prove a key step
toward the development of next-generation memory storage, superb magnetic field
sensors and many other applications long dreamed about. But commercial devices
are a long way off; no devices have yet been made using this material. The
Cornell experiment was conducted at an extremely cold temperature—about 4
degrees Kelvin (-452 Fahrenheit). The team is already working on materials that
are predicted to show such properties at much higher temperatures.
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