Available online 16 October 2010.
A thin film of europium titanate stretched across a substrate of dysprosium scandate is the strongest simultaneously ferroelectric and ferromagnetic material synthesised since 1966. For 44 years nickel boracites held the crown. The new two-faced materials could find applications in the development of highly sensitive magnetic memory, sensors or microwave-based devices.
The new material, devised by an international team of 24 researchers based in Czech Republic, Germany and USA, [Darrell Schlom et al., Nature (2010) 466, 954; doi: 10.1038/nature09331], is a thousand times as powerful as its nickel predecessor at a chilly temperature of just 4 degrees above absolute zero. The new material was designed on the basis of theory to find a suitable elemental combination to produce simultaneous ferroelectricity and ferromagnetism a property that is exceedingly rare among natural materials and coveted by electronics visionaries.
The weak properties of single-phase multiferroics has meant that researchers have turned to composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components, some of these are fairly close to applications, but a single-phase material would be an enormous improvement.
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Cornell researchers made a thin film of europium titanate ferromagnetic and ferroelectric by depositing it on an underlying substrate with a larger spacing between its atoms.
(Image courtesy of Darrell Schlom).
“Previous researchers were searching directly for a ferromagnetic ferroelectric,” explains team member Darrell Schlom of Cornell University, “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.” The team's earlier work had suggested that “squashing” a material by this alignment approach would induce the dual properties, but with this particular case “stretching” was the successful approach. Indeed, other groups have exploited strain to enhance the mobility of transistors and increase superconducting, ferromagnetic and ferroelectric transition temperatures.
The crystal structure of the europium titanate is under strain because of the tendency of the atoms of europium titanate to align with the underlying arrangement of atoms of the dysprosium scandate single crystal upon which they are deposited. The stretch due to spin-lattice coupling amounts to an expansion of just 1%, although this is a far greater stretch with which the bulk material could cope without cracking.
The researchers suggest that their strategy could lead to additional ferromagnetic ferroelectrics that function at higher temperatures, although that could still be a long way off and no commercially viable devices are yet on the horizon. “Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics,” the team concludes.
The Cornell team worked with colleagues at Penn State University, Ohio State University, Argonne National Laboratory and elsewhere.