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Super-lattice design realises elusive 'multiferroic' properties

22 August 2015

A team at Northwestern University is attempting to combine ferromagnetism and ferroelectricity, which rarely coexist in one material at room temperature.

Super-lattice structure of lithium osmate and lithium niobate (illustration courtesy of McCormick School of Engineering/Northwestern University/James Rondinelli)

From the spinning disc of a computer’s hard drive to the varying current in a transformer, many technological devices work by merging electricity and magnetism. But the search to find a single material that combines both electric polarisations and magnetisations remains challenging.

This elusive class of materials is called multiferroics, which combine two or more primary ferroic properties. Northwestern Engineering’s James Rondinelli and his research team are interested in combining ferromagnetism and ferroelectricity, which rarely coexist in one material at room temperature.

“Researchers have spent the past decade or more trying to find materials that exhibit these properties,” says Rondinelli, assistant professor of materials science and engineering in the McCormick School of Engineering at Northwestern. “If such materials can be found, they are both interesting from a fundamental perspective and yet even more attractive for technological applications.”

In order for ferroelectricity to exist, the material must be insulating. For this reason, nearly every approach to date has focused on searching for multiferroics in insulating magnetic oxides. Rondinelli’s team started with a different approach. They instead used quantum mechanical calculations to study a metallic oxide, lithium osmate, with a structural disposition to ferroelectricity, and sandwiched it between an insulating material, lithium niobate.

While lithium osmate is a non-magnetic and non-insulating metal, lithium niobate is insulating and ferroelectric but also non-magnetic. By alternating the two materials, Rondinelli created a super-lattice that — at the quantum scale — became insulating, ferromagnetic, and ferroelectric at room temperature.

“The polar metal became insulating through an electronic phase transition,” Rondinelli explains. “Owing to the physics of the enhanced electron-electron interactions in the super-lattice, the electronic transition induces an ordered magnetic state.”

An article describing the research appears in the August 21 issue of Physical Review Letters.

This new design strategy for realising multiferroics could open up new possibilities for electronics, including logic processing and new types of memory storage. Multiferroic materials also hold potential for low-power electronics as they offer the possibility to control magnetic polarisations with an electric field, which consumes much less energy.

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