In 1921 scientists noticed a curious feature of certain materials, which they dubbed ferroelectricity. Fifteen years later an explanation was provided, proposing hypothetical particles named hysterons. It's taken 80 years, but these particles have now been confirmed, and their behavior explained.
Ferroelectric materials contain electrically polarized subregions. The direction of polarization can be changed using an external electric field, and will persist after the field is removed. Consequently, ferromagnetic materials placed in a field will have the direction of polarization of their constituent regions align. The process is analogous to magnetizing iron in a magnetic field, prompting the term ferroelectric, even though most materials that behave themselves are not ferrous (that is they contain no iron).
Uses for ferroelectric materials include data storage and scientific equipment, but our understanding of what causes ferroelectric activity has lagged. Now Professor Martijn Kemerink of Linköping University, Sweden has shown why two materials are ferroelectric.
In Nature Communications, Professor Kemerink reports on the discovery that the snappily-named ferroelectrics P(VDF-TrFE) and trialkylbenzene-1,3,5-tricarboxamide contain charged cylindrical stacks of layered materials a nanometer (40 billionths of an inch) wide and several times as high.
The German physicist Franz Preisach provided a theoretical model for ferromagnetism in 1935, and then realized it could also be applied to ferroelectrics. Preisach reasoned these materials were made up of units he called "hysterons", which had oppositely charged poles. Without being exposed to a field, the hysterons would be randomly aligned, and their effects would cancel out.
However, when a lump of ferromagnetic material is placed in an electric field the hysterons start to line up with the field. The stronger the field, the more hysterons align. When the field is removed the alignment remains.
Although Preisach's theory explained observations, no one knew what hysterons really were, or why they formed in particular materials, until now.
The observed ferroelectric effects occur because hysterons, or particles, of slightly different heights and widths form into cylindrical stacks that then interact with each other.
"We could prove that these stacks actually are the sought-after hysterons," Kemerink said. "The trick is that they have different sizes and strongly interact with each other since they are so closely packed." It's the nanostructure that dictates the way ferroelectric materials behave.
Once the dimensions of composite hysterons have been identified, Kemerink and co-authors could predict the response curve of a ferromagnetic material to a supplied electric field.
The two materials studied are representative of very different classes of ferroelectrics, so the authors think their explanation probably applies more generally.
The work may have many applications, but most immediately in building more flexible and efficient memory devices.