In 1935, Franz Preisach postulated that a type of "particle" could be invoked to explain the phenomenon of ferroelectricity, wherein some materials have a spontaneous electric polarization that can be reversed by the application of an external electric field. Now, researchers from the universities in Linköping and Eindhoven have observed these "particles" in organic ferroelectric materials for the first time. The new work might guide the design of materials for multi-bit memory devices. [I. Urbanaviciute et al., Nature Commun., (2018); DOI: 10.1038/s41467-018-06717-w]
Ferroelectricity is the sibling phenomenon to ferromagnetism. Metals such as iron, cobalt, and nickel are ferromagnetic and their electrons behave as tiny magnetic dipoles. In ferroelectric materials the dipoles are electric rather than magnetic having a positive and negative pole rather than a north and south. In the absence of an applied magnetic or electric field, the orientation of the dipoles in both classes of material is random. Apply an adequate field though and the dipoles become aligned with. In a ferroic material, the alignment is maintained even after the field is removed and can only be reversed by application of the opposite field that is stronger than the original critical, or coercive, field. This phenomenological hysteresis makes such materials useful in rewritable computer memory, such as hard disks.
In an ideal ferroelectric, polarization is universal, uniform and almost instantaneous. In a real material, polarization takes place differently at different points in the material and at different speeds. Understanding this non-ideality is critical to developing future memory devices. Unfortunately, the particles thought to be involved in the process, the hysterons proposed more than eighty years ago had not been observed until now.
Martijn Kemerink and his colleagues at Linköping and Eindhoven believe they have identified the nature of the hysterons in two organic ferroelectric model systems:the semi-crystalline copolymer P(VDF-TrFE) and the polycrystalline molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA). They showed that the molecules in the materials like to lie on top of each other, forming cylindrical stacks about one nanometer across and several nanometers long.
"We could prove that these stacks actually are the sought-after hysterons," Kemerink explains. "The trick is that they have different sizes and strongly interact with each other since they are so closely packed. Apart from its own unique size, each stack therefore feels a different environment of other stacks, which explains the Preisach distribution," he adds. Moreover, non-ideal switching of a ferroelectric material depends on its nanostructure and more specifically how many stacks interact with each other and the details of the way in which they do this.
"We had to develop new methods to measure the switching of individual hysterons to test our ideas. Now that we have shown how the molecules interact with each other on the nanometer scale, we can predict the shape of the hysteresis curve," Kemerink adds. "This also explains why the phenomenon acts as it does. We have shown how the hysteron distribution arises in two specific organic ferroelectric materials, but it's quite likely that this is a general phenomenon."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.