Researchers validate 80-year-old ferroelectric theory

Researchers have successfully demonstrated that hypotheses are proposed by Franz Preisach in 1935 actually exist. In an article published in Nature Communications, scientists from the universities in Linköping and Eindhoven show why ferroelectric materials act as they do.

Ferroelectricity is the lesser-known twin of ferromagnetism. Iron, cobalt and nickel are examples of common ferromagnetic materials. The electrons in such materials function as small magnets, dipoles, with a north pole and a south pole. In a ferroelectric, the dipoles are electric rather than magnetic, and have a positive and negative pole.

In the absence of an applied magnetic (for a ferromagnet) or electric (for a ferroelectric) field, the orientation of the dipoles is random. When a strong field is applied, the dipoles align with it. This field is known as the critical (or coercive) field. Surprisingly, in a ferroic material, the alignment remains when the field is removed, and the material is still polarized. To change the direction of the polarization, a field at least as strong as the critical field must be applied in the opposite direction. This effect is known as hysteresis-the behavior of the material depends on what has already happened to it. Hysteresis makes these materials highly suitable as rewritable memory, for example, in hard disks.

In an ideal ferroelectric material, the whole piece switches its polarization when the critical field is reached and it does so with a well-defined speed. In real ferroelectric materials, different parts of the material switch polarization at different critical fields, and at different speeds. Understanding this non-ideality is key to application in computer memory.

A model for ferroelectricity and ferromagnetism was developed by the German researcher as an early modernization of the early 19th century. Each hysteron shows ideal ferroic behavior, but has its own critical field that can differ from hysteron to hysteron. It has been agreed that the model gives an accurate description of real materials, but scientists have not understood the physics on which the model is built. What are the hysterons? Why do their critical fields differ as they do? In other words, why do ferroelectric materials act as they do?

Professor Martijn Kemerink's Research Group, in collaboration with researchers at the University of Eindhoven. Cylindrical cylindrical stacks of around nanometre wide and several nanometers long.

"We could prove that these stacks are actually sought after." The trick is that they have different sizes and strongly interact with each other. of other stacks, which explains the Preisach distribution, "says Martijn Kemerink.

The researchers have shown that the non-ideal switching of a particular material depends on its particular nanostructure-in, 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 effects of nanoparticles." Now that we have shown how molecules interact with each other on the nanometer scale, we can predict the shape of the hysteresis curve. the phenomenon acts as if it is shown in the hysteron distribution in two specific organic ferroelectric materials, but it is quite likely that this is a general phenomenon.I am extremely proud of my doctoral students, Indre Urbanaviciute and Tim Cornelissen, who have managed to achieve this, "says Martijn Kemerink.

The results are a guide to the design of new materials, so-called multi-bit memories, and are a further step along the path to the small and flexible memories of the future.

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More information:
Indrė Urbanavičiūtė et al, Physical reality of the Preisach model for organic ferroelectrics, Nature Communications (2018). DOI: 10.1038 / s41467-018-06717-w