«Structure and dynamics of interfaces in ferroic materials»

The term Ferroics is intimately related to crystals. Crystals are natural or synthetic materials with a high degree of order and symmetry. Ferroic expresses a certain functionality.

[Fer01] — egular ordered structure of a crystal (left corner), resembles some similarity with e.g. Portuguese tiles.

Symmetry plays an important role in nature, in technique, as well as in art. Who would not be impressed by the famousTaj Mahal (on the right), which was finished in 1648 in loving memory of Mumtaz Mahal, the wife of the Indian Großmogul.

The symmetry largely determines the properties of crystals. If the symmetry is lowered, e.g. due to the presence of a phase transition at a critical temperature Tc, large changes in physical properties occur over a rather narrow temperature range. This symmetry breaking leads to an enormous increase of functionality (Ferroelectricity, Ferromagnetism, etc.) of a material, which can be used for technical applications. E.g. high sensitive thermal detectors for the search of buried people, etc.

In the present project we go further and study the properties of tiny parts of crystals, the domain walls (picture below). Domain walls are inherently connected to symmetry breaking, i.e. whenever symmetry breaking occurs, there appear different variants of the low symmetry structure with adomain wall between.

[Fer03] — Sketch of a domain wall (yellow) between two domains (red and green).

Domain walls are about 1-10 nm (0.000001 – 0.00001 mm) thick, which means that about 10000 domain walls fit into the thickness of a human hair. The thing that makes such tiny nano-objectsfascinating is, that they can have completely different properties as their surrounding. For example such domain walls can conduct current (even superconducting) when the surrounding is insulating, they can be magnetic with non-magnetic surrounding, and for this reason very small devices can be built, which work on the basis of domain wall functionality.

[Fer04] — Sketch of a domain wall moving in a random energy landscape.

What technicians – looking for such application – do not like so much, is the fact, that such domain walls can be pinned by defects, because such pinning slows down any dynamic process in a device.

In the presence of defects the domain wall feels a potential energy that is very irregular and resembles an alpine landscape(image on the right).

Very often in physics it happens that a given subject or problem turns out to be very similar to another one, which at a first glance had looked very different. This happens also in the present case. It turns out that the movement of domain walls in a random potential can be used to better understand earthquakes. Why this is so?

Roughly speaking earthquakes happen when the built up stress- due to the movement of tectonic plates – is abruptly released when week parts of the earth crust break. If an inhomogeneous material breaks it produces a specific noise, called crackling noise.

Crackling noise

The image on the right shows the irregular structure of the crack line in a torn piece of paper. This is the simplest experiment for a system that producescrackling noise. The released energy follows a power-law distribution.

[Fer06] — Measured power-law distribution of crackling noise in a natural mineral (Schist) from the Großglockner region in Austria.

The simplest experiment for a system that produces crackling noise is to disrupt or crumple a piece of paper. The irregular movement of the crack front releases energy which is power-law distributed. This is very similar to the famous Gutenberg-Richter law in earthquakes. It means that – fortunately – earthquakes with higher magnitude are much less frequent compared to low energy earthquakes.

Measurements of earthquakes over many decades have shown that this law is very nicely fulfilled. The physics behind it – which serves that thisdamping of high energy events happens – is strongly connected to the physics of interfaces in random potentials.

Meanwhile, within the present project we have studied quite different systems that producecrackling noise under slow compression, including nano-porous silica, as well as natural porous materials like charcoal and even mica schist from the Großglockner region. All of them showing nice power-law behaviour of the corresponding energy distributions.

[Fer07] — Crackling noise produced during the compression of a nanoporous material when it cracks.

Read more...

Similarities between the compression of a porous material and earthquakes
Strain intermittency due to avalanches in ferroelastic and porous materials
Crackling noise from moving domain walls
Dynamic Mechanical Analysis measurements

Ultimately, the main goal of this research is two-fold: on one hand, understanding the couplingmechanisms that are responsible for the functionality of domain walls. Because then we could deliberately learn how to address domain walls for e.g. data storage. On the other hand, a deeper knowledge of the origin of the observed dramatic slowing down of the dynamics of domain walls, with the aim to finally avoid it.

MD Simulation

A very nice example of a molecular dynamic simulation of a molecular crystal(KSCN) showing the thermal fluctuations with heating above the critical temperatureTc, and the evolution of domains anddomain walls with cooling below Tc.

[Fer09] — Handwriting of recent calculations on domain wall problems (to be published)

The methods we use include Microscopic methods with various resolution, fromoptical microscopy (10-6 m) to electron microscopy (10-10 m), as well as group theory and Landau-Ginzburg-Devonshire models, which finally lead to differential equations.

Lead zirconate

This movie was taken with anoptical polarizing microscope during cooling a PbZrO3 crystal (leadzirconate) and shows the movement of domain walls.

Data Schist & Schist energy with time

This data set gives the energy distribution of a slow compression experiment performed on a mineral (schist from Großglockner) with time.

Upon this data, the power-law distribution was calculated (see picture). You can download the original Data Files here (.dat) and here (.obj).

Research team

Principal Investigator: Prof. Wilfried Schranz

Source Material - Attribution

[Fer05] Crackling nose (torn paper sound) Publicly shared under CC BY 4.0.[Fer06] Großglockner © Wilfried Schranz, Universität Wien