What is ferroelectric property of the material

Multiferroics: A material with many properties

The new material class of multiferroics combines properties as diverse as magnetism and ferroelectricity, the coexistence of which in one material creates completely new physical phenomena. Understanding the complex interplay of the individual effects and optimizing the behavior of the material for technical applications poses a complex challenge for research.

It was already known in ancient China that certain minerals seem to be able to attract and repel each other without the use of external forces. Left to their own devices, they always align themselves in the same way with regard to the cardinal points. One of the oldest technical applications of this phenomenon known as magnetism is in the compass.

Magnetism - a Ferroic order

The physical cause of magnetism is very well understood today. The intrinsic angular momentum of the electrons in an atom, the so-called spin, causes a microscopic magnetic moment, similar to a tiny bar magnet. In a crystal, the individual magnetic moments can interact with each other, which - if the interaction is strong enough - leads to a common, unidirectional alignment of all individual moments: The "bar magnets" point in the same direction and the crystal as a whole becomes magnetic.

In physics, this type of magnetism is called "ferromagnetism", based on the Latin word "ferrum" for the magnetic element iron. So-called "antiferromagnetism" was discovered very late. As with ferromagnets, the countless atomic magnetic moments in antiferromagnets are collectively aligned, but in such a way that all moments compensate each other. In the simplest case, two adjacent magnetic moments are simply antiparallel to one another. However, there are also highly complex structures. The spectrum ranges from triangular or tetrahedral arrangements to long spiral or helical structures.

Magnetism is also a temperature-dependent phenomenon. Because heat makes the orientation of the many individual atomic magnets "fidget". This induced movement competes with the pursuit of collective alignment; the magnetic moments are disordered. Only when the temperature falls below a certain material-specific value does the substance change to an orderly state known as "ferroic". Above this temperature it is "paramagnetic". It only shows its magnetic properties when it is exposed to a magnetic field.

The useful variety of the Ferroic orders

In addition to magnetism, there are other such ferroic ordering phenomena in nature as ferroelasticity and ferroelectricity. Ferroelasticity describes a collective elastic deformation of the crystal lattice of a material. Depending on the temperature, the building blocks of the material take on different arrangements, which in turn affect the physical properties of the material as a whole.

Ferroelectricity of barium titanate

The individual atoms, which are arranged in a crystalline material like a lattice, are also charged differently, positively or negatively. If these charges shift against each other, atomic electrical dipoles appear in the material. In ferroelectricity, these dipoles align themselves uniformly and permanently. Analogous to the magnetic field of ferromagnetism, the alignment of the dipoles in ferroelectricity creates an electric field.

The technical applications of such ferroic materials are extremely diverse. Whether magnetic tapes or hard drives - permanent data storage is largely based on magnetic materials. In addition, ferro and antiferromagnets play a major role in electronics and measurement technology and even in optics. Ferroelectrics are almost as widespread. In addition to their classic use in electronic components, they are becoming increasingly relevant in modern telecommunications based on the propagation of light.

Multiferroica - the combination of Ferroic orders

Multiferroics now combine several ferroic phenomena of order and thereby open up completely new technical application possibilities. Fifty years ago, the Soviet Union at that time developed the idea of ​​combining various ferroic properties in a material in a single phase, i.e. at the same temperature. Above all, the combination of ferromagnets and ferroelectrics, which at that time were still called "ferroelectromagnetics" or "ferromagnetoelectrics", stimulated the scientific imagination. The so-called magnetoelectric effect of such materials is particularly interesting: the ferromagnetic order can be changed directly with an electric field and, conversely, the ferroelectric order can be changed with a magnetic field. In concrete terms, this means that if the material is placed in a magnetic field, it responds with an electrical voltage. This effect also exists in simple magnetic materials, but only very weakly. In multiferroics, however, the effect should be amplified many times over due to the mutual alignment of the dipoles and magnetic moments.

Technologically, the magnetoelectric effect is extremely interesting. Magnetic memories are conceivable which would be resistant to magnetic stray fields, but which could be written with low electrical fields. Research has also recently been carried out on the development of efficient currentless magnetic field sensors which convert the magnetic field strength directly into an electrical voltage. In the future, such sensors will measure tiny brain waves in medicine, for example, with a precision that has not been achieved before.

Magnetic field sensors for measuring brain activity

Learned from the past

For decades, however, these considerations remained mostly pure thought games. Few materials have been found that actually show magnetic and electrical order in one phase. In the meantime, it has been possible to physically understand why the effect is so weak in these materials. Because the electrically charged particles in the crystal apparently do not “want” to build up an ordered magnetic moment and permanent electrical charge separation at the same time.
With the theoretical understanding of why the classical approach did not lead to the goal, new ideas could be developed as to how the initially incompatible phenomena can be combined in one material.
For about a decade, the concept of multi-ordered materials under the name “multiferroics” has been the focus of research interest again. If it is possible to couple strong ferroelectricity with ferromagnetism, the way is free for a completely new generation of high-performance sensors, data memories and electronic components.

The second part of our article deals with the ingenious approaches used by scientists to develop the new super material and to use its properties technologically.