Magnetism Without Magnets

November 2, 2021

Electric current is deflected by a magnetic field; in conductive materials the so-called Hall effect occurs. This effect is often used to measure magnetic fields. Now a surprising discovery has been made at the Technical University of Vienna, in collaboration with scientists from the Paul Scherrer Institute (Switzerland), McMater University (Canada) and Rice University (USA): an exotic metal made from cerium, bismuth and palladium and the material was found to produce a giant Hall effect, in the total absence of any magnetic field. The reason for this unexpected result lies in the unusual properties of electrons: They behave as if there were magnetic monopoles in the material. These discoveries have now been published in the scientific journal PNAS.

A voltage perpendicular to the current

When an electric current flows through a metal strip, electrons move back and forth. If a magnet is placed next to this band, a force acts on the electrons, the so-called Lorentz force. The path of electrons through the metal band is no longer straight, but bends a bit. So there are now more electrons on one side of the metal strip than the other, and this creates a voltage – perpendicular to the direction the current is flowing. This is the classic Hall effect, as it has been known for many years.

“Measuring the intensity of the Hall effect is one of the ways to characterize materials in our laboratory,” explains Professor Silke Bühler-Paschen, from the Institute for Solid State Physics at the Technical University of Vienna. “You can learn a lot about the behavior of electrons in the solid state from such an experiment.” When Sami Dzsaber, who was working on his thesis in the Bühler-Paschen research group, examined the Ce3Bi4Pd3 material, he took his task very seriously and also carried out a magnetic field-free measurement. “This is actually an unusual idea, but in this case it was the decisive step,” says Silke Bühler-Paschen.

The measurement revealed that the material exhibits a Hall effect even without an external magnetic field, and not just a normal Hall effect, but a huge one. In normal materials, a Hall effect of this intensity can only occur with huge electromagnetic coils. “So we had to answer another question,” says Silke Bühler Paschen. “If a Hall effect occurs without an external magnetic field, is it extremely strong local magnetic fields that occur on a microscopic scale inside the material, but can no longer be felt outside?”

For this reason, research was carried out at the Paul Scherrer Institute in Switzerland: With the help of muons – elementary particles especially suitable for investigating magnetic phenomena – the material was examined more closely. But it turned out that no magnetic field could be detected even on a microscopic scale. “If there is no magnetic field, there is also no Lorentz force that can act on the electrons in the material, but a Hall effect was nevertheless measured. This is really surprising, ”says Silke Bühler-Paschen.

Symmetry is what counts

The explanation for this strange phenomenon lies in the complicated interaction of electrons. ‘The atoms of this material are arranged according to very specific symmetries, and these symmetries determine the so-called scattering ratio, that is, the ratio between the energy of the electrons and their momentum. The scattering relationship tells us how fast an electron can move when it has a certain energy, ”says Bühler-Paschen. “It’s also important to note that you can’t look at the electrons individually here: there are strong quantum mechanical interactions between them.”

This complex interaction gives rise to a phenomenon that mathematically seems that there are magnetic monopoles in the material, that is, solitary north and south poles, which do not exist in this way in nature. “But it actually has the effect of a very strong magnetic field on the movement of electrons,” says Bühler-Paschen.

The effect had already been theoretically predicted for simpler materials, but no one had been able to prove it. The breakthrough came with the investigation of a new class of materials: “Our material with the chemical composition Ce3Bi4Pd3 is characterized by a particularly strong interaction between electrons,” explains Bühler-Paschen. This is known as the Kondo effect. It makes these fictitious magnetic monopoles have exactly the right energy to influence the material’s conduction electrons with extreme force. This is the reason why the effect is more than a thousand times greater than theoretically predicted.

The new giant spontaneous Hall effect has some potential for next-generation quantum technologies. In this field, for example, non-reciprocal elements are important that produce a direction-dependent dispersion, without the need for an external magnetic field; could be done with this effect. “The extremely non-linear behavior of the material is also of great interest,” says Silke Bühler-Paschen. “The fact that the complex phenomena of many particles in solids give rise to unexpected application possibilities makes this field of research especially exciting.”

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
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