. Scientific Frontline: Surprise from the quantum world

Tuesday, March 21, 2023

Surprise from the quantum world

The ferromagnetism of the topological isolator manganese-bismuth-telluride only arises when the atomic structure fails. To do this, some manganese atoms (green) must be moved out of their original position (second green atomic plane from above). Only when there are manganese atoms in all levels with bismuth atoms (gray) is the magnetic orientation of the manganese atoms so contagious that ferromagnetism arises.
Illustration Credit: Jörg Bandmann / ct.qmat

The Würzburg-Dresden Cluster of Excellence ct.qmat has designed a ferromagnetic topological isolator - a milestone on the way to energy-efficient quantum technologies.

As early as 2019, an international research team around the material chemist Anna Isaeva - then junior professor at the Würzburg-Dresden Cluster of Excellence ct.qmat - complexity and topology in quantum materials - succeeded in producing the first antiferromagnetic topological isolator manganese-bismuth-tilluride. (Mn2Te4) a little sensation.

This miracle material no longer needs a strong external magnetic field - it brings its own inner magnetic field with it. This offers the opportunity for new types of electronic components that magnetically encode information and transport it on the surface without resistance. This could make information technology more sustainable and energy-saving in the future, for example. Since then, researchers worldwide have been analyzing different facets of this promising quantum material.

With MnBi6Te10 Milestone succeeded

Based on MnBi2Te4 a team from the Cluster of Excellence ct.qmat has now tailored a topological isolator with a ferromagnetic order - MnBi6Te10.

Ferromagnetic means: All magnetic moments of the individual manganese atoms point in the same direction. In contrast to the antiferromagnetic predecessor MnBi2Te4, in which only the magnetic moments within a single material layer point in the same direction.

The small difference in the composition of the crystal from individual chemical elements actually causes great things, because the ferromagnetic topological insulator MnBi6Te10 has a robust and stronger own magnetic field than its antiferromagnetic predecessor.

“We were able to use the quantum material MnBi6Te10 produced in such a way that it becomes ferromagnetic at 12 Kelvin. Even if these minus 261 degrees Celsius are still too low for components, this is the first step on a long way to go,” explains Professor Vladimir Hinkov from Würzburg. His research group demonstrated ferromagnetism with measurements on the material surface - where the magnetic topological isolator conducts electricity without loss, while isolating it inside.

Race for the miracle material

The ct.qmat research team was not the only one who worked on a ferromagnetic topological isolator in the laboratory: “After the great success of MnBi2Te4 was immediately searched for other candidates for magnetic topological isolators worldwide. In 2019, a total of four groups had MnBi6Te10 synthesized as a new hope - however, the miracle material was only ferromagnetic with u,” says Isaeva, today professor for experimental physics at the University of Amsterdam.

Mismatch in the system

In almost detective work, the Dresden material chemists around Isaeva found out how such a crystalline material can be produced.

They made an astonishing discovery: some atoms have to be repositioned from their actual atomic layer, i.e. They leave their ideal arrangement in the crystal. "By distributing manganese atoms in all crystal layers, the surrounding manganese atoms are stimulated to turn their magnetic moment in the same direction - the magnetic order is contagious," says Isaeva.

“Atomar misorder, as it prevails in our crystal, is usually considered disruptive in chemistry and physics. Orderly atomic structures can be calculated more easily and understood better, but do not always lead to a result,” adds Hinkov. “For us, this misorder is the crucial mechanism for MnBi6Te10 becomes ferromagnetic,” emphasizes Isaeva.

Network for cutting-edge research

Cluster researchers from the Technical University (TU) Dresden, the Julius Maximilians University (JMU) Würzburg and the Leibniz Institute for Solid State and Materials Research (IFW) Dresden were involved in the research. The crystals were manufactured by material chemists around Isaeva (TU Dresden). Ferromagnetism was then demonstrated in the volume of the samples at the IFW Dresden. Here Dr. Jorge I. Facio also developed a comprehensive theory that the ferromagnetism of the misordinate MnBi6Te10 as well as the antiferromagnetic competitors. The Hinkov team from JMU Würzburg was responsible for the decisive surface measurements.

The researchers are currently working on the fact that ferromagnetism occurs at significantly higher temperatures. The first results are already available for approx. 70 Kelvin. At the same time, the ultra-low temperatures must be increased, in which the exotic quantum effects are shown, because the lossless power line only starts at 1 to 2 Kelvin.

Published in journal: Advance Science 

Source/CreditUniversity of Würzburg

Reference Number: qs032123_02

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