TUM-Physicists discover new magnetic order:

Skyrmion Lattice in a chiral Magnet

Sebastian Mühlbauer, doctoral student with Professor Peter Böni at the TUM Department for Experimental Physics, was originally meant to measure something completely different in relation to the metallic compound of manganese-silicon. However, Professor Christian Pfleiderer, who works together with Mühlbauer, forgot to ask him to alter the measuring arrangement. This meant that the magnetic field for the measurements with the MIRA instrument of the FRM II was set up parallel to the neutron beam, rather than perpendicular as planned. And Mühlbauer measured something very peculiar: “When I suddenly saw a ring of six points on the screen instead of the two points that I had expected, I immediately called Christian”, the 28-year old recounts. By chance, this happened on April 1, leading Pfleiderer to initially believe that his doctoral student was playing an April Fool’s joke on him. However, the hexagonal pattern really was there - and this at a temperature of minus 245° Celsius and a magnetic field of 0.2 Tesla, which roughly corresponds to the field of a strong permanent magnet.

On making this discovery, Pfleiderer, Mühlbauer and their colleagues experienced a sense of euphoria. “This was true team work”, is how Pfleiderer describes the weeks during which the physicists in Garching conducted their decisive measurements. “Sebastian was in charge.” The solid state physicists from the TUM discussed their observations per video conference with theoretical physicists from Cologne University, headed by Professor Achim Rosch. The Science publication thus became a combination of experiment and theory. The Cologne-based physicists theoretically calculated the magnetic vortices, which the physicists from Munich had measured with neutrons.

Even nine months after the discovery, Pfleiderer’s and Mühlbauer’s enthusiasm is still written all over their faces. “The vortex filaments are already unusual in themselves. It is as if the metal were a magnetic soup in which stable quantum knots form. What is even crazier is that these vortex filaments always align themselves along the magnetic field. They don’t care about the crystal structure at all.” says Pfleiderer. “They behave like particles that can move freely in a solid body.”

The 43-year old and his colleague Achim Rosch from Cologne have also come up with an explanation as to how the vortices are produced. The magnetic moments in manganese silicon normally form a helix. Laying three such helical structures on top of each other like a star, however, produces the vortices. Pfleiderer has already been researching the hard and brittle manganese silicon for 18 years as it is just right for his magnetic measurements and easy to produce as single crystals. Several years ago, during research residencies at Cambridge University, the Grenoble Research Center and Karlsruhe University, the Stuttgart-born physicist discovered that manganese silicon has anomalous metallic properties; he reported this finding in a series of three publications in the specialist journal Nature and a further article in Science. The discovery of vortices may also explain the origin of the anomalous metallic behavior.

The magnetic vortices are primarily of interest for quite different reasons, however. As early as in the 1950s, Werner Heisenberg, the Munich Nobel laureate, suggested that scientists search for a theory for the building blocks of the universe, which described these building blocks as knots in a medium. This idea was seized upon by the British physicist Tony Skyrme in the 1960s – the particles he proposed are thus called skyrmions. From a purely mathematical point of view, the magnetic vortices discovered by Pfleiderer and colleagues are precisely such skyrmions.

The most important point regarding the magnetic vortices is that the discovery by Pfleiderer and Mühlbauer promises to have many new applications. Pfleiderer suspects that manganese silicon is not the only magnetic material to form vortices. Twenty years ago, Professor Alex Bogdanov, then in Donetsk in Ukraine, now at the Leibniz Institute for Solid State and Materials Research in Dresden, was already predicting that the magnetic knots, which Mühlbauer and Pfleiderer have now discovered, should be found in many substances. “We have already found another material since our first discovery in manganese silicon”, divulges Pfleiderer. If it becomes clear how to control the formation of knots, completely new methods can be developed for using magnetism to process and store information.

The mathematical discipline that is used to describe skyrmions is called topology. It concerns itself with geometric bodies that are not changed when stretched, compressed or twisted. Topology states, for example, that a donut can be transformed into a coffee cup. It is only very recently that topology has been experiencing its breakthrough in material sciences and solid state physics. In the same issue of Science, scientists at Princeton University, for example, report the discovery of a topological insulator on the surface of a bismuth antimony mixture. The surface of the metal becomes electrically insulating because the electron motion forms topological knots. As Prof. Jan Zaanen from Leiden University comments enthusiastically in Science, Mühlbauer’s and Pfleiderer’s vortices thus also have, in the broadest sense, a counterpart in the electronic structure, which again promises completely new applications - in quantum computing, for example.

The TUM physicists carried out their measurements with neutrons using the MIRA instrument at the Heinz Maier-Leibnitz (FRM II) research neutron source of the TUM. The name MIRA stands for a variable star formation. It plays on the fact that the measuring instrument can be easily altered for very different measurement methods. In MIRA, neutrons carrying a magnetic moment impinge on the sample as a beam. They are deflected by the magnetic moment in the sample, the vortices. Their deflection provides information on the magnetic structure within the sample. Sebastian Mühlbauer and his colleagues were thus able to make the unusual vortices in manganese silicon visible.

Pfleiderer values the possibilities at the FRM II, which is only a few meters away from his office in the TUM’s Department of Physics: “This means fantastic access to a large installation, which you don’t have anywhere else in the world. We can take the crystals that are produced in our laboratories in the physics department straight across the way and measure the magnetic structure, its dynamics and many other properties with neutrons.”

Sebastian Mühlbauer started as a working student at the research neutron source in Garching. “I have simply always been interested in fiddling around with such large devices”, he says with a grin. After completing his diploma at the FRM II it was clear to Mühlbauer, who hails from Dachau near Munich, that he also wanted to do his doctoral thesis there. The physicist has been working on his doctorate since 2005 and would like to complete it this year. “Sebastian really has enough material for three doctoral theses”, says Pfleiderer. But Mühlbauer modestly denies this. Right now he is looking forward to the publication in Science, something for which other scientists have to work their whole life long.

Original publication:
Skyrmion Lattice in a Chiral Magnet; S. Mühlbauer, B. Binz, F. Jonietz, C. Pfleiderer, A. Rosch, A. Neubauer, R. Georgii, P. Böni
Science, February 13, 2009, Vol 323, Issue 5916 – DOI-Nr.: 10.1126/science.1166767

Imagery:
http://mediatum2.ub.tum.de/?cfold=683612&dir=683612&id=683612

Contact:
Dipl-Phys. Sebastian Mühlbauer
Technische Universität München
Department of Physics, Chair E21
James Franck Str. 1, 85748 Garching
Tel.:  +49.89.289.12515
E-Mail: sebastian.muehlbauer@frm2.tum.de

Prof. Christian Pfleiderer
Technische Universität München
Department of Physics, Chair E21
Tel.:  +49.89.289.14720 / 14712 (Secretary)
E-Mail: Christian.Pfleiderer@frm2.tum.de

Andrea Voit
Public Relations
Technische Universität München
Research-Neutronsource FRM II
Lichtenbergstr. 1
85748 Garching
Tel: +49.89.289.12141
E-Mail: andrea.voit@frm2.tum.de