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Posted: Aug 01, 2011
Lattice of magnetic vortices
(Nanowerk News) Physicists at Hamburg and Kiel University and the Forschungszentrum Jülich have found for the first time a regular lattice of magnetic skyrmions – cycloidal vortex spin structures of exceptional stability – on a surface. This fascinating magnetic structure was discovered experimentally at the University of Hamburg by spin-polarized scanning tunnelling microscopy and imaged on the atomic scale. Theoreticians at the Christian-Albrechts-Universität zu Kiel and the Forschungszentrum Jülich were able to explain this magnetic state with the help of quantum mechanical calculations performed on supercomputers. As the scientific magazine Nature Physics reports online on July 31, 2011 ("Spontaneous atomic-scale magnetic skyrmion lattice in two dimensions"), the researchers discovered the magnetic skyrmions, which consist of 15 atoms, in an atomic layer of iron on the surface of an iridium crystal. This discovery could give new impetus to the area of spintronics.
The tiny cycloidal vortices composed of only approximately 15 atoms form a regular, almost square lattice. In the right section of the illustration the magnetic measurement by spin-polarized scanning tunnelling microscopy is shown as a gray-scale image. The square cutout represents a single skyrmion. The colored cones show the magnetic direction of the individual, hexagonally arranged iron atoms of the metal film. (Image: M. Menzel, University of Hamburg)
About 50 years ago the theoretical physicist, Tony Skyrme, studied quantum mechanical field theories and to his surprise found stabile and localized configurations that interact with each other and can arrange themselves in a lattice in the same way as atoms. Due to these properties he identified these vortex-like solutions as elementary particles. These skyrmions named after their discoverer later appeared in many different fields of physics and developed into an important concept. The possible formation of skyrmions in magnetic materials had already been predicted 20 years ago and was also confirmed experimentally in bulk materials.
The magnetic skyrmion lattice discovered in Hamburg occurs in an atomically thin film on a surface. The diameter of the vortices is only a few atoms and is thus at least one order of magnitude smaller than the previously known magnetic skyrmions. As it is often the case chance also played a major role in this discovery. "It is known that iron can sometimes form unusual magnetic structures. Still it was a great surprise when we found this almost square magnetic structure on the nanometer scale which is not really compatible with the hexagonal system of the iron atoms", said Dr. Kirsten von Bergmann, member of the experimental research group headed by Prof. Roland Wiesendanger in Hamburg. The fact "that a sophisticated variation of the experimental setup gives data that can be compiled to yield the complicated magnetic structure" fascinates also Matthias Menzel, a postgraduate student.
In order to understand this intriguing spin structure and the exceptional symmetry breaking between magnetic and atomic order, the theoreticians at the University of Kiel and the Forschungszentrum Jülich had to develop a model for the spin structure and carry out complex quantum mechanical calculations on supercomputers at Jülich. These provided the confirmation that stable magnetic skyrmions form on this metal surface. Professor Stefan Heinze, Head of the research group in Kiel: "With the help of our model we were able to specify the precise spin structure in the iron film and identify it as a skyrmion lattice. The comparison with the experimental data provided the ultimate proof for our discovery."
The interplay of various magnetic interactions is the cause for the occurrence of this complex structure. While the canting of atomic spins with a certain rotational sense is caused by the antisymmetric Dzyaloshinskii-Moriya interaction, the skyrmions found here can only be induced by the so-called four-spin interaction with the participation of four magnetic atoms.
The magnetic skyrmions found open up completely new possibilities for future applications, for example in the field of spintronics, but at the same time raise new questions: How does electric current interact with the skyrmions and can we deliberately move the magnetic vortices in a specific manner?