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Forschungszentrum Jülich - Annual Report 2011

30 Forschungszentrum Jülich | Annual Report 2011 Magnetic Spirals Comprising Fifteen Atoms Magnetic data storage is getting smaller and smaller while simultaneously becoming more efficient. To facilitate further development in this direction, scientists are con- stantly searching for even tinier objects in which magnetization remains stable, thus allowing the basic states of all computer languages – “zero” and “one” – to be re- corded and read out as reliably and energy-efficiently as possible. On this quest, a team of researchers from Jülich, Hamburg and Kiel stumbled upon very promising structures on the surface of a material, namely skyrmions. They published an article on their findings in Nature Physics. The cubic peephole represents a single skyrmion comprising fifteen atoms and their spins. The red, orange and green arrows in this computer simulation show the upward or downward orientation of the spins. The skyrmions form a regular lattice. S kyrmions were named for Brit- ish physicist Tony Skyrme. He discovered these stable spiral or knot-like structures almost 50 years ago – albeit not as real objects but as a mathematical solution in cer- tain field theories. Fields in the physical sense can be used to describe the forc- es of nature and matter itself. “Follow- ing this discovery, skyrmions became a general concept in physics that is im- portant for elementary particles, liquid crystals, Bose-Einstein condensates and quantum Hall magnets, for example,” says Jülich physicist Prof. Stefan Blügel, Director at the Peter Grünberg Institute and at the Institute for Advanced Sim- ulation. More than twenty years ago, theo- rists predicted that skyrmions also oc- cur in magnetic materials. However, their existence was first proven in 2009: a group from TU Munich discov- ered lattices made of magnetic spiral- like structures in a weak magnetic field in manganese silicide. Skyrmions can be distinguished from these structures in a number of ways, as outlined by the Jülich team headed by Blügel and re- searchers from the universities of Ham- burg and Kiel. First of all, they also ex- ist without an external magnetic field. Secondly, their diameter is no bigger than a few atoms. Thirdly, they are found on the surface of an extremely metals. When they analysed iron on irid- ium at the University of Hamburg using a scanning tunnelling microscope and a special technique, they noticed regular magnetic patterns that did not agree with the crystal structure of the metallic surface. In order to understand the pat- tern and the unusual break in symmetry between the magnetic and atomic order, the theorists from Jülich and Kiel had to develop a physical model and perform complex quantum-mechanical calcula- thin film and not inside a material. The scientists from Jülich, Hamburg and Kiel detected them in an atomic layer of iron on an iridium crystal. Physical model developed Originally, the researchers wanted to ap- ply an atomic layer of chromium to iridi- um in order to investigate another mag- netic state that was assumed to exist here. However, as the experiments were unsuccessful, they decided to test other