The University of Hamburg was founded only 100 years ago, but research there was flourishing as early as the 1920s. For example, the later Physics Nobel laureate Otto Stern performed his studies and taught at the former Physikalisches Staatsinstitut (state institute of physics) and made his research institution in Jungiusstraße one of the world’s leading centres of atomic, molecular and nuclear physics, despite the difficult economic climate of the time.
In the 1920s, the University of Hamburg’s Physikalisches Staatsinstitut hosted one top scientist after another. Developments such as magnetic resonance imaging and atomic clocks are based on the research activities and findings in Hamburg at that time. Unfortunately, this period did not last long: together with many other Jewish scientists, Otto Stern was driven out of Germany after the National Socialists’ rise to power in 1933, and never came back.
Since the beginning of the 1990s, Hamburg has become an important and internationally acknowledged science centre, especially in the field of nanotechnology. This is supported by the many world-renowned research groups in physics and chemistry that focus on every aspect of nanoscience and nanotechnology, from fundamental research to product development.
A good illustration of this rapid rise is the University of Hamburg’s physics institutes, where Otto Stern used to work and where research is now being conducted into completely new concepts for magnetic data storage.
The search for new storage media is driven by our hunger for more and more digital data, the importance of which has grown exponentially within just a few years. Be it documents, company presentations, reference works, navigation data, games, electronic books, photos, videos or music – today, we want to access everything digitally. Selling and leasing digital data via the Internet has now become an important economic factor and is already replacing traditional data carriers such as compact discs, DVDs, Blue-ray discs and even printed books. However, these vast amounts of data need to be securely stored somewhere. Conventional data storage media have developed rapidly over the last few decades, but mobile devices such as smartphones, tablets and notebooks in particular often do not provide sufficient storage space.
Storing more and more information in an ever smaller space is only possible if the read-write heads and the tiniest magnetic fields (bits) on computer hard drives become smaller and smaller (see Fig. 1). But this is exactly where conventional magnetic storage technologies will soon reach their physical limits.
This is why we need to develop new ideas for processing and storing of digital data. At the moment, magnetic moments – the spins – are arranged either parallel or antiparallel, i.e. collinear, to each other. However, non-collinear spin structures where adjacent spins can be arranged in far more ways may also be possible. These new spin structures have only recently become the subject of intensive research and open up a variety of new applications. The breakthrough in the investigation of such complex spin structures came with the use of atomic-resolution magnetic microscopy techniques. These were developed at the University of Hamburg twenty years ago, and the institution is still the world leader. The technologies include spin-resolving scanning tunnelling microscopy  and magnetic exchange force microscopy . The use of spin-resolving scanning tunnelling microscopy in Hamburg recently resulted in a spectacular finding: the discovery of skyrmions in ultra-thin magnetic layers. What is that all about?
Around 50 years ago, the theoretical physicist Tony Skyrme found stable and localised states in quantum mechanical field theories, which he identified as elementary particles. Named after the man who discovered them, these skyrmions can be thought of as the knot of a vector field. In a magnetic system, this would correspond to a complex magnetisation distribution, as shown in Fig. 2.But how can magnetic skyrmions help to store digital data? Thanks to their knot structure, magnetic skyrmions are particularly robust and stable against external influences. They can be assigned a kind of charge, called the topological charge, which makes it possible to use a skyrmion to show the bit state “1” (there is a skyrmion) and “0” (there is no skyrmion). A few years ago, our group of researchers in Hamburg succeeded for the first time in demonstrating that skyrmions like this can be observed in ultra-thin magnetic layers made from simple, common metals such as iron  and can be extremely small: just one nanometer in diameter – a world record!
In 2013, the physicists at the University of Hamburg succeeded in generating and deleting individual magnetic skyrmions in a targeted way for the first time . To do this, the researchers used a film made from palladium and iron, just two layers of atoms thick, on an iridium surface. When this sample is moved into a magnetic field, it is possible to observe individual, localised skyrmions with the help of the spin-resolving scanning tunnelling microscope. Each skyrmion is composed of no more than around one hundred atoms. These skyrmions can be written with a small electric current from the magnetic tip of the microscope and then – when the direction of the current is reversed – deleted again (see Fig.3).
Admittedly, a data storage medium with the mechanically movable tip of a scanning tunnelling microscope has the same problems as the current hard drives: the mechanical control of the skyrmion bits is slow, the mechanics are subject to the usual wearing processes and the storage medium has to be in a vibration-free position during operation, in order to prevent defects and losses of data. This would make this innovative skyrmion storage medium unsuitable for mobile systems at least. However, there is a solution in sight for this problem: instead of moving the read-write head across the surface mechanically , the skyrmions can also be moved underneath the surface using an electric current (see Fig. 4). To do this, a thin strip of iron in which the skyrmions can be generated can be used. A read-write head is installed above a point on the strip and is able to read, delete and write the skyrmions that are moved underneath it. The tiny knots remain stable despite the movement, providing the basis for a robust data storage medium without moving parts. This can be used to store huge quantities of data reliably, and is about as indestructible as a conventional USB stick.
With the help of the magnetic knots, in future it will be possible to pack the zeros and ones on magnetic data carriers thousands of times closer together than can be done using conventional technology. Another key point is the significantly reduced energy consumption of skyrmion storage media compared to current storage systems, making continuing research and development in skyrmion storage media an important contribution to the green IT technology of the future. The European Union has now recognised this. From September 2015, it provides millions of euros of funding to the skyrmions project at the University of Hamburg – in cooperation with the Parisian research group led by Physics Nobel laureate Albert Fert and numerous other partners from science and industry – as part of the Future Emerging Technologies (FET) programme. It is hoped that initial prototypes for a new generation of magnetic data storage media will result over the next three years.  R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009). ,  U. Kaiser, A. Schwarz, R. Wiesendanger, Nature 446, 522 (2007).,  S. Heinze et al., Nature Physics 7, 713 (2011).,  N. Romming et al., Science 341, 636 (2013).  Ch. Hanneken et al., Nature Nanotechnology 10,1039 (2015).
Prof. Dr. Dr. h.c. Roland Wiesendanger
Born in 1961, the author studied at the University of Basel and was appointed to the Chair of Experimental Solid State Physics at the University of Hamburg in 1992. Prof. Wiesendanger has led the Interdisciplinary Nanoscience Centre Hamburg and been spokesman for the DFG Collaborative Research Centre 668 “Magnetism from the Single Atom to the Nanostructure” since 2006. Since 2009, he has led two European Research Council excellence projects on the topics of “Atomic Magnetism” and “High-temperature Supraconductors”.