New method for generating magnetic vortex

Magnetic vortices, so-called skyrmions, are regarded as the hope of a more efficient storage technique and are being intensively researched. Scientists have now discovered a method for generating skyrmions that can be integrated directly into the memory chip and works reliably up to the gigahertz range. They have generated the small nano vertebrae by means of short current pulses at predetermined locations and then moved in a controlled manner. By holography with X-rays, they have the skyrmions imaged and directly detected. Participants included the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), the Massachusetts Institute of Technology (MIT) and other German research institutions. The research results appeared in "Nature Nanotechnology".

Scientists have produced skyrmions using sandwich structures of platinum, a magnetic alloy of cobalt, iron and boron, and magnesium oxide. Dr. MIT explains: "Due to the spin-reverb effect, a quantum mechanical effect, and a special interaction of the atoms at the interfaces of the materials, skyrmions can be selectively generated by current pulses, which is directly possible in so-called racetrack structures at predetermined points, which is essential for controlled data writing. " The racetrack structures are nanometer-thin wires of stacked magnetic materials. The researchers were able to pinpoint the exact location of the magnetic vortex by adding a small additional narrowing in the wire.

The fact that the special skyrmion magnetic vortices were actually generated and pushed into the racetrack wire with another current pulse was proven by the scientists at the German electron synchrotron DESY in Hamburg using X-rays. "X-ray holography makes it extremely sensitive to detect these very small magnetic structures, so the magnetization vertebrae can be imaged with a resolution of about 20 nanometers," explains Dr. med. Bastian Pfau, one of the scientists of the MBI team.

The scientists have been able to follow in their investigations how skyrmions are generated with individual current pulses, which are then moved with further pulses. It was important to understand the basic processes: what happens in the few nanometer thin layers of the material and at the interfaces, when single short current pulses with a duration in the range of nanoseconds are passed through the material? How do electrons from the platinum layer during the current pulses affect the magnetization in the adjacent cobalt alloy to form skyrmions with a certain sense of rotation? For this purpose, the team has compared its observations with micromagnetic simulations, in which the processes are simulated in the computer. "These insights into the microscopic mechanism will decisively help us to further develop the concepts and materials for future data storage technologies," says Büttner.

Fig. 1: Schematic representation of a racetrack wire. This consists of a stack of 45 layers, each only about a nanometer in diameter. The scheme shows only three of the 45 layers. Skyrmions (shown in blue) are created in this special material system behind the bottleneck created by the slots when strong current pulses are sent through the wire. The skyrmions can then be moved along with other weak current pulses for storage along the wire. The presence or absence of a skyrmion then encodes a bit "1" or "0". In the background a section of an x-ray hologram can be seen, as the researchers have recorded to depict the skyrmions. (Graphic: Moritz Eisebitt)

Save data in three dimensions

"Our data is in the cloud" - whoever says that sometimes forgets that the data is ultimately stored on hard drives, in large data centers of companies like Google and Facebook. The individual data bits are stored in the magnetization of thin magnetic films. The bits are written with a mechanically moving read / write head with magnetic field pulses on a fast rotating disk, the actual hard disk. In order to be able to store more data in the same space in the future, scientists are working on moving from this inherently two-dimensional storage method to a three-dimensional method. In such so-called racetrack memories, the bits should also be stored as magnetization patterns, but now in a wire-like structure. There they can be read and written like on a racetrack - hence the name - very quickly. In contrast to today's hard disks, the writing and shifting of the bits should only be done with the help of very short current pulses and therefore the use of moving parts should be avoided altogether. Since the racetrack wires, like many parallel straws, can be packed tightly in three dimensions, significantly higher storage densities would be possible.

A candidate for the representation of single bits are the nanometer-small vortices in the magnetization of the magnetic material, the skyrmions. They fascinate the researchers because they can be moved by electricity and are very stable. The presence or absence of a skyrmion would then represent bits "0" and "1" in the future. In order to produce individual skyrmions in a controlled manner, however, up to now very expensive equipment was necessary - the current research results show a new path here.

Original publication

Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques

F. Büttner, I. Lemesh, M. Schneider, B. Pfau, C. M. Günther, P. Hessing, J. Geilhufe, L. Caretta, D. Engel, B. Krüger, J. Viefhaus, S. Eisebitt, G. S. D. Beach

Nature Nanotechnology 12 (2017) 1040–1044

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