In today’s unbiquitous hard disks, data are stored in thin magnetic films on rotating discs. Writing and reading out of the data is done via mobile read-write heads. This mechanical method limits the access speed enormously. Faster accesses and higher storage density promise the “Racetrack” -memory. Instead of two-dimensional magnetic films, the bits and bytes are deposited on thin wires as a magnetization structure and written in and read out without any mechanics. In contrast to hard disks, the writing and moving of the bits is to be done exclusively with the help of very short current pulses and the use of moving parts is not necessary. Since the racetrack wires can be tightly packed in three dimensions, much higher storage densities would be possible.
Small magnetic vortices, so-called scyrmions, play a central role in this technique. They can be moved around by means of current, but are at the same time very stable. The presence or absence of a skyrmion could be interpreted as logical “0” and “1”. However, in order to produce individual scyrmions in a controlled manner, very complex apparatuses were previously necessary. The current research results show a new way forward.
Scientists have now found a method for generating scyrmions that can be directly integrated into the memory chip and works reliably up to gigahertz frequencies. They succeeded in generating the small nano-vortices by means of short current pulses at predetermined locations and then moving them in a controlled manner along the wire. Using holography with X-rays, they were able to image the scyrmions and thus detect them directly. The Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI, Berlin), the Massachusetts Institute of Technology (MIT) and other German research institutions were involved.
The scientists produced scyrmions by using sandwich structures made of platinum, a magnetic alloy of cobalt, iron and boron as well as magnesium oxide. Felix Büttner from MIT explains: “Due to the spin-hall effect, a quantum mechanical effect, and a special interaction of the atoms at the interfaces of the materials, scyrmions can be produced. With our method, this is possible directly in so-called racetrack structures, at predetermined locations, which is essential for a controlled writing of data.” Racetrack structures are nanometer-thin wires made of superimposed magnetic materials. The researchers were able to determine the exact location of the magnetic vortices within the racetrack structures by means of a small additional constriction in the wire.
The fact that the Skyrmion magnetic vortices were actually generated and pushed into the racetrack wire with a further current pulse was demonstrated by the scientists at the German electron synchrotron DESY in Hamburg, using X-ray radiation. X-ray holography allows the detection of these very small magnetic structures in a highly sensitive manner. The magnetization vortices can thus be imaged with a resolution of about 20 nanometers.
The scientists were able to observe how scyrmions are generated with individual current pulses, which are then moved with further pulses. They could determine what happens in the layers of the material, which are only a few nanometers thin, and at the interfaces when individual short current pulses are sent through the material for a period of several nanoseconds. Likewise, they found out how do electrons from the platinum layer influence magnetization in the adjacent cobalt alloy during the current pulses, so that scyrmions with a certain spin are formed. The team compared its observations with micromagnetic simulations in which the processes are simulated in the computer. “These findings on the microscopic mechanism will help us decisively to further develop the concepts and materials for future data storage technologies,” the researchers believe.
The results of their research were recently published in the magazine “Nature Technology,” under the title “Field-free deterministic ultrafast creation of magnetic skyrmions by spin-orbit torques” (Felix Büttner, Ivan Lemesh, Michael Schneider, Bastian Pfau, Christian M. Günther, Piet Hessing, Jan Geilhufe, Lucas Caretta, Dieter Engel, Benjamin Krüger, Jens Viefhaus, Stefan Eisebitt and Geoffrey S. D. Beach). Nature Nanotechnology. DOI 10.1038/nnano.2017.178