Domain walls as new information storage

A scanning electron micrograph of a w=500 nm wide and 30 nm thick permalloy ring with radius r=2 μm overlayed with a scanning transmission X-ray microscopy (STXM) image showing the in-plane magnetic contrast. © André Bisig, Johannes Gutenberg Universität Mainz

Domain wall motion imaging: at high speeds, material defects no longer play a role

Physicists at the Johannes Gutenberg University Mainz have directly observed how magnetic domains behave within ultrasmall, curved nanowires. Their work involved using the MAXYMUS x-ray microscope of Max Planck Institute for Intelligent Systems  at Berlin-based electron storage ring BESSY II, which is operated by the HZB. In doing so, they succeeded at capturing these processes in the form of image sequences. In this way, they were able to not only experimentally confirm theoretically predicted effects but also to observe and understand  new properties that promise interesting potential applications in the area of information technology, including as information storage devices or as position sensors. Applications based on the principle of magnetic domain walls are already being used in sensor technology.

Ferromagnetic materials break up in domains, which are regions with uniform magnetization. At the boundary of two different domains a domain wall forms. These walls are mobile and this mobility can be exploited for applications. Prof. Dr. Mathias Kläui‘s work group at the Johannes Gutenberg University Mainz has studied these domain walls, which form inside tiny magnetic rings some 4 micrometers in diameter. These rings consist of permalloy, a type of ferromagnetic nickel-iron alloy that can be easily magnetized.

To this end, the Mainz physicists worked closely with the team of Prof. Dr. Gisela Schütz from MPI for  Intelligent Systems, Stuttgart and Prof. Dr. Stefan Eisebitt’s team of scientists, who is head of the joint functional nanomaterials research group of the HZB and the TU Berlin. Through measurements at synchrotron sources BESSY II at the HZB and the Advanced Light Source in Berkeley, USA, they were able to directly observe the movements of the domain walls via such specialized X-ray microscopes as MAXYMUS, operated by the department of Prof. Dr. Gisela Schütz, MPI for Intelligent Systems, Stuttgart.

The researchers managed to specifically move the domain walls inside the ring using pulsed, rotating magnetic fields. “The faster we rotate these domain walls, the easier controlling them becomes,” says Dr. André Bisig, member of the Kläui team and first author of the study. In the process, they also discovered a new effect: The domain walls’ speed oscillates during rotational movement as the domain walls’ internal magnetic structure changed periodically.

Yet another observation concerned the effects of irregularities within the nanowires on the domain wall motion. According to the results, the faster a domain wall is rotated, the lower is the impact of material defects on this motion. “These findings expand our basic understanding of magnetic domain structures’ dynamic behavior,” Stefan Eisebitt explains. “They also illustrate the importance of being able to directly ‘watch’ functional nanostructures ‘at work’ using cutting-edge X-ray microscopes in order to develop new applications from these basic insights.” And Markus Weigand, leading scientist at the MAXYMUS-Beamline explains: “Our scanning-x-ray-microscope at BESSY II is currently the most powerful instrument for directly time resolved images of magnetisation dynamics.  It can show such processes in extreme slow motion, ten billion times slower than in nature.”

André Bisig et al.
Correlation between spin structure oscillations and domain wall velocities
Nature Communications, 27. August 2013
DOI: 10.1038/ncomms3328

arö / Uni Mainz