Fig.: Shadow XMCD PEEM images showing the magnetic configuration in nanowires after current pulses in opposite directions. Due to the choice of focus conditions the wire itself is invisible here, while the contrast is actually coming from the shadow region of the synchrotron beam that the wire cast onto to the supporting substrate. The diagonal black/white patterns between domains allow the identification of the domain walls to be of the Bloch-point type. After opposite current pulses they have opposite circulation sense, as indicated by the central diagonal stripe’s color (upper panel dark, lower panel bright) in the highlighted boxes.
Cerdanyola del Vallès, 25th November 2019. The future of spintronics relies on the development and study of 3D nanostructures as they may have a great impact in the development of new magnetic data storage systems like racetrack memories. In this proposed new concept, information or bits would still be encoded by magnetic domains or the walls between them. But rather than moving the material mechanically to the read / write head (like for a spinning hard disk), only the magnetic pattern would be moved along the magnetic strip or wire by the driving force of an electric current, called spin transfer torque. However, the fast and reliable motion of the domain walls still remains a major challenge.
Due to their 3D nature, cylindrical magnetic nanowires host two types of magnetic domain walls with discrete topologies, including the so-called Bloch-point wall, which does not exist in 2D flat nanostrips such as produced by conventional clean-room technology. Now it was shown that these unique Bloch-point walls can achieve very fast speeds while remaining stable.
"The current makes the domain wall move and the OErsted magnetic field created by the current stabilizes it. So, domain walls are not being transformed as happens in flat systems, which suffer intrinsically from the resulting instabilities", says Michael Foerster, scientist at the CIRCE beamline of ALBA.
An international group of scientists, led by Michael Schöbitz and Olivier Fruchart from Spintec laboratory in Grenoble (France) and Sebastian Bochmann and Julien Bachmann from the Friedrich-Alexander Universität Erlangen-Nürnberg (Germany), came to this conclusion after studying current-induced domain wall motion in nanowires. The results of the study show that, when applying electrical current pulses, the Bloch-point walls move at a very high speed, achieving a new record for this type of materials.
Although they could already observe the fast magnetic domain wall motion in cylindrical wires using magnetic force microscopy in their laboratory, it is only by using the shadow photoemission electron microscopy (PEEM) technique with X-ray Magnetic Circular Dichroism (XMCD) that it was possible to resolve the exact wall type. With this additional information they could determine the reason for the fast speed. The most striking discovery was that the OErsted field surrounding the nanowire during current pulses transformed all domain walls into the Bloch-point wall type, which can be understood as they display the same symmetry as this OErsted field. This configuration then does not transform anymore, in contrast to previously investigated 2D systems where continuously oscillating transformations slow down the domain walls (Walker breakdown).
The experiments were performed at the CIRCE beamline of the ALBA Synchrotron and the Nanospectroscopy beamline from Elettra (Italy), using the "shadow" XMCD PEEM for magnetic contrast. "These results are the first to highlight the important role that the current-induced magnetic (OErsted) field plays in the speed and stability of domain walls in cylindrical nanowires and open the path for the continuous research on these fascinating nanostructures. In the near future, it would be very interesting to try to monitor the details of the domain wall dynamics during the current pulse, i.e. combined microscopy with time resolution", says Michael Schöbitz, first author of the study.
