Dynamics of current-driven magnetic domain walls and skyrmions

• Main Author: Indra Purnama
• Other Authors: Lew Wen Siang
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Science::Physics::Electricity and magnetism
• Online Access: https://hdl.handle.net/10356/65534
• Institution: Nanyang Technological University

The dynamics of current-driven magnetic domain walls and skyrmions are both topics of high interest in the magnetism community due to the possibility of employing them in novel high-density non-volatile memory devices. One such device is the well-known racetrack memory, where the information is stored in a ferromagnetic nanowire as compared to the conventional harddisk memory, while the operation is mediated via the injection of current to the nanowire. In a domain wall-based racetrack memory, the information is represented by magnatic domains that are separated by domain walls, while in a skyrmion-based racetrack memory, the information is represented by the presence of the skyrmions themselves. However, to be able to realize such devices, it is very important to have a complete understanding of both the domain wall and dynamics under the application of current. At present, the investigation of domain wall dynamics is mainly focused on finding additional driving mechanisms to improve its mobility. In the case of skyrmion dynamics, the research is still in the very early stage, although it has been shown that it is possible to inject individual skyrmions individually which brings skyrmion-based devices closer to realization. In this thesis, the dynamics of current driven magnetic domain walls and skyrmions in various nanostructures are investigated. We show by micromagnetic simulations that it is possible to drive multiple domain walls in a multi-nanowire system by just applying current to one of the nanowires. The phenomenon is made possible due to the magnetostatic coupling between the domain walls. When materials with in-plane anisotropy are considered, the coupling is realized when the nanowires are placed parallel to each other, and the technique can be further utilized to drive several domain walls in the current-free nanowire. When materials with strong perpendicular magnetization anisotropy are used, the phenomenon is realized by stacking the nanowires vertically, and the coupling between the domain walls is found to be strong enough to change the domain wall configurations and increase their current-driven speed. As compared to the domain wall, a current-driven skyrmion is found to move at an angle with respect to the intended conduction electron flow direction due to the presence of the Magnus force. Here, we show that it is possible to guide the movement of a skyrmion by surrounding and compressing the skyrmion with strong local potential barriers. The compressed skyrmion also receives higher torque contribution from the conduction electrons, which results in a significant increase of the skyrmion speed. Our research work can then be said to contribute to three areas. First, our work on a system of multiple nanowires with in-plane magnetization anisotropy has made it possible to design a memory device where the data is remotely driven instead of being directly driven by current injection. Such device will then benefit from increased data retention as the data will experience no heating due to the absence of direct current injection. From the physics point of view, our work on the multiple nanowire system has also shed a light on the spring-like nature of the Domain Wall- Domain Wall (DW-DW) interaction, which can potentially be used as a framework to investigate the mass property, i.e. the inertia, of DWs in ferromagnetic systems. The second contributions can be found in our work on the coupled DW system in vertically-stacked nanowires with perpendicular magnetization anisotropy (PMA). Our results shows it is possible to optimize the speed of the DWs in PMA nanowires by locking the DWs in a configuration that is least hindered when driven by current. Lastly, our work on the skyrmion research provides a framework to solve the issue of skyrmion annihilations. We show that any skyrmion-based device that are designed in the future will require the inclusion of artificial potential barriers at the edges of the device to prevent the skyrmion to be annihilated, which we accomplished by introducing curbs to the design of the skyrmion device.