Topological Spin Textures in Three Dimensions: Dynamics, Stability, and Emergent Electromagnetics

  • Yu Li

Student thesis: Phd

Abstract

Magnetic materials can host a rich variety of nanoscale magnetic structures with non-collinear spin orientations, known as topological spin textures. A plethora of intriguing phenomena entwines magnetism and topology, especially where a Dzyaloshinskii-Moriya interaction is present. Exploitations of the robust topological stability inherent in these textures promise applications in magnetic data storage and nanocomputing devices. To fully realise the potential of these spin structures, fundamental studies exploring the underlying mechanisms in real systems are essential with specific foci, including their formation, stability, transition, detection, and annihilation. The work presented in this thesis explores topological spin textures in the context of three-dimensional ferromagnets and addresses the ramifications of field-induced transitions, thermal stabilities, and associated emergent electromagnetic fields. In particular, from the three-dimensional perspective, fruitful insights are spotted by considering the depth-resolved complexity of magnetisation profiles. The first aspect of this study lies in the complex dynamics of extremely small topological defects - Bloch points, during the temporal evolutions of single topological spin textures, such as skyrmion tubes in bulk magnets and skyrmion chains in multilayers. Under the application of an external magnetic field, the transition mechanism is dominated by the combined effects of the Dzyaloshinskii-Moriya interaction and magnetostatic interaction. During the process, the Bloch-point propagation speeds are km/s. Meanwhile, the speed is periodically modulated by regular ''washboard'' potential wells that originate from the atomic arrangement, and oscillates with a THz frequency. Moreover, we report the thermal stabilities of such systems via exploration of minimum energy paths to other possible equilibrium states. It seeks to build upon the knowledge that topological spin textures can be stabilised as metastable states in a wide parameter space. The results show that the transitions are mediated by Bloch-point dynamics, where the presence of saddle points in the energy landscape is associated with the creation/destruction of Bloch points. It is notable that due to the discrete nature of magnetic systems, the topological protection property partially breaks, which leads to finite magnitudes of energy barriers. The resulting spatial and temporal non-uniformities of the magnetisation within the topological structure create emergent electromagnetic fields that act on conduction electrons. We reveal a phenomenological connection between the emergent magnetic field and three-dimensional topology, allowing unambiguous recognition of different textures based on their topological Hall signals. The transitions also bring drastic modifications to local emergent magnetic fields. It is shown that the ultra-fast propagation of a Bloch point radiates solenoidal emergent electric fields with MV/m magnitude and THz oscillation frequency. Using the understandings of the studies in this thesis, we propose several designs that seek to enable tuneabilities of thermal stabilities and ultra-fast transition phenomena. Relevant techniques are compatible with existing nanofabrication technologies. Thus, validation by future experiments should be within reach.
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorVasileios Pavlidis (Supervisor) & Christoforos Moutafis (Supervisor)

Keywords

  • topological Hall effect
  • atomistic simulation
  • micromagnetic simulation
  • nudged elastic band method
  • emergent electromagnetism
  • spintronics
  • topological spin texture
  • magnetic skyrmion
  • Dzyaloshinskii-Moriya interaction
  • magnetism

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