Spin Moiré Engineering and Emergent Electromagnetism in Topological Spin Crystals

This book provides a comprehensive theoretical investigation into the engineering of topological properties, emergent electromagnetic phenomena, and magnetic and electrical functionalities of materials associated with topological spin textures. Topological spin textures in bulk magnets such as skyrmions form their periodic arrangements called topological spin crystals. They exhibit unique magnetic properties and quantum transport phenomena due to their noncollinear and noncoplanar spin textures. One of the challenging problems in these magnets is flexible generation, annihilation, and control of the spin textures with different topology, leading to their novel functionality useful for next-generation devices. Although such a problem has been discussed in several specific cases, a generic viewpoint was missing in the previous works. To facilitate the systematic discussion on the control of the topological spin textures, the author proposes the "spin moiré" picture, taking particular note of the observation that topological spin crystals are often represented as superpositions of multiple spin density waves. Moiré is an interference fringe generated by a superposition of waves and it exhibits the periodicity different from the superposed waves. Importantly, a variety of moiré patterns are realized in many ways by changing a number of parameters, e.g., periods, amplitudes, phases, propagating directions, and the number of superposed waves. Given the analogy with the conventional moiré patterns, one can conceive a further variety of spin patterns and their continuous modulations by regarding the topological spin crystals as moiré patterns of the vector (spinor) fields. Given the growing recognition of topology's significance in describing the state of matter, especially in magnetic materials, the findings presented in this book establish a versatile framework for controlling topological magnetism, which does not only advance the fundamental understanding of topological properties in magnets but also opens new pathways for exploring emergent electromagnetic phenomena and designing magnetic functionalities. Intended for students and researchers in the fields of condensed matter physics and material science, this book serves as a valuable resource for those interested in fundamental theory and advanced discussions of topological magnetism and its control.