Static and Dynamic Properties of a Bloch Point in Soft Magnetic Nanostructures

Abstract

A Bloch point is a magnetic singularity in a ferromagnet. While the two-dimensional (2D) magnetic structures such as vortices and skyrmions have long been the central theme of research in magnetism, their zero-dimensional (0D) counterpart in the three-dimensional (3D) boundary condition, the Bloch point, has been mostly a subject of theoretical investigation due to the difficulty in achieving stable Bloch points within magnetic nanostructures. Unlike other spin textures, a Bloch point has a unique feature – the local magnetization at a Bloch point completely vanishes. It has also been theoretically proposed to bear critical roles in the dynamic behavior of the one-dimensional (1D) and 2D spin textures such as domain wall motion, vortex core switching, and skyrmion dynamics. Besides, the spin-wave coupled phenomena, such as spin wave emission, spin wave scattering, and spin wave-Cherenkov effect, and ultrafast dynamics of the Bloch point are promising effects for the ultrafast and ultradense device application. The novel physical behavior is rooted in the energy barrier at the atomic lattice point, which is derived from the discrete atomic lattice. However, the physical behavior of a Bloch point associated with the energy barrier at the atomic lattice point has not yet been experimentally explored due to the difficulty to obtain stable Bloch points in the nanostructure. This thesis treats a stable Bloch points embedded within nontrivially distorted magnetic vortex cores in the rectangular-, elliptical-, and asymmetric-shaped permalloy (Ni80Fe20, Py) disks of 100 nm thickness which can stabilize the Bloch point. The stabilized Bloch point is observed by utilizing the magnetic transmission soft x-ray microscopy (MTXM), combined with micromagnetic simulations. Based on the Py nanostructure, this thesis provides the experimental demonstration of the existence of the energy barrier at the atomic lattice point which is associated with the physical properties of the Bloch point by observing the dynamics of the Bloch point. Furthermore, it shows the interaction of the Bloch point-Bloch point and Bloch point-edge defect interaction, as well as the manipulation method of the Bloch point driven by magnetic field pulse. This thesis an important scientific breakthrough for a complete understanding of singularities by providing the details of static and dynamic properties of Bloch points. The finding of influence of the energy barrier at the atomic lattice point on the dynamics of Bloch point, and the fundamental study of the interaction of the Bloch point, and manipulation method of Bloch point would immediately generate many follow-up activities in fundamental aspects of magnetic singularities as well as device applications of them.