| dc.description.abstract | Magnon spintronics explores the solid-state devices that utilize electric control of spin currents carried by magnons, the quanta of spin waves. The magnon, the dynamic eigen-excitation of magnetically ordered materials, offers promising opportunities for next-generation information processing and communication systems. It can be engineered through various parameters, such as the orientation and magnitude of applied magnetic fields, choice of magnetic material, and sample geometry, enabling versatile engineering of wave-based computing technologies.
Beyond the conventional ferromagnetic magnons, the final goal of this thesis is to obtain a better understanding of antiferromagnetic dynamics, which gains huge attention due to its unique characteristics including ultrafast magnetization dynamics, low damping, and negligible dipole fields. To achieve this, the study focuses on the magnetization dynamics within magnetic materials ranging from ferromagnet, ferrimagnet, and antiferromagnet, using Brillouin Light Scattering (BLS) spectroscopy. Starting from the understanding of ferromagnetic magnetization dynamics, the interface-driven effects on magnon physics are studied in ferromagnetic insulator/metal bilayers. The study reveals previously unidentified interface-driven changes in magnetization dynamics that can be exploited to locally control and modulate magnonic properties in thin-film heterostructures.
As the ferromagnetic film transitions towards antiferromagnetic behavior, notable changes in BLS spectra are observed, including significant linewidth broadening. These experimental results highlight the crucial role of antiferromagnetic exchange interactions in multi-sublattice magnetic materials' magnon dynamics. Motivated by the observed reduction in magneto-optical (MO) signal for ferrimagnetic thin films, we conducted a comprehensive study on canted antiferromagnet, focusing on thin hematite (𝛼 — Fe₂O₃) film. Through both experimental and theoretical analyses of the MO properties of hematite films, we demonstrate that the canted net magnetic moment induced by the Dzyaloshinskii-Moriya interaction (DMI) generates a significant linear MO effect, while the quadratic MO signal arises from the Néel vector. Attributing to the observation, we further demonstrate that the secondary MO effect from dynamical net magnetic moment induces the significant BLS signal, in both theoretical and experimental manners. Furthermore, we demonstrate the strong correlation between magnetic domain structure and local magnon spectra, which is identified for the first time in thin 𝛼 — Fe₂O₃ film.
Our study extends to the direct observation of a non-thermalized magnon state with spin current injection, indicative of a characteristic excitation mechanism with linearly polarized modes in an easy-plane antiferromagnet. The magnon thermalization and relaxation processes are further elucidated between the two linearly polarized modes. The result shows the significant contribution of high k magnons to long-distance spin transport in hematite using non-local electric measurements. Finally, we characterize the magnetic anisotropy within hematite films, distinguishing each contribution to its origin. These findings offer insights into magnetization dynamics and the manipulation of AFM spin textures via optical methods. | |
