Abstract
This thesis investigates the behavior of skyrmion and spin wave in inhomogeneous systems in two aspects: (1) skyrmion pinning and motion in inhomogeneous chiral magnetic films. (2) spin wave amplification through superradiance at the interface between two ferromagnetic regions.
Based on minimal energy principle, skyrmion pinning position in a magnetic film with artificial disks is investigated. A skyrmion structure consists of two primary components: skyrmion core and skyrmion wall. The magnetizations within the wall tend to remain in areas where exchange stiffness and anisotropy are minimal, and Dzyaloshinskii-Moriya interactions (DMI) are substantial. Conversely, the magnetizations within the core prefer to align parallel to each other and remain in regions where anisotropy is substantial. Based on the core-to-wall ratio, skyrmions are classified into two categories: core-dominate and wall-dominate skyrmion. The pinning location of a skyrmion can vary based on several factors, including the type of skyrmion, type of disk, and relative size of the disk in relation to the skyrmion. Specifically, a skyrmion may be pinned either at skyrmion core or skyrmion wall, depending on these aforementioned variables. When the disk size is comparable to the size of the skyrmion, the skyrmion undergoes significant deformation, causing it to either shrink or expand in order to fill the entire disk.
The dynamics of skyrmions in inhomogeneous films with disorders are heavily influenced by the skyrmion structure, the applied driving force, and the strength of the disorder. Unlike the motion of a typical body driven by a force, the motion of a currentdriven skyrmion is not only in the direction of the force (longitudinal motion), but also in a direction perpendicular to the force (transverse motion). This unique behavior is origin from the topological structure of the skyrmion, which is determined by its topological charge, dissipative dyadic, and damping coefficient in magnetic films. For a particular electric current, there is a critical level of disorder strength that is dependent on the density of current. Furthermore, there exists a critical level of damping that is dependent on both the current density and the disorder strength. In the event that the disorder strength surpasses the critical value, the skyrmion becomes immobilized and cannot move in the non-uniform film. If the disorder strength falls below the critical value, the transverse motion of the skyrmion can be improved if the damping is below the critical value, or impeded if the damping is above the critical value. However, the longitudinal motion of the skyrmion is always impeded by the disorder. The root of these phenomena is the force on the skyrmion that is brought by the disorder, which can be categorized as either static or kinetic, similar to the force of friction in Newtonian mechanics. During the pinning phase, the static force acts in opposition to the direction of the skyrmion’s motion, which is crosswise to the current for spin-transfer-torque-driven motion. During the boosting and impeding phases, the kinetic force acts in the opposite direction to the velocity of the skyrmion.
In the realm of spin wave-based devices, a significant challenge lies in the rapid decay of spin wave amplitude due to damping. To address this issue, amplification of spin wave through superradiance has been achieved, the spin wave amplitude can be amplified at the interface between a normal and a current-flow ferromagnetic region. Spin wave spectrum inversion is observed in the current-flow ferromagnetic region, such that the phase velocity and group velocity of spin waves are opposite. The interface between the normal ferromagnetic region and current-flow ferromagnetic region can be likened to a super-mirror, with a spin wave reflection coefficient exceeding unity. The refracted wave in the current-flow ferromagnetic region is backward propagating into the normal region and interfering with the reflected wave. It appears that the current-flow region is coherently emitting waves and energy to the reflected wave, thereby amplifying spin waves. Additionally, the Snell law of spin wave reflection and refraction at the interface is further investigated.