Micromagnetic Studies Of Domain Walls And Vortices

The spin-transfer torque (STT) induced by spin-polarized current was found to be an efficient method to manipulate the magnetization in magnetic nanostructures. With-out the aid of external magnetic field, different types of dynamical states induced by STT can occur, such as magnetization reversal, domain wall motion, and persistent pre-cession, etc. The research on magnetic properties in nanostructures not only makes it possible to understand the magnetic dynamics, but also important for potential appli-cations. A series of applications, such as racetrack memory, microwave oscillator, and magnetic sensor, have been achieved with the deeper studying on the properties in mag-netic nanostructures.In this work, dynamics of domain walls and vortices in different nanostructures have been studied by microniagnetic simulation. For the research on domain wall, we focus on an important phenomenon in domain wall motion, known as Walker Break-down. We discuss possible solutions to suppress Walker breakdown and enhance the domain wall velocity. In another part of the work, we performed a systematic study on the dynamics of vortices, as well as the phase locking between two vortices. Following is the specific contents.1. Domain wall motion in magnetic nanostripsAt first, artificial defects or stray fields can serve as pinning sites for domain walls, and it hinders the motion of domain wall. On the other hand, the domain wall struc-ture can be modified by the pinning. It is possible to promote domain wall motion under suitable deformation of spin structure in domain wall. The domain wall motions in mag-netic nanostripe-nanobars structure have been investigated to discuss the contradictory effects of domain wall pinning. The results show that the remote pinning is much weaker than the geometric pinning, and the domain wall structure can be modified by a series of remote pinning sites. The domain wall velocity is also increased owing to the modified domain wall structure. Additionally, theoretical analysis reveal that the driving force for 180° domain wall motion comes from magnetic stray fields, and the stray fields directly related to confined magnetic geometries, micromagnetic study shows that by changing the cross-section, the velocity of domain wall motion in nanostrip can be significantly enhanced.Secondly, the dynamics of 360° domain wall in magnetic nanostrip are micomag-netic studied.360° domain wall is a metastable state consisting two 180° domain walls, and the magnetization directions on both sides of domain wall are consistent. There-fore, the static magnetic field is not responsible for the long-range 360° domain wall displacements, while it can be driven by STT. Due to the unsteady structure, the 360° domain wall can be annihilated easily. The bias-filed assisted 360° domain wall motion is proposed based on theoretical analysis and hence the structural stability as well as domain wall velocity can be enhanced.At last, the Dzyaloshinskii-Moriya interaction (DMI) coming from the effect of structural inversion asymmetry in magnetic materials and the strong spin-orbit coupling, is responsible to the properties of domain wall. In this part of our work, we present a numerical study of static and dynamic features of domain wall affected by the DMI.2. Phase locking of two coupled vorticesThe ability of spin-polarized current to excite the vortex gyrotropic motion through the transfer of spin angular momentum has led to the demonstration of spin-torque nano-oscillators (STNOs). The STNO array should be archived in order to improve the output power. The phase locking of coupled vortices has been studied in this part of work.For the case of dipolarly coupled vortex-based STNO, when the dynamics of vor-tices induced by a spin-polarized current injected via point contact, the range of cur-rent density for phase locking can be broaden significantly. By presenting the results of calculation and comparing them with the theoretical analysis, the impact of coupling strength on the parameters of phase locking was investigated. Then when the two STNO connected by a magnetic bridge, the coupling between the two vortices comes from the contributions of the exchange energy and demagnetization energy, and the coupling strength is further improved. The time required for phase locking is also greatly re-duced. But due to the bridge coupling, the phase difference between the two vortices is no further zero. We demonstrate that the phase difference can be also adjusted by the Oersted field distribution and the spin valve size. The systematic study of phase locking process is important for the designs and applications of vortex-based STNOs.