Numerical Analysis On Domain Wall Dynamics In Magnetic Films And Nanowires

In the past years, there are much applications of the achievements in the study of thin-film and nanowire magnetic systems, i.e., the high-density storages and high-sensitive sensors. The dynamic processes of domain walls far from equilibrium has attracted much attention, especially the phase transitions of magnetic systems. The numerical method based on the short-time dynamic scaling is of great advantage in the analysis of critical behavior of domain wall dynamics, as it does not suffer from the critical slowing down. In this paper, we numerically investigate the relaxation dynamics of domain walls under different low-dimensional magnetic systems,additionally with the assistant of some analytical resolutions. The study includes the explanation of phase transitions,the analysis of scaling behaviors, the measurement of critical exponents, the observation of spin wave propagations, the discussion of the Walker breakdown phenomenon, etc.With Monte Carlo simulations, we investigate the relaxation dynamics of domain walls at the Kosterlitz-Thouless phase transition, taking the two-dimensional XY model as an example.Firstly we consider a magnetic system starting with two domains with the magnetizing angle of 180°. The dynamic scaling behavior is carefully analyzed, and a domain-wall roughening process is observed. Two-time correlation functions are calculated and ageing phenomena are investigated.Inside the domain interface, a strong logarithmic correction to scaling is detected. The results verify that the Kosterlitz-Thouless phase transition is topological due to the free vortices and vortex pairs, and provide a quantitative method for precise measurement of the relevant critical exponents.Secondly, if the angle between the magnetization of the domains slightly deviates from 180°, we detect that the initially tiny component of the magnetization behaves a power-law increase. The relevant dynamic exponent is analyzed, in addition to the analytical deductions that lead to similar results. This phenomenon is not observed in Ising model or ?4 model, which indicates that the XY model exhibits different relaxation dynamics and further study is required to understand its magnetic mechanism.The domain wall dynamics in magnetic nanowires is numerically studied with the Landau-Lifshitz-Gilbert equation. When the driving field is below the Walker breakdown threshold, the domain wall exhibits the stable Walker propagation, while above the threshold presents the ret-rograde breathing mode. To understand the latter case where the Walker's assumption of small driving field is invalid, we theoretically and numerically analyze how the oscillation period is con-trolled by the external field, the damping constant and the anisotropy. More importantly, the phase transitions are not systematically studied in the micromagnetics. In this paper, the dynamic effects of quenched disorder on the domain wall motion are explored. A continuous pinning-depinning phase transition is reliably detected. The dynamic scaling form is analyzed with the data collapse of the domain wall velocity, the static and dynamic critical exponents are measured respectively,which provides important resources for the study of the phase transitions in the micromagnetic systems.