The Magnetization Dynamics And Magnetization Reversal Of Magnetic Nanostructures

Abstract:The magnetic storage and information processing devices realize the information record and processing through the manipulation of magnetization of magnetic nanodots or trilayer magnetic nanostructure. The speed of information processing depends on the magnetization reveasal time. It requires an ultrafast magnetization switching and a GHz information processing frequency which is with the same order of spin wave eigenfrequency, for the development of forthcoming nanoscale magnetic information devices. Therefore, the magnetization dynamics of magnetic nanostructure is important for the information devices. In addition, the magnetization reversal is closely related to the magnetization dynamics in the nanostructure. It is necessary to understand the magnetization dynamics, magnetization reversal mechanism and the relationship between them in the nanostructures. Achieving controllable magnetization reversal processes is a key issue for the development of forthcoming magnetic information devices.The precessional magnetization reversal is a promising approach toward the ultrafast magnetization switching. It is necessary to control accurately the pulse duration. There is only a small time window to reliable precessional reversal. Besides after the precessional reversal, a residual magnetization ringing increases the magnetization reversal time and magnetization stability, which harms the application of magnetic information devices. To solve these key problems, based on the micromagnetism thory, in this thesis we study as follows:Firstly, the magnetization dynamics is systematically investigated in the nanodots and the trilayer nanostructure magnetic element where two ferromagnetic layers (FM) are separated by a nonmagnetic (NM) spacer. There are many quantized and localized spin wave modes due to the three-dimensional confinement of finite-size, such as the edge mode, the fundamental mode, the backward modes, the Damon-Eshbach modes and the "mixed" modes exhibiting both the backward and the Damon-Eshbach modes features. The spin wave eigenmodes properties can be manipulated by the modulation of different structural parameters in the nanostructure magnetic. In FM/NM/FM trilayer nanostructure magnetic, the presence of coupling between the layers causes the appearance of modes which are either in-phase (acoustic mode) or out-of-phase (optical mode) coupled spin wave modes in the two layers. The acoustics modes frequency is almost not affected by the coupling strength between the layers while the optical modes frequency decreases with the increasing of the coupling strength. If these two ferromagnetic layers have different thickness or magnetic parameters, there are frequency locking phenomena through the copling between the two FMs. The magnetization dynamics properties and its modulation mechanism have been interpreted based on the micro-magnetism and the spin wave theory.Secondly, we focus on the magnetization reversal of magnetic nanostructure. The process of magnetization reversal can be tuned through the shape of nanostructure. We find that the magnetization reversal process is related to the magnetization dynamic. When magnetization reversal occurs, it is accompanied by a soft magnetic mode whose spacial symmetry determines the initial steps (onset) of the microscopic magnetization phase transitions path. In rectangular nanodot, the edge mode goes soft inducing that the magnetization reversal initiates at the ends and undergoes domain nucleation and wall propagation, while in the structures with tapered ends, the F spin mode goes soft inducing that the magnetization reversal starts in the center and uniform switches to reversal state. In CoFeSiB/NM/CoFeSiB, if two FM layers have same thickness, magnetization reversal initiates at the ends and the OP-S-EM spin wave softens triggering the two FM layers reversal at the same time. There are four magnetic state transitions during the process of magnetization reversal triggered by four different soft spin modes when the two FM layers’thickness is not same.Finally, the ultrafast precessional magnetization reversal in magnetic nanostructure is explored. A strategy is presented using a small DC spin-polarized current to assist the precessional magnetization reversal. The accurate control of the pulse duration is necessary to realize a reliable precessional reversal for the pulse field-induced precessional reversal without the spin-polarized current. There are time windows for the pulse duration. When the pulse duration is in the range of the time windows, a successful precessional reversal is realized. In addition, after the precessional reversal, a residual magnetization oscillation or ringing around the new equilibrium state persists for several nanoseconds. This increases the magnetization reversal time. The pulse time window can be effectively broadened and the magnetization ringing can be significantly suppressed if a small DC spin-polarized current is applied to the magnetic nanostructure during the precessional reversal. The ultrafast precessional magnetization reversal is studied by a perpendicular polarizer spin current. The switching is realized only if the field pulse duration is accurately chosen. There is also a time window for the pulse duration, which depends on the type of pulses. There is a small pulse time window by applying a square shape wave pulse spin polarizer current while the time window can be effectively broadened by applying a cosine spin wave polarized current. The reason for this is that spin transfer torque manipulation on magnetization affects differently for the two type pulses current.