Micro - Magnetic Simulation Of The Effect Of Anisotropy On Current - Driven Domain Wall Motion

Current-driven manipulation of magnetic domain walls in patterned magnetized thin-film stripes via the spin-transfer-torque effect has attracted much interest because of its potential applications for next-generation solid-state memories, logic and microwave devices. The motion of domain walls due to a spin transfer torque of electrons has been well studied theoretically and experimentally. An open question is that the threshold current density required to move a domain wall is too large for practical applications. Recently, it was shown that the current-driven motion of the transverse wall (TW) can be enhanced by tuning the perpendicular magnetic anisotropy (PMA). However, the underlying physical origins for the effect are not very clear. In magnetic nanostripe system, vortex wall (VW) is one of the basic domain walls. As for the VW, the current-driven motions are more complex than that of the TW, because the horizontal motion of VW along the stripe is accompanied by the vertical motion of the vortex core. Theoretically, the study on the current-driven motions of the VW may clarify more underlying physical origins than that of the TW. In high-data-density design, where the nanowires will be placed very close to each other, the interaction between the DWs from adjacent nanowires is important. In this letter, we discussed the current-driven VW motions in magnetic nanostripe system with PMA and the current-driven motion of the coupled VWs in a two-nanostripe system with PMA by micromagnetic simulations.The first chapter presents the development of magnetic materials in digital information storage field. The second chapter introduces the simulation method I used in this paper--spin dynamics simulation.The third chapter presents in detail the effect of every term in modified Landau-Lifshitz-Gilbert equation on the current-induced VW motion. The relationship of vortex core vertical motion and the polarizations of vortex core and TWs are discussed.In fourth chapter, the current-driven VW motions in magnetic nanostripe system with PMA are studied. The numerical results show that the driving forces for both the horizontal motion and the vertical motion can be monotonously enhanced by the PMA and the current. However, the confining force induced by the confining potential energy firstly increases then decreases with the increasing of the PMA. As a result, the velocity of the horizontal motion firstly decreases then in-creases with the increasing of the PMA when the current is small. However, the velocity of the horizontal motion just increases as the PMA increases when the current is larger than a critical value. On the other hand, the velocity of the vertical motion increases with the PMA in a monotonous way. The underlying physical origins for these effects are quantitatively provided.The last chapter concerns the spring-like oscillatory motion of the coupled VWs occurring when current is applied to both nanostripes in opposite directions. The effects of the perpendicular anisotropy on the spring-like oscillatory motion are studied. We show that the magnetostatic interaction between the two VWs can be changed by the perpendicular anisotropy.