Nano-magnetic Domain Wall Bar Regulation And Current Drive

Magnetic materials have been major carriers in digital information field from last century. As the requirements of higher storage density and faster reading velocity, domain wall and domain wall motion have been entered into peoples’ eyes and attracted a lot of interest. The digital information is stored by the magnetic domain, the transition zone between two adjacent domains is called domain wall. Results show that steady domain wall structure is depends on material parameters. By moving data units to reading and writing device, instead of traditionally rapidly rolling disk, it is optimistic to realize higher reading velocity and storage density in promising three dimensional race track memory. Magnetic field can be used to displace domain wall, but uniform magnetic fields can make neighboring domain walls move in opposite directions and annihilate each other, causing the information lost. However, we can avoid this situation by using spin polarized current to displace domain walls, which makes the domain wall motion induced by polarized current become one of most active subfields of spintronics. In this paper, Ⅰ study the approach to induce different types of spin vortex structure and the current-induced domain wall motion in ferromagnetic nanostrips.Chapter Ⅰ presents the development of magnetic materials in digital information storage field.Chapter Ⅱ introduces the simulation method I used in this paper--spin dynamics simulation. Also it gives a presentation of calculating effective field by fast Fourier transform (FFT) and a new approach to find the steady state quickly.In chapter Ⅲ, the approach to induce different types of spin vortex structure in ferromagnetic nanostrips is discussed. The results show that in isotropic or week anisotropic system, the static magnetic structure favors single vortex walls. The approach of pinning the boundary spins is effective to control the types of single vortex walls in ferromagnetic nanostripes. This approach can be used as auxiliary means to write the digital information with concrete types of spin vortex structure into ferromagnetic nanostripes due to the induced SVS keeps its stabilization when the pinning is removed, e.g., for a certain digital information, we can control a confirmative magnetic domain configuration to represent the digital information by pinning the boundary spins. Moreover, the stabilization of SVS will be enhanced by pinning the spins in its boundary. However,for long nanostripes, there may are only two edges, say, along the y direction, but there are no boundaries along thexdirection. In this cases, the located magnetic field pulse along theydirection can be set to play the role of the boundary pinning. The results will have a potential application in switching and manipulation of ferromagnetic nanostripes as data storage media and logic devices.In chapter IV discusses the domain wall motion under the insert of polarized current. By analyzing the Landau-Lifshitz-Gilbert(LLG) equation with adiabatic and nonadiabatic terms,I discuss factors that have contribution to the wire direction and transverse direction motion.The results show that the adiabatic term makes the domain wall moving along the wire direction, while both nonadiabatic term and inner force in system itself have contribution to the transverse direction motion, one element compete with or assist the other,depending on numerical relationship between nonadiabatic constant β and damping constant α. But the results indicate that in the same system,by adjusting the nonadiabatic constant to balance the inner force effect, one can achieve a steady domain wall motion along wire direction without transverse motion.The inner force includes exchange interaction between neighbour magnetization,dipole-dipole interaction,et.at. It is not easy to distinct each effect on domain wall motion. Another unexpected results is that muti-vortexes can move steadily along the wire direction in a natural system.