Study On Spin Dynamics Of Transverse Domain Wall Movement Under Temperature

With the rapid development of information in this society, people’s demand of data storage and data read speed is increasing. How to store large amounts of data and use less space become the focus of attention. IBM’s parkin proposed racetrack memory in 2008. It is a magnetic device to store information in three-dimensional space, which uses magnetic domain to store data in magnetic nanoribbons. racetrack memory can improve the storage density, so magnetic domain wall motion has great potential applications in terms of data storage and logic devices, which makes the domain walls and domain wall movement of magnetic nano structures get widespread concern and research in the past few decades. Dynamic behavior of magnetic domains in the magnetic nanomaterials has become the focus of scientific inquiry. Currently domain wall motion induced by the spin-transfer torque effect which comes from free electrons has gained sufficient research in the theoretical and experimental. Now a major problem in the practical applications we faced is temperature, material defects can not be avoided, their impact on the domain wall motion need to be researched. This paper studies the influence of temperature on the speed of domain wall motion, and when the domain wall is pinned.In the introduction section, we introduce the history of magnetic materials and the powerful advantage in terms of magnetic materials applications and storage in today’s society. Also it introduces some concepts of magnetic domains and magnetic domain walls, and information about the magnetic domain wall motion.The second chapter describes the basic methods and its realization process of numerical simulation that used in this paper, we used the spin dynamics simulation method.In chapter 3, we use micromagnetic simulation method to study the regular of lateral movement of domain walls driven by a current temperature, and study the physical mechanism of domain wall motion deeply. The results show that:current drive always accompanied by domain wall deformation, and the deformation influence on the speed of the domain wall is most direct. Temperature just to affect the domain wall deformation. In conclusion, there are two cases for temperature influence on current-induced domain wall motion. For small current density without the Walker breakdown, the temperature fluctuation has a few influence on domain wall motion. But for large current density case where the Walker breakdown occurs, the temperature fluctuation can suppress the Walker breakdown, and the effect is more obvious with the increasing of temperature. The reason is that, in the small current density case, the out-of-plane magnetization (MZ) of the domain wall induced by the current can keep its saturation states, so the domain wall motion can almost independent on the temperature fluctuation. But for large current density case, the domain wall cannot keep its Mz which can closely control the domain wall motion. Interestingly, the MZ can flip periodically with the current increase. So the domain wall motion has a regular changing where Walker breakdown occurs. Interestingly, in this case, the MZ periodic inversion can be suppressed by temperature fluctuation, and the suppression become obvious with the increasing of the temperature. Finally, the MZ inversion disappears, the domain wall motion approach stabilization, the Walker breakdown has been suppressed completely. This helps us to understand the movement of domain walls, and how the temperature effects the domain wall motion.In chapter 4, we study the effect of random material defects on domain wall dynamics, while also taking into account the role of local Joule heating. On the one hand we find that MZ of domain walls will change when passing through the defect, which lead to domain wall speed changes. When the current is small, less than the critical current, the domain wall velocity has a little reduction because of MZ oscillations which caused by defects. When the current is greater than the critical current, the domain wall periodical flip (Walker breakdown) will be destroyed because of defects, which makes the domain wall motion much faster. On the other hand, due to the presence of defects, the problem of pinning domain wall cannot be ignored. When the defect in the narrow domain region, the domain wall defects size account for a larger proportion will have a greater pinning effect. When the defect is in the wide domain region, domain wall defect occupies a smaller proportion, it easy for domain walls to across the defect. Moreover the distance from the domain wall to defect also had a significant effect in domain wall pinning. Studies have shown that this is the MZ caused, when the domain wall far away from the defect, MZ has time to grow, at this time domain wall can across the defects. When the domain wall near the defects, MZ has not turn to the larger value, the defect could pin the domain wall. In this case we can make domain wall across the defect by enlarging the current. Finally, we compared the effects of uniform temperature and temperature fields containing local Joule heat for domain walls crossing the defect. Found that uniform temperature field has no significant effect of depinning, but by adding local joule heat can achieve obvious depinning effect.