Magnetic Structure Regulation And Current Driving Of Multi - Domain Walls In Magnetic Nanobelts

The researches of devices based on domain walls have attracted much attention because of its potential applications in next generation memories, magnetic logic devices, magnetic oscillator and so on. In these devices, domain wall motion is a key technology. There are several methods to drive domain wall motion, such as magnetic field, spin polarized current, spin wave and temperature gradient. The current driven motion due to spin transfer torque can realize the synchronized motion of multiple domain walls in the nanostripe without disturbing the original sequence and can achieve higher domain wall speed. Therefore, the current driven motion of domain wall has been well studied theoretically and experimentally in recent years. At present, the motion of single domain wall driven by current has been well studied in detail. However, for practical applications, there are usually some problems related to the motion of multiple domain walls. For instance, in order to increase storage density, the racetrack memory is made into a three dimensional structure. The storage density depends on the distance between adjacent nanowires and the density of domain walls in single nanowire, and the reading or writing speed of the memory depends on the domain wall velocity. However, there are magnetostatic interactions between domain walls in the magnetic nanowire, which may affect the dynamic behavior of domain wall motion and even lead to information distorted. So it is essential to control the stability of multiple magnetic domain wall structures in the magnetic nanostripe. In this paper, we discussed the stable manipulation of multiple magnetic domain walls and its current-driven motion in a magnetic nanostripe by micromagnetic simulations.The first chapter mainly introduces the application and development of magnetic storage technology, the related knowledge of domain wall, the methods to drive the domain wall motion, and the research status of the domain wall motion in theory and experiment.The second chapter introduces micromagnetic simulation, which is the numerical simulation method used in this paper, including the traditional spin dynamics method and the LLG equation with spin transfer torque.The third chapter studies the manipulation of multiple transverse wall structures and its current-driven motion in the single magnetic nanostripe. The results show that the interaction between two transverse walls is related to the orientation of the center of domain wall. There is only long-range attraction between two transverse walls with same orientation, and two domain walls will be annihilated at last. However, when two transverse walls are arranged with opposite orientation, there are both long-range attractive interaction and short-range repulsive interaction. When the two interactions counteract with each other, a metastable structure will be formed. If a number of transverse walls with opposite orientation are arranged in turn, a complex domain wall structure will be formed. The formed complex wall is called multiple 360° domain wall structure (M360S) since they includes multiple 360° substructures. Besides, the M360S may behave like single domain wall under an applied current. When the current is larger than a critical value, the M360S will annihilate. The critical current for the annihilation strongly depends on the number of 360° substructures in M360S, which exhibits a parity effect. And we find that the parity effect may be relevant to the out-of-plane magnetic moment of the M360S.The fourth chapter discusses the influence of anisotropy on the current-driven motion of 360° domain wall. The research is mainly focused on three cases, which are magnetic system with uniaxial anisotropy, magnetic system with cubic anisotropy and magnetic system with both uniaxial and cubic anisotropy. The result shows that only the case of magnetic system with cubic anisotropy, the annihilation current of 360° domain wall can be effectively improved without changing the structure of the domain wall and the velocity of 360° domain wall increases linearly with the applied current at first and then decreases, which is similar to the behavior of Walker breakdown. The main reason is related to the periodic variation of out-of-plane magnetic moment of 360° domain wall.