Numerical Investigations Of Magnetic Skyrmions With Ultra-high Density Information Carrier In Micromagnetic Framework

To overcome the bottleneck in the development of the current storage technology,the study of spin transfer torque effect,racetrack memory and magnetic skyrmions have attracted considerable attention among the people.Magnetic skyrmions can provide a solution to the information of fast and steady transmission,high-density storage,low energy consumption due to their topological stability,small size and the possibility of moving them with current of small density.The magnetic medium is solid state in magnetic skyrmions-based racetrack memory.Thus racetrack memory possesses the best of both Hard Disk Drive and Solid State Drive and has the vast significance to the development of the storage industry.Before we realize the magnetic skyrmions-based devices in practical applications,it is absolutely critical to master the key technology of magnetic skyrmions creation and motion in nanostructure.This paper simulates magnetic skyrmions creation and motion by injection of spin-polarized current in magnetic material and the creation and motion of magnetic skyrmions in nanostructure are studied in detail.In this thesis,we first investigate the effect of the spin-polarized current density,current duration and injection size to create magnetic skyrmions by micromagnetic simulator OOMMF.We calculate the threshold of current density in the context of different injection size.We provide solutions to the reduction of the threshold of current density to create magnetic skyrmions and get to know that topological number of magnetic skyrmions can be a reference for the current duration.Magnetic skyrmions can be created by the whole injection of spin-polarized current,but its magnetic moment direction reverses to the local injection.This result proves that we can create two chirality directions of magnetic skyrmions from one initial magnetic configuration.Magnetic skyrmions size can be controlled by changing the model dimension and geometric form and has nothing to do with the spin-polarized current density,current duration and injection size.The smaller size of the model has more confinement effect on the size of magnetic skyrmions and nanodisk have more confinement effect than nanosquare.Then,we study the application of magnetic skyrmions in racetrack memory.This paper proposes to divide the racetrack into three parts: creation region,motion region,annihilate region due to the difference problems in the process of magnetic skyrmions creation,motion,annihilation,so that we can have optimum design of racetrack memory depending on the feature of every part.The threshold of current density to create magnetic skyrmions can be reduced by expanding the creation region in the racetrack memory with local current injection.We inject current in the whole creation region,a properly designed current pulse that tends to convert Neel walls to a stable magnetic skyrmions.This method will make further reduction of current density.Finally,we study the current-driven motion of magnetic skyrmions in the racetrack.In the motion,magnetic skyrmions will appear escape on the upper track border and get clogged at the end of the racetrack sometimes.We set a barrier in the motion region and a notch in the annihilate region,which effectively resolves the magnetic skyrmions escape and congestion.The study indicates that the value of magneto-crystalline anisotropy constant determines the ability of the barrier and the form of the notch determines the ability of annihilation.When multiple magnetic skyrmions in the racetrack driven by the current,the later magnetic skyrmions can push the front one to the end of the racetrack and the front one can be annihilated at an intermediate current density.The effect of Skyrmions-Skyrmions repulsion is not so well as the notch.This paper discusses the effect of the model dimension and geometric form of the nanodisk and technological parameters on the creation and motion of magnetic skyrmions in details.Those results provide solutions to the reduction of the threshold of current density to create magnetic skyrmions in creation region and improvement the velocity of magnetic skyrmions in motion region and also effectively resolve the magnetic skyrmions escape on the track border and congestion at the end of the racetrack and other key techniques.Those have reference significance for the application of racetrack memory.