Current-induced Magnetic Skyrmion Dynamics For Memory And Oscillator

Magnetic skyrmion is a spin structure with the property of topological protection.In recent years,magnetic skyrmion has been demonstrated to have great potential in the applications to spintronic devices such as Racetrack memories and Spin-transfer nano-oscillators(STNOs)due to its own advantages.However,there are still many problems that need to be addressed before the commercial application.In this work,we investigated the dynamics of magnetic skyrmion by means of micromagnetic simulations,and proposed a series of methods to optimize the application in Racetrack memories and STNOs.In addition,we also compared the static properties and dynamic processes of magnetic domain walls and magnetic vortices.For the research on Racetrack memories,because the in-plane 360 degree domain wall has better ability to resist magnetic interfering field,we studied the influence of Dzyaloshinskii-Moriya interaction(DMI)on the current-induced 360 degree domain motion in the third chapter.It is found that the maximum critical velocity of 360 degree domain wall is increased by 19.87%,and the domain wall becomes easier to pass notches.Compared with magnetic domain walls,magnetic skyrmions as storage units in the Racetrack memory have the advantages of small size,high stability,and low drive current density.Therefore,we study current-induced motion of four types of skyrmions in the fourth chapter.In addition to the Twisted skyrmion,which can avoid the skyrmion Hall effect under certain conditions,the other three types of skyrmions deviate from the direction of the conduction current when driven by the spin transfer torque effect or the spin Hall effect,which limits the application of skyrmions in the Racetrack memory.Therefore,we have proposed two methods.One method is to increase the perpendicular magnetic anisotropy of the nanotripe boundary as a potential barrier,and found that the maximum velocity of skyrmion increases more than 2.7 times.The other method is to replace ferromagnetic skyrmions with antiferromagnetic skyrmions,the antiferromagnetic skyrmions can move without deflection when driven by the spin Hall effect due to the zero topological number.Moreover,antiferromagnetic skyrmions are faster than ferromagnetic skyrmions at the same current density.For the research on STNOs,earlier researches showed that magnetic skyrmions can be rotated in a nanodisk by the perpendicular spin-polarized currents,which provides a guarantee of skyrmion-based STNOs.In the fifth chapter,we first studied the influence of a perpendicular magnetic field on the skyrmion-based STNO,and found that the frequency of the STNO increases from MHz to GHz with the presence of the perpendicular magnetic field.The frequency tunability of the STNO can be further improved to more than ten GHz when an annular groove is added to the free layer.The increases in the frequency tunability in the above two cases are both due to the increased skyrmion potential.However,the low output power of a single STNO is one of the key restrictive factors for its commercial application.Therefore,we designed a dumbbell-shaped STNO,whose work principle is based on the repulsion between the domain wall and the skyrmions.On that basis,the STNOs can be integrated into arrays,which are expected to greatly increase the total output power.Compared with the spin-polarized currents,microwave or static magnetic field can avoid the generation of Joule heating when manipulating the magnetization distribution of magnetic materials.Therefore,in the sixth chapter,we first investigated the topological motion of the skyrmions in a nanodisk driven by an in-plane microwave magnetic field.It is found that the frequency,phase,and amplitude of the microwave magnetic field have an effect on the rotation modes,the trajectory size,and the off-centered process of the skyrmion,respectively.Following,we studied the formation and the polarity reversal of skyrmion lattice under a static magnetic field.Finally,we studied the static properties and ferromagnetic resonance of double magnetic vortices in an isosceles triangle structure,and found that the resonance frequency of the two vortex cores is shifted by an in-plane magnetic field.