Micromagnetic Research Of Skyrmion-Based Spin-Torque Nano-Oscillator

Magnetic skyrmion is a particle-like spin structure,which has the advantages of stable structure,small unpinning current density and multi-field manipulation.It has attracted intense attention because of its wide application prospects in many spintronic devices such as racetrack memory devices,spin-torque nano-oscillators,transistor-like devices,spin logic devices.The micromagnetic research of the skyrmion-based spin-torque nano-oscillator frequency performance and the micromagnetic research of a skyrmion-based spin-torque nano-oscillator with a nanoring-like boundary region acting as barrier,are introduced.Before discussing the works above,the related research backgrounds are reviewed,such as magnetism and magnetic materials,spintronics,magnetic skyrmion and skyrmion-based device applications.Previous studies have shown that the skyrmion in the nanodisk of magnetic multilayer films can be excited by a vortex-like spin-polarized vertically injected current into a steady state of circular motion along the nanodisk edge.A system with such the skyrmion oscillation state can outputs a frequency-adjustable voltage signal through the principle of giant magnetoresistance effect,thus such a system can be used as the application of a spin-torque nano-oscillator.However,when the injected current density increases to a critical value,the skyrmion in the nanodisk may be expelled from the nanodisk and finally annihilate near the nanodisk edge,therefore,frequency performance of this device can no longer be further boosted by increasing the current density.In this thesis,a nanodisk with an enhanced perpendicular magnetocrystalline anisotropy edge region,is introduced.In the proposed structure,when the driving current density is larger,the skyrmion can still be confined in the nanodisks.Thereby,the proposed structure can improve the frequency performance of this skyrmion-based spin-torque nano-oscillator to meet the level of practical applications.The micromagnetic simulations results show that the upper threshold of the skyrmion oscillation frequency can be increased by 75% compared to the original structure without enhanced edge.