Dynamics Of Magnetic Skyrmion And Its Applications In Spintronics

Magnetic skyrmion is a basic magnetic structure with topological property.The study of magnetic skyrmions is focused on its fundamental phenomenon,such as skyrm-ion Hall effect and intrinsic spin wave excitations.Moreover,lots of applications in spintronics have also been proposed,including skyrmion-based racetrack memory,spin-transfer nano-oscillators,skyrmion-based Brownian computing devices and so on,where an efficient driven method could be necessary in skyrmion motion.In particular,the study of novel skyrmion-based spintronic applications is also a necessary requirement.In this thesis,by combining micromagnetic simulations and experiments,we investi-gated the spin excitation of skyrmion-like structures with different skyrmion number,skyrmion dynamics under electric fields and thermal noises,and the application and optimization of skyrmion in spintronics.The conclusions are drawn as follows:The spin excitations of typical k? skyrmion(k = 1)include two in-plane ex-citation modes which are clockwise and anti-clockwise rotation,and an out-of-plane breathing-type mode.In the third chapter,for k? skyrmion(k = 1,2,3)with differ-ent topological properties,we find that the number of spin excitations in both in-plane direction and out-of-plane direction increases with increasing k.The clockwise mode and anti-clockwise mode appear alternately with increasing frequency for a skyrmion with higher k.Moreover,we show that a perpendicular magnetic field can induce a transformation between k? skyrmions as well as changing of corresponding excitation modes and frequencies.Skyrmion Hall effect limits its application in spintronic devices,in particular the racetrack memory.Meanwhile,the Joule heating is inevitable when using electric cur-rent as a driving force.In the fourth chapter,we investigate the dynamics of anti-skyrmion,skyrmionium(2? skyrmion)and antiferromagnetic skyrmion under electric fields.Through voltage controlled magnetic anisotropy(VCMA)effect,we find the an-tiskyrmion propagates under the perpendicular microwave electric field and an in-plane magnetic field,its velocity depends on the amplitude and frequency of microwave elec-tric field,and the damping constant as well as the field strength.The antiskyrmion can move along field direction and the skyrmion Hall effect disappears as well when the field is applied in a particular direction.In addition,skyrmionium and antiferromag-netic skyrmion with zero topological number show zero skyrmion Hall angle,which could be promising information carrier candidates.We show skyrmionium and anti-ferromagnetic skyrmion can move linearly along the direction of the anisotropy gra-dient induced by electric field,and corresponding velocities increase with increasing anisotropy gradient.We also propose several skyrmion applications in spintronic devices as well as its optimization,including skyrmion-based multi-channel racetrack memory,the pinning and rotation of a skyrmion in the presence of nanoengineered point and ring defects,and radio frequency mixer based on magnetic skyrmion.In the fifth chapter,we depict that skyrmions propagation in racetrack could be in different channel depending on electric current density,magnetic anisotropy under voltage gate and the ON/OFF states of voltage gate.Moreover,the skyrmion Hall effect vanishes and its velocity increases by introducing voltage gate.We also show the pinning and rotation of skyrmion in the spin transfer nano-oscillator depends on the type and geometry of defects,where the oscillation frequency can be tuned in a large range.Based on skyrmion oscillator,the skyrmion based radio frequency mixer,which integrates both input signals and output signal functionality with nanometer size,shows frequency-modifying process similar to the conventional radio mixer.In the application of spintronic devices,the thermal noise can be a severe problem that can even be a dominant effect determining the skyrmion nucleation,stability and addressability.In contrast,non-deterministic thermal dynamics of skyrmion has been exploited recently for alternative types of devices that aim at skyrmions-based proba-bilistic computing.In the sixth chapter,we show the commensurability between ele-ment symmetry and the number of skyrmions governing skyrmion diffusion in confined geometries.The arrangements and diffusion depend on the number skyrmion and the symmetry of geometry structure.In geometries with lower symmetry,the mean squared displacements of skyrmion reach a plateau within the observed time.The phenomenon of commensurability is corroborated by simulations of a quasi-particle model.