| Magnetic skyrmions are topologically stable structures,typically 10-100 nm in size,which are highly stable and can be driven by lower current densities than those driving magnetic domain walls.These properties make skyrmions reliable carrier for high-density information storage.Since skyrmion was proposed and has shown great potential for application,there has been a proliferation of re-lated research.Some researchers have predicted and observed a number of other spin textures that can replace conventional skyrmions,so as to compensate for the shortcomings of skyrmion in practical applications and further enhance the applicability of the skyrmion family in spintronic devices.Biskyrmion and syn-thetic antiferromagnetic skyrmion(SAF skyrmion)are two of these spin textures.Biskyrmion is composed of two Bloch skyrmions with opposite helicity,and its stability results from the competition between dipole interaction and perpendicu-lar magnetic anisotropy,but does not depend on Dzyaloshinskii-Moriya interaction(DMI),which reduces the material limitation for the application of the skyrmion family.SAF skyrmion refers to a pair of antiferromagnetically coupled magnetic skyrmions in synthetic antiferromagnets,whose magnetization distribution is com-pletely antiparallel,so the effective topological charge number of SAF skyrmion is zero.The feature enables the SAF skyrmion to move in a straight line along the direction of current propagation without deflection.Therefore,when SAF skyrmion is used as a carrier to transmit information,the information is not eas-ily lost and the transmission efficiency is high.This paper mainly studies the dynamics of biskyrmion and SAF skyrmion by means of micromagnetic simu-lation,including the spin excitation modes of biskyrmion,the oscillation charac-teristics of biskyrmion and construction of a multistate memory device based on SAF skyrmion helicity.Firstly,the spin excitation modes of biskyrmion in a nanodot were investi-gated,an oscillation mode at the lower frequency and mixed modes of breathing mode and rotation mode at higher frequencies are observed.In addition,the ef-fects of applying static magnetic fields along out-of-plane and in-plane directions on spin excitation spectra for biskyrmion were also investigated respectively.It is found that the frequencies for the mixed modes are strongly dependent on the changes of biskyrmion magnetization distribution caused by the out-of-plane static magnetic field,but these excitation frequencies are insensitive to the changes of biskyrmion orientation induced by the in-plane static magnetic field.Our result indicates novel excitation modes which enrich the theory of eigenmodes for spin textures and offer a direct mean of detecting and characterizing biskyrmion in ex-periment.Subsequently,in order to investigate the potential of biskyrmion in practical applications,we investigated the biskyrmion oscillation characteristics in a circu-lar spin valve structure.When the current passes through the pinning layer po-larized along the z-axis and injects vertically into the free layer containing the biskyrmion,the biskyrmion makes a stable circular motion in the center of the disk driven by the spin transfer torque.This motion makes the magnetoresistance oscillate periodically,and then outputs a stable AC signal.The current density,the applied magnetic field and the size of disk can regulate the oscillation frequency of biskyrmion.In addition,when the magnetization distribution in the pinned layer is vortex structure,the biskyrmion driven by the uneven spin transfer torque can precession from the center of the disk and make a rotational motion at the edge of the disk.Moreover,the helicity of the vortex in the pinned layer will affect the precession trajectory and oscillation frequency of the biskyrmion.To solve the obstacle of skyrmion Hall effect on the development of racetrack memory,we also investigated the dynamics of bilayer skyrmion in synthetic anti-ferromagnets.Firstly,the formation,in-plane-current-driven motion and the case of passing through impurities for SAF skyrmions in the absence of DMI were in-vestigated.We also studied the spacing variation between continuously arranged skyrmions and the reading of skyrmions with different helicity.It is found that SAF skyrmions can move in a straight line along the direction of current propagation,and continuously arranged several skyrmions with different helicity can maintain a certain distance between each other and each skyrmions would not annihilate.In addition,we proposed that skyrmions with different helicity could be distin-guished by the difference of magnetoresistance signals.Based on these results,we proposed to construct the skyrmion helicity-based racetrack memory devices in the SAF system,which have the advantages of high density and low energy consump-tion. |
