Micromagnetic Simulation Based On Skyrmion Spin Nano-oscillator

Skyrmion is a kind of quasi-particle and topologically protected spin structure.At present,it can be observed in many magnetic materials with broken spatial symmetry.The spin moment nano oscillator(STNO)is a new type of microwave oscillator which can output microwave by using the spin polarization current to induce the magnetic moment precession of the free layer in the magnetic nanostructure and combined with the magnetoresistance effect.It has the advantages of small size;oscillation frequency can be controlled by current and so on.In this paper,we use the micromagnetic simulation method to investigate the dynamics of the two semi-skyrmions excited by the spin transfer torque effect and the spin orbit torque effect,respectively.As well as the influence of STT and SOT effect on the frequency of skyrmion oscillation,the following conclusions are drawn:In the first part,we first studied the kinetic behavior of skyrmion only under the STT effect,obtained the kinetic equation of two-semi-skyrmions in the free layer of circular nanopillar and the velocity equation of skyrmion oscillation,and obtained the oscillation frequency of skyrmion under different conditions.At the same time,through the method of micromagnetic simulation,we change the external field and magnetic material parameters,and obtain the correlation phase diagram which may affect the oscillation frequency.It is found that current density,damping coefficient and exchange interaction constant have the greatest influence on skyrmion frequency.The frequency of two-semi-skyrmions is more than two times higher than that of a single-semi-skyrmion oscillations.In the second part,we consider the dynamic behavior of magnetic moment under STT effect and SOT effect.Based on LLG equation containing STT moment term and SOT moment term,the dynamics equation of skyrmion is analyzed.The influence of SOT and STT effects on skyrmion oscillation frequency is controlled by the deflection direction of spin polarization current.The results show that the oscillation frequency of skyrmion decreases with increasing SOT effect.SOT effect plays a certain role in inhibiting skyrmion oscillation.Although it reduces the excitation current of skyrmion and has a great advantage in storage,SOT effect does not have a great advantage over STT effect in inducing skyrmion oscillation.Secondly,by comparing the presence or absence of in-plane magnetic field in the free layer,the weak in-plane magnetic field also has a certain promoting effect on the spin structure of the fixed skyrmion.When there is an in-plane magnetic field,the tilt Angle of the polarization current is about 10° higher than that of the non-existent magnetic field.In the third part,we investigate that the skyrmion oscillation can be excited in the free layer only under the action of SOT.The results show that the two-semi-skyrmions spin structure generated by the SOT effect is not stable and cannot exist in a large range of current density.However,it effectively reduces the current density required to excite the skyrmion oscillation.Therefore,skyrmion oscillator based on SOT effect can be used where the required oscillator frequency is low.Experimental results show that the current density is negatively correlated with the skyrmion frequency.In summary,skyrmion-based spin torque nano-oscillators have great potential.Microwave oscillators induced by STT have better performance compared to those induced by SOT,providing a good idea for developing the next generation of skyrmion oscillators.