Micromagnetic Simulations Of Spin Torque Nano-Oscillators And High Frequency Microwave Dynamics

Magnetic components play an important role in the application of microwave technology and have been used in passive equipment such as phase shifter and circulator.Spin torque nano oscillator（STNO）based on spin transfer torque（STT）or spin orbit torque（SOT）effect can convert DC current into high-frequency microwave signal,which provides technical possibility for the development of nano magnetic active microwave components.In the STNO with[Co/Ni]multilayers with perpendicular anisotropy（PMA）or ferromagnetic films such as Co Fe B as the excitation layer,if the nano contact current injection design is adopted,a spin periodic oscillation called droplet type can be excited,and the frequency is in the GHz range,which provides an idea for the development of new-type STNO.In order to improve the precession frequency,synthetic antiferromagnetic structure（SAF）or antiferromagnetic material（AFM）can be used to realize sub-terahertz and terahertz high-frequency oscillation.Antiferromagnetic materials have the unique advantages of zero stay field,anti magnetic field disturbance robustness and ultrafast dynamics.They are expected to be widely used in the next generation of spintronic devices such as STNO.Therefore,it is very important to deeply study and reveal the dynamic characteristics of these materials and devices.In this paper,micromagnetic technology is mainly used to carry out a series of research on the dynamic characteristics of magnetic droplet nano oscillator,terahertz signal of SAF structure,ferromagnetic resonance characteristics of antiferromagnetic thin films and other related problems.The main research results are as follows:Firstly,the dynamic behavior of magnetic droplet type STNO under the combined action of electric field and current is studied.By applying an electric field to tune the perpendicular magnetic anisotropy of the free layer,the spectrum of magnetic moment precession is obtained.According to the different magnetic moment oscillation modes,it can be divided into three frequency regions:localized magnetic droplet state,distorted magnetic droplet state and propagating spin wave state.The results show that the precession frequency of the magnetic droplet is less than the ferromagnetic resonance frequency of the system,and decreases with the decrease of the magnetic anisotropy field.When the precession frequency is close to the ferromagnetic resonance frequency,the magnetic droplet state changes to a distorted magnetic droplet state.With the further increase of the applied electric field,the system evolves into a propagating spin wave mode,and a series of frequency jump branches appear on the spectrum.The frequency jump comes from the round-trip motion caused by the reflection of the spin wave from the sample boundary under the condition of limited system size,and then the spin wave superposition produces the standing wave phenomenon.Through theoretical analysis,we obtain the node number of spin wave state.Finally,the dependence of oscillation mode on current,external magnetic field and sample size is established.This electric field current coordinated regulation mode provides a new idea for the design and development of spin nano oscillators from localized oscillation state to propagating spin wave state.Secondly,the influence of Oersted field on localized magnetic droplets and propagatable spin wave modes is studied.The distribution of Oersted field caused by perpendicular current is studied.It is found that due to the action of Oersted field,a new magnetization distribution state is produced in the system at low current,which is called spiral mode.When the current increases beyond the threshold,the system will still produce magnetic droplet state;Then,both the droplet state and the spiral state can be transformed into a propagating spin wave state by increasing the electric field.In addition,the effects of current and external magnetic field on spin wave state are analyzed.It is found that current can control the symmetry of spin wave state,and the frequency is linear with the magnetic field.We also explore the influence of magnetic damping.These results will be helpful to establish the parameter details from the localized state of the magnetic droplet to the propagating spin wave state.Thirdly,the spin valve dynamics of a perpendicular magnetized synthetic antiferromagnetic（SAF）structure as a free layer is studied.The magnetization precession and spin wave excitation driven by the current STT without assistance of magnetic field.We found that two different modes of magnetic moment stable precession can be excited at an appropriate current density,and fast Fourier analysis found that their frequencies are in the sub-terahertz range.Further study shows that there is an inflection point between the excited microwave frequency and current density,which happens to be the transition state of the two modes.Furthermore,the dependence of the excitation frequency on the antiferromagnetic coupling strength between the two ferromagnetic layers and the film thickness is established,and the phenomenological expression of the frequency is given.At the same time,we also studied the magnetization reversal without external field driven by SOT effect in the synthetic sandwich structure;It is found that the field free magnetization reversal can be realized by means of interlayer DMI between the two ferromagnetic layers,whether ferromagnetic coupling or antiferromagnetic coupling.These results provide a new idea for exploring terahertz nano oscillator and MRAM.Fourthly,for the antiferromagnetic material crystal thin film material（taking Mn F₂as an example）,the antiferromagnetic resonance phenomenon is studied by atomic scale micromagnetic method.The simulation results have a highly agreement with the experiments and related theories.In the low field,there are two frequency response branches,one of which is a right-handed mode that increases linearly with the external magnetic field,and the other is a left-handed mode that decreases linearly with the external magnetic field.When the external magnetic field is larger than the spin flop critical field determined by perpendicular magnetic anisotropy and exchange coupling,it degenerates into a right-handed response mode.We compare the antiferromagnetic resonance phenomenon with the coupled pendulum model in classical mechanics,and analytically derive a perfectly descriptive quasi coupled pendulum theoretical model from the LLG equation,which reveals the simulation results well.In conclusion,the use of STT or SOT effect to explore the effective method and device design of exciting high-frequency microwave signals has opened up a new way for the development of various new-type spin nano oscillators with high-frequency oscillations.