Micromagnetic Modeling And Simulation Of Static And Dynamic Properties Of Magnetic Memory Devices

Data storage device is one of the main applications of ferromagnetic materials.There are two main types of memory devices based on ferromagnetic materials,one is the well-known hard disk drive(HDD)and the other is the magnetic random-access memory(MRAM)which is under intensive investigations in recent years.The characteristics of these two types of data storage device can be well explained by Landau-LifshitzGilbert-Slonczewski(LLGS)equation and micromagnetics.As a result,related numerical simulation plays a very important role in understanding the device behavior,trade-off between key performance parameters,as well as the design and verification of the advanced schemes.Based on the above research background,this thesis investigated the characteristics data storage devices including HDD and MRAM data by micromagnetic modeling and finite element electromagnetic field analysis.The main research contents and the conclusions are as follows: Based on the above research background,the main research contents and results of this thesis are as follows:(1)According to the micromagnetic theory and numerical method of differential equations,Micromag,a micromagnetic simulation software based on the finite difference method was developed.In this part,firstly,the torques,free energies and corresponding effective fields involved in LLGS equation were introduced,and then the discrete expressions suitable for the finite difference method were given.Secondly,the group of differential equations solved by Micromag was demonstrated,along with the comparison of suitable numerical methods.Finally,the calculation results of the program were verified and analyzed based on the micromagnetic standard problems #2 and #4.Micromagnetic model is one of the main theoretical tools for the following research work.(2)A method combining micromagnetic simulation,finite element electromagnetic field analysis and response surface methodology was proposed for comprehensive analysis and optimization of magnetic writing performance.This method intuitively reflects the comprehensive influence of multiple design parameters of the magnetic recording system on the response target through a second-order polynomial model.Based on the method,two rounds of Center Composite Design(CCD)were used to analyze the influence of the trailing gap width,side gap width,shielding slot depth,and the anisotropy energy density of the recording media on effective writing field gradient in the shingled magnetic recording system.It is found that the effective writing field gradient is mainly affected by the trailing gap width and media anisotropy energy density,secondarily affected by side gap width,and is not affected by the shielding slot depth.In addition,the influences from the trailing gap width and the media anisotropy energy density is strongly correlated,which illustrates the necessity of comprehensive analysis of the writing performance of the magnetic recording system.(3)The geometrical dependence of the key performance indicators of in-plane SOT-MRAM,including the threshold current density,thermal stability,and spin transfer efficiency,were investigated based on the analytical analysis of LLGS differential equations,and verified by the numerical results from both macrospin model and micromagnetic model.Firstly,the expression of the threshold current density that determines the dynamics of free layer magnetization was obtained based on the analytical analysis of LLGS equation.Then the analytical model of the threshold current density with respect to the cell geometry was established by introducing the demagnetization factor of the elliptical thin film magnet,and the correctness was verified by comparison with the numerical results from macrospin model and micromagnetic model.On this basis,the geometric dependence of the key performance indicators of in-plane SOTMRAM cell with the Co Fe B/Mg O/Co FeB/W structure was studied from a theoretical perspective.It is found that the length of the long axis of the free layer nonlinearly determines the instable threshold current density and the switching threshold current density,where as it linearly determines the cell thermal stability.In addition,the optimal ellipticity of the in-plane SOT-MRAM cell is about 3,which is consistent with the previous empirical results.(4)A perpendicular SOT-MRAM based on interlayer antiferromagnetic coupling was proposed.The functional part of memory cell is composed of perpendicular MTJ,heavy metal layer(HM)and ferromagnetic assisting layer(FAL).The free layer of the MTJ is adjacent to the HM and coupled antiferromagnetically to the FAL through interlayer exchange.The magnetization switching of the free layer is driven by inertia and exhibits unique and rich switching characteristics under excitation of current pulses with different widths.More importantly,through fully utilizing the spin current generated by spin Hall effect and the interlayer antiferromagnetic coupling,this scheme realizes bias field free switching within tens of picoseconds,and the cell thermal stability is also improved.In particular,the threshold switching current,switching time,and thermal stability of the memory cell were analyzed based on both coupled macrospins and micromagnetic model,and the requirements for the cell structure and material properties were further investigated.