Micromagnetics Simulations On Microwave Circulator Based On M-type Barium Ferrite

M-type barium ferrite?Ba-M?is an excellent ferrite permanent magnetic material,which has hexagonal structure and has the advantages of high saturation magnetization and strong magnetocrystalline anisotropy,etc.Compared with the bulk or films,M-type barium ferrite nanowires arrays show very strong shape anisotropy fields because of their extremely high aspect ratio,which makes their effective magnetic anisotropy,remanence ratios and coercivity values larger.Meanwhile,their magnetic properties can be controlled by adjusting multiple size parameters of nanowires array,which are expected to be widely used,such as self-biased microwave ferrite devices,perpendicular magnetic recording,and microwave absorbing materials.However,as the techniques of experiment and measurements still need to be improved,the research results on Ba-M nanowires array are few,and the research methods are only limited to experimental means and relatively single,which results in slow research progress.Based on the above facts,the research on Ba-M nanowires array has carried out by means of micromagnetics simulations,and the effect of many factors on its magnetic properties have been investigated in this thesis.In the first part,the effects of three polycrystalline factors,grain size,grain orientation and damping factor on the magnetic properties of Ba-M nanowires array have been investigated.The results show that the change of grain size and orientation affects the spontaneous magnetization,hysteresis effect and complex permeability of nanowires array,especially the grain orientation,which plays a decisive role in the magnetic properties of nanowires array.The damping factor determines the type of magnetic spectrum of nanowires array.In the second part,the effects of the length,diameter and distance of nanowires and the magnetization direction on the magnetic properties of Ba-M nanowires array have been studied.The results show that the shape anisotropy of the nanowires array increases with the increase of the length of nanowires.When the aspect ratio is greater than 10,the magnetic properties of the nanowires array tend to be stable.The diameter and distance of the nanowires affect the interaction field between the nanowires,thus affecting the magnetic properties of the nanowires array.When the nanowires array is magnetized in the z direction,it is easier to reach the saturation magnetization state,indicating that the z direction is the easy magnetization axis.Finally,a self-biased circulator has been designed based on the Ba-M nanowires array,and the simulating results.The design has been optimized.The final optimization results show that,at 40.71 GHz-41.98 GHz,the circulator can work well without additional bias magnetic field.The return loss(S₁₁)and isolation(S₃₁)are greater than 20 dB,the working bandwidth is 1270 MHz,and the insertion loss(S₂₁)is 0.86 dB at the center frequency of 41.4 GHz.In addition,the effect of grain orientations on the performance of circulator has also been studied.The results show that,as the orientation angle???increases,the center frequency of circulator deviates from the preset frequency.The overall performance of the circulator becomes worse with increasing the orientation angle???,which is due to the decrease of remanent magnetization of Ba-M nanowires array.