Study On Uniaxially Oriented Hexagonal Ferrite Materials And Its Applications

With the rapid development of mobile communication systems,the trend towards miniaturization,high frequency,broadband,and integration of microwave systems is inevitable.Circulators are important components in communication systems,used to achieve non-reciprocal transmission of signals.Their characteristics are based on the asymmetry of the off-diagonal components of the tensor magnetic permeability of ferrite materials.To achieve miniaturization and broad bandwidth of circulators,the requirements for ferrite materials include low loss,higher saturation magnetization,and the use of anisotropic magnetic fields to replace external bias magnetic fields,thereby reducing device volume.The high magnetic crystalline anisotropy field of hexagonal ferrites allows their magnetization axis to lie out of plane,thus replacing part or all of the external magnetic field in circulator implementation,achieving devices with either no external field or low external field,thereby realizing miniaturization and planarization of devices.Currently,hexagonal ferrites face the following issues in practical applications:1.The magnetocrystalline anisotropy of undoped M-type hexagonal barium ferrite is excessively high.It is necessary to make the anisotropy field adjustable To adapt different operating frequencies;2.The ferromagnetic resonance linewidth of oriented polycrystalline hexagonal ferrite is too high,leading to significant insertion loss in the device.Therefore,it is necessary to regulate the magnetocrystalline anisotropy of the material through ion substitution.Additionally,process optimization is required to improve the microstructure of the material,aiming to reduce porosity and increase orientation while maintaining a high remanence ratio,thus optimizing the ferromagnetic resonance linewidth.In addressing the above issues,this dissertation conducts doping modification studies on M-type and U-type barium ferrites,aiming to control the magnetic crystalline anisotropy field of the materials,improve remanence ratio,and reduce the iron magnetic resonance linewidth.The dissertation also aims to improve material orientation,enhance the material’s microstructure to reduce porosity,increase remanence ratio,and decrease the iron magnetic resonance linewidth through process optimization for different ion substitution formulas.Finally,it analyzes the requirements of high-frequency,broadband circulators on materials,studies the relationship between operating frequency,bandwidth,saturation magnetization,and magnetic crystalline anisotropy field of materials,and designs and simulates a self-biased circulator operating at 29.5 GHz with a bandwidth of5.2 GHz.The main research contents and achievements of the dissertation are as follows:Doping modification experiments were conducted on M-type barium ferrite.Firstly,a study was conducted on the complete substitution of Ba²⁺with Sr²⁺in the Sr_xBa_1-xFe_11.5O₁₉ system.The substitution of Sr²⁺slightly increased the saturation magnetization and,to some extent,reduced coercivity.Then,Cobalt ions Co³⁺were then used to replace Fe³⁺in Sr_0.5Ba_0.5Fe_11.5O₁₉.A series of Sr_0.5Ba_0.5Fe_11.5-xCo_xO₁₉ samples have been prepared,characterized,and analyzed.When the Co³⁺content reached x=1,X-ray diffraction showed the appearance of impurity peaks,The doping of Co³⁺had a significant impact on the magnetic properties of M-type ferrites,with saturation magnetization initially increasing slightly and then decreasing significantly.Coercivity exhibited a significant decrease with increasing doping,macroscopic magnetic anisotropy decreased significantly,and the remanence ratio reached its minimum at x=0.5.Co³⁺effectively reduced the magnetic crystalline anisotropy field from 17 k Oe to 12.5 k Oe.Finally,In the Ba Fe_12-xCo_xO₁₉ system,smaller initial particle sizes were used for secondary ball milling during sintering,and no impurity phases were found after sintering.In the x=0 to 0.9range,all final sintered samples exhibited good remanence ratios.Co³⁺effectively regulated the magnetic crystalline anisotropy field,achieving control within the range of17 k Oe to 9 k Oe.First-principles calculations were performed on the Ba Fe_12-xCo_xO₁₉system.The total binding energy of different crystal site structures was used to calculate the doping energy of Co³⁺at various crystal sites.The calculation results indicate that Co³⁺preferentially occupies the 2a and 12k sites,which have majority spin.The electron configuration of Co³⁺is 3d⁶,and in a strong coordination field,where the orbital splitting energy is greater than the electron pairing energy,it exhibits a low-spin state.The magnetic losses of Ba Fe_12-xCo_xO₁₉ samples were characterized by ferromagnetic resonance linewidth measurements.The results showed that the introduction of Co³⁺lowered the frequency of ferromagnetic resonance,consistent with the decrease in the magnetic crystalline anisotropy field due to Co³⁺doping.All samples exhibited large ferromagnetic resonance linewidths,exceeding 1398 Oe,primarily due to insufficient material density,high porosity,and insufficient orientation.Apart from controlling the magnetic crystalline anisotropy field by doping Co³⁺into M-type barium ferrite,another option is to choose U-type ferrite with lower magnetic crystalline anisotropy field as the research target.U-type ferrite(Ba₄Zn₂Fe₃₆O₆₀),as an axial anisotropic ferrite,has a lower magnetic crystalline anisotropy field than M-type ferrite,and its crystal structure is more complex.This complexity provides more lattice sites for doping modifications,offering great potential for achieving high-performance self-biased ferrimagnetic ferrites.However,there is limited literature on U-type ferrites,with only a few laboratories successfully preparing U-type ferrite films for characterization analysis.Additionally,achieving pure-phase U-type ferrites using traditional oxide methods to prepare ceramic blocks is challenging.The magnetic anisotropy is even smaller for U-type ferrite.To achieve higher remanence,further improvement of the microstructure of the samples is needed.In this study,a series of U-type ferrites were prepared,and the crystal structure was characterized by XRD refinement.The microscopic morphology of non-oriented U-type ferrites was analyzed,and the magnetic properties of the materials were tested.Traditional transmission resonator methods were attempted to measure the ferromagnetic resonance linewidths in the X-band.The results showed that Co²⁺doping in Zn₂U could lower the magnetic crystalline anisotropy of the material and reduce the ferromagnetic resonance linewidth.In the Ba₄Zn_2-xCo_xFe₃₆O₆₀（x=0.4,0.6,0.8,1.2）system,when the doping level of Co²⁺is x=1.2,the ferromagnetic resonance linewidth reaches a minimum of 658 Oe.The dissertation finally studies the design of a self-biased circulator,presenting and comparing the deviations between the approximate solutions of the electromagnetic field truncated at different Fourier expansion orders and the actual results.The dissertation verifies the feasibility of a wideband design scheme for self-biased millimeter-wave circulators.Based on the solution results,the numerical relationship between the device performance parameters and the magnetic properties of ferrite materials is provided.The simulation results of the wideband millimeter-wave self-biased circulator indicate that when the magnetization intensity is selected as 4000 Gs and the magnetocrystalline anisotropy field is 9 k Oe,a self-biased circulator design can achieve a bandwidth of 5.2GHz,an insertion loss of less than 0.8 d B,and an isolation of greater than 20 d B at a center frequency of 29.5 GHz.