Fabrication And Properties Of Low Dimension Ferrite Used In Microwave Circulator

With the development of electronics and communication technologies,electronic devices are required to be multifunctional and miniaturized,which will result in even more higher requirements on the selection,preparation,and design of materials.As a passive device,microwave circulator can regulate the direction of the electromagnetic wave using the nonreciprocal properties of ferrites.In order to meet the development trend of magnetic devices,circulator should be thinner,more functions integration,higher operation frequency and self-biased.Hexagonal M-type barium ferrite(BaFe₁₂O₁₉,Ba-M)with high uniaxial anisotropy field was deemed to be the preferred materials for realizing the high operation frequency and self-biased circulators.Meanwhile,yttrium iron garnet（YIG）ferrite has lower ferromagnetic resonance linewidth.Thus,it is of great significance to prepare high quality of Ba-M and YIG films for the miniaturization of circulators.In this dissertation,the work will be focused on preparing high saturation magnetization,high magnetocrystalline anisotropy field and high squareness ratio of BaM films and low ferromagnetic resonance（FMR）linewidth of YIG films,which are thought to be the materials for the film circulators.Firstly,Ba-M films were deposited on Al₂O₃,MgO and Si substrates separately.The microstructures and magnetic properties of the films were investigated.The relationships between magnetic properties and deposition parameters have also been discussed.The optimized deposition parameters were obtained.Secondly,high quality YIG films were deposited on Si and GGG substrates.The mechanism of broadened FMR linewidth was discussed.Thirdly,the ferrite nanowires array was synthesized into the porous silicon templates fabricated by ourselves.The growth mechanism and magnetism properties of nanowire were analyzed.Fourthly,ferrite nanodots were deposited on substrates by the PLD technique employing ultrathin AAO templates,their magnetic properties was also discussed.The main research results and discussions can be organized as follow.1.Preparation of BaM films and their properties controlling.（1）Ba-M targets with high density of 98.6% and the grain size of 2.2 mm was prepared by the two-steps sintering route.（2）Using Al₂O₃ as the substrates,the relationships between deposition parameters and microstructures of film were analyzed.The relationships between microstructures and magnetic properties were also discussed.The static magnetic properties of films were found to be greatly improved by Pt buffer layer.Consequently,a Ba-M film with Ms of 357 emu/cm3,Hk of 16.5 kOe and lower than 20 % of squareness ratio was prepared with a Pt buffer layer of 20 nm.（3）Using the MgO substrates,the effects of annealing temperature and Pt buffer layer on the microstructures and magnetic properties of Ba-M films were studied.The diffusion of atoms between substrates and films was revealed.Pt buffer layer plays an important role in decreasing the crystallization temperature and atoms diffusions.Lower temperature and longer annealing time can improve the degree of crystallization and magnetic properties of Ba-M films.Finally,the Ba-M film with Ms of 358 emu/cm3,Hk of 16.5 kOe and squareness ratio（Mr/Ms）of 50.2% has been obtained.（4）Using Si substrates,the effect of buffer layer thickness on magnetic properties of films was studied.Ferrite films with Ms of 317 emu/cm3,Hk of 15 kOe and squareness ratio of 68.6% was deposited with 200 nm Pt as the buffer layer and under 3 hours annealing at 900 oC.（5）The films on different substrate were compared and analyzed by first-order-reversal curves（FORC）,it is found that the multi domains were found in the films prepared on Al₂O₃ and MgO substrates,single domain was found in the films prepared on Si substrate.The interaction field satisfies the relation: Al₂O₃ > MgO > Si.2.Preparation of YIG films and their ferromagnetic resonance linewidth.（1）Fabrication of YIG target with high quality by two steps sintering.（2）Tailing the magnetic properties of YIG films by adjusting buffer layer.The FMR linewidth of YIG films on Si substrate was successfully decreased to 53 Oe by using CeO2/YSZ combined with three low temperature buffer layers（YAG/YAIG/YIG）,and the thickness of films without cracks formed can be also increased to 500 nm.（3）The submicron YIG films were deposited on GGG substrates.The relationships between FMR and microstructures of film were discussed.A conclusion can be made based on the multi peaks in the measured graphs and the thickness dependence that the film was composed of more than one region.It is also found that YIG films on GGG substrate with intrinsic linewidth of 7 Oe and non-intrinsic linewidth of 25 Oe by the angle dependence of FMR measurements.3.Preparation of ferrite nanowires array and FORC studies.（1）The porous silicon templates were synthesized by electroless chemical deposition Cu particles and self-assembly Au dots.Porous silicon templates with 500 nm of diameter and distance between pores were prepared by two-steps etching in HF and H2O2 solution.（2）The Ba-M and CFO nanowire arrays were fabricated in porous silicon templates by sol-gel method.A thin layer of SiO2 was formed on the surface of PS by oxidization which can improve the intake volume of gel.The SEM images show the diameter of ferrite nanowire array was about 200 nm and the length can controlled by the injection volume of gel.The magnetic properties show that the Ba-M nanowires array was isotropic with coercivity of 2560 Oe.The Co-ferrite nanowire array was also isotropic seen from the hysteresis loops,while the FORC map shows that localized coercivity of 1000 Oe has the highest probability,and that localized interactions field can be inferred from the FORC graphs.4.Fabrication of ferrite nanodots array and its magnetic properties.（1）YIG nanodots arrays were deposited on GGG and Si substrate and its magnetic properties were compared.It is observed that the microstructures,the diameters of nanodots with 350 nm were the same as the ultrathin AAO template used.It is also found that the coercivity of nanodots array was larger than that of continuous thin film,which can be ascribed to its higher anisotropic field.The reason of multi peak in FMR spectrum and broadened FMR linewidth can also be ascribed to its non-uniform of nanodot height and pores in the film and incoherent rotation of dynamic magnetization process.（2）Ba-M nanodots array were deposited on Si and Al₂O₃ substrates and its properties were studied.It is observed from XRD pattern that high orientation of nanodots array has been obtained on Al₂O₃ substrate.Hysteresis loops show that the nanodots array has high remanence ratio than that of continuous film.FORC map shows that BaM nanodots array on Al₂O₃ substrate has single domain properties.It is found that the interaction between nano dots was smaller than that of continuous thin film,which can be ascribed to the non-continuous microstructures in the films with nanodots.