The dissertation is dedicated to the development of the synthesis technique of nanosized ferromagnetic materials with the structure of garnet and spinel by the method of coprecipitation from aqueous solutions and to the development of magnetically tuned composite resonant elements consisted of a nonmagnetic high-quality dielectric resonator and of a magnetic film based on synthesized magnetic particles. This elements can be used as non-reciprocal elements (circulators, phase shifter), in wireless communication systems: GSM, Wi-Fi, 5G, in radar systems.
Ferrite materials were synthesized in the dissertation by the method of coprecipitation from aqueous solutions at constant pH. In the synthesis of ferrite with the structure of the garnet Y3AlFe4O12 four approaches were used, this approaches were related to the order of deposition of the corresponding metals hydroxides. Comparative studies of the properties of synthesized ferrite nanoparticles with the structure of the garnet Y3AlFe4O12 were carried out. Studies of the obtained particles showed that named modifications of the synthesis process strongly affect the filtration coefficient, affecting a time of the process of filtering and washing of particles from unwanted products of reaction (Na+ ions). With the simultaneous deposition of Fe and Al hydroxides the filtration coefficient increased threefold compared with the case of simultaneous precipitation of all metals. Particle washing was performed to eliminate the sodium ions in the product as they increase the loss at high frequencies.
Ferrite ceramics were obtained on the basis of the synthesized particles. Due to the properties of the synthesized ferrite particles it was possible to sinter ceramics at a temperature of 1350-1400oC, that is 100-150oC lower than the temperatures that are usually used in the sintering of ferrite ceramics with garnet structure. It allowed to reduce the requirements to the necessary equipment. All synthesized ceramics samples have a high density, similar electrophysical parameters, but according to the width of the ferromagnetic resonance (FMR) the best samples made from particles obtained by simultaneous deposition of Fe and Al hydroxides (approach 3, 4). The most uniform distribution of metal atoms was for ceramics made from the material synthesized by the 3rd approach. In this case the investigations of magnetic properties indicate that the magnetization and the coercive force for all ceramic samples differ little, despite the notable difference in the magnetization of the particles. This information indicate that the structure of garnet during heat treatment was formed faster when when the particles synthesized by the approach №3 were used.
Ferrites with the spinel structure Ni1-xZnxFe2O4, x = 0, 0.25, 0.5, 0.75, 0.8, 0.85, 0.9, 0.95, 1 were synthesized by precipitation from aqueous solutions. Studies showed that the highest magnetization have ferrite particles of composition Ni0.5Zn0.5Fe2O4. Using synthesized ferrite particles with a spinel structure Ni0.5Zn0.5Fe2O4 magnetically tuned composite resonators were fabricated in the form of a BaTi4O9-ZnO-based resonator with an end face film consisting of nanosized ferrite particles and a photopolymer. The films were polymerized by ultraviolet (UV) light. The thickness of the magnetic films was optimized. Electrophysical properties of the obtained nonlinear elements were first investigated. It is shown that such structures has the ability to change the resonance frequency due to the influence of an external constant magnetic field (ferromagnetic resonance) while maintaining a sufficiently high value of Q-factor. The resonance frequency change reached the value of 71 MHz at a frequency of 11 GHz with a change in the magnetic field strength in the range H = 0-3500 E. Q-factor of the resonant element with a magnetic film decreased compared to a resonator without a magnetic film from 6100 to 1700 at the frequency of 12.77 GHz.
Studies showed that the obtained composite resonators are non-reciprocal elements. The measuring cell and composite resonator were modeled using Ansys HFSS. It is determined that with its optimal placement in the measuring cell (at the point of the maximum of the magnetic field) it is possible to achieve direct losses of 0.2-0.8 dB, while the non-reciprocity of the energy transmission spectrum at FMR frequency (from 34 dB) remains large. It is shown that a non-magnetic dielectric resonator concentrates energy in the volume of a magnetic film, causing a small volume of ferrite film to affect significantly stronger the properties of the composite resonance element. Therefore, we can talk about the synergy of a work of the composite resonant element components.
Compared to other non-reciprocal elements, the resulting resonators have such advantages as simplicity of manufacture, extensive miniaturization capabilities and compatibility with the planar technology.
