Controlling the Electromagnetic Properties of Magnetic Composites and Metamaterials

Abstract

Ferrites are a class of magnetic oxides with superior electromagnetic (EM) properties at microwave frequencies when compared to conventional metallic magnetic materials for use in antenna miniturization and radar absorbers. Metamaterials are also a special group of materials, which are known to provide EM responses not found in nature due to subwavelength structuring. In this thesis, a range of ferrite composite materials and metamaterial structures are exploited to develop new methods for controlling permittivity and permeability up to 4 GHz with a view to producing high refractive index materials and to demonstrate broadband impedance matching to free space. The rst section of the thesis uses composites of powdered MnZn ferrite (as the ller) and PTFE (as the matrix), fabricated by a novel cold pressing technique, to produce composites for a range of volume fractions of MnZn ferrite (between 0-80% vol.). The EM properties for all composites were determined as a function of % vol. and the results were found to be in agreement with the Lichtenecker mixing formula. This study is the rst convincing con rmation of the Lichtenecker mixing formula over a broad range of volume fractions (0-80% vol.). The cold pressing method was found to produce composites with reproducible EM properties, and was extended to use aluminium, barium titanate (as llers) and also cellulose as an alternative matrix. Importantly, with regard to the study of cellulose composites, our work is the rst to explore volume fractions of up to 85% and, the rst to con rm the Lichtenecker mixing formula with these materials. The ferrite particle size, as well as the volume fraction of ferrite, impacts the EM response of the composites. Both the permittivity and permeability increase as a function of ferrite particle size; however, the permeability increases at a much faster rate than the permittivity with particle size. It is shown that by controlling the ferrite particle size in conjunction with the volume fraction of ferrite, broadband impedance matching to free space can be realised for tailored values of refractive index. This is the rst study that demonstrates independent control of the permittivity and permeability of ferrite composites by controlling the ferrite particle size. Alternatively, by adding a third component to the two part composite it is demonstrated that broadband impedance matching to free space can also be realised with a refractive index of 16.1 (between 10-50 MHz). This is the rst time, to the authors knowledge, that three part composites have been used to achieve high refractive index materials that are impedance matched to free space. The second section of this thesis takes the concept of metamaterials to structure ferrite composite material with a further view to gain independent control over the permittivity and permeability. By tailoring the EM response of this metamaterial, which is comprised of anisotropic arrays of ferrite cubes, broadband impedance matching to free space is demonstrated. The refractive index over the impedance-matched frequency range is also very high (9.5). The metamaterial also acts as an excellent non-re ecting subwavelength thickness absorber up to 200 MHz. An analytical description of the permittivity and permeability dependence on the metamaterial parameters is developed to predict the EM response of this metamaterial, and of similar systems. In the last part of this thesis, the concept of cubic metamaterials is extended to more complex metallic meta-atoms, where the permittivity and diamagnetic response of the metamaterial are independently tailored to demonstrate how the refractive index can be tuned over a broad frequency range. By understanding the role of individual cube parameters, the diamagnetic response can be controlled between near zero and unity, which greatly alters the refractive index. The results are the rst experimental validation for showing `design' control of the permittivity and permeability of these metamaterials via geometry tuning of the meta-atom design.