Calculations of Microwave Permeability, Permittivity and Absorption Properties of Magnetic Particle Composites

Abstract

This thesis presents a general approach in the design of magnetic absorber through the calculations of magnetic permeability and electric permittivity. The emphasis of this thesis is to calculate and design the intrinsic and extrinsic materials property of particle composite materials. The intrinsic materials property of particle composite materials refers to its effective permeability or permittivity, while the extrinsic materials property of composite materials refers its power reflection coefficient.

The first step of the design of magnetic absorber is to select good magnetic filler(s) for the magnetic absorber. The intrinsic permeability of several magnetic metallic materials is investigated using a model developed from the Landau-Lifshitz-Gilbert (LL-G) equation. In this study, we are able to answer why Fe is used to make the magnetic RAM from L to X band (1-12 GHz). The effect of saturation magnetisation, anisotropy field, damping coefficient, particle size and conductivity are studied. The importance of the spatial orientation of the magnetic domain on its intrinsic permeability is demonstrated.

The second step is to compute the effective permeability and permittivity of the composite containing the magnetic fillers. This is to check if our results obtained in first step matches well with microwave measurements, and therefore to validate the models used.

The calculation of materials property of particle composite in the second step is meant for design purpose as some of the composite processing parameters could be unidentified in practice. Thus, there will be some discrepancies in the calculated and measured permeability and permittivity. Consequently, the measured permeability and permittivity are used to design the microwave absorber. The third step is to develop a model to extrapolate or interpolate the measured permeability or permittivity for other volume fractions, and this makes establishing a huge database of permeability and permittivity for different volume fraction unnecessary.

The last step is to design the microwave absorber using the Dallenbach or the Salisbury Screens.

At the end of the thesis, a new approach to calculate the effective permeability of carbonyl iron and magnetite composite and in general, magnetic material composite has been developed. The approach uses Landau-Lifshitz ferromagnetic resonance (LL FMR) equation to derive an analytical formula. With the understanding of LL FMR equation, an algorithm to extract the effective magnetic parameters of magnetic thin films has been formulated and validated.