Magnetically tuning microwave propagation parameters in ferrofluids

The paper reports on the frequency (f) and static magnetic field (H) dependencies of the microwave propagation parameters, in the ranges 0.1-6 GHz and 0-90.7 kA/m, of a kerosene-based ferrofluid with magnetite particles, filtered in magnetic field gradient. In the investigated range, the sample exhibits ferromagnetic resonance phenomenon and Maxwell-Wagner dielectric relaxation. Unlike the usual way of studying the propagation of microwaves through different media, in this paper we have defined an overall reflection coefficient, Rw(f, H), of a material with thickness, w, deposited on a total reflective support, which takes into account both the attenuation of wave within the material and the reflection at the air-material interface. Based on the measured relative magnetic permeability, [Formula: see text], and relative dielectric permittivity, [Formula: see text], a comprehensive and meaningful set of microwave propagation parameters are determined. Apart from Rw(f, H), this set of parameters of ferrofluid includes the attenuation constant of the electromagnetic wave, [Formula: see text](f, H), the phase constant [Formula: see text](f, H), the real, n'(f, H), and imaginary, n"(f, H), components of the refractive index, the reflection coefficient at the interface air-material, R(f, H), and the quarter wavelength in material, [Formula: see text](f, H). Based on the theoretical considerations and characteristics of ferrofluid, simplified and practical formulas of the propagation parameters are given and also possible applications of the results are suggested (such as electromagnetic absorber, phase shifter, microwave lenses and vibration sensor). This connection between theory and experimental results offers an example for the preliminary design of microwave applications of ferrofluids and, by extension, for any material consisting of magnetic nanoparticles dispersed in a dielectric matrix.