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.

The concept of ferrofluid preheating in the treatment of cancer by magnetic hyperthermia of tissues.

The frequency dependence of the complex magnetic susceptibility, χ(ω) = χ'(ω) - i χ″(ω), of a water-based magnetic fluid with magnetite particles, over the frequency range 500 kHz to 2 MHz and at three different temperatures of 30 °C, 40 °C and 50 °C are presented. Based on these experimental measurements, the dependence on frequency, f = (ω/2π) Hz, of the heating rate, ΔT/Δt, of the ferrofluid has been evaluated at these stated temperatures and alternating magnetic field of different amplitudes, Ho = 200 A/m, 400 A/m, 600 A/m, 800 A/m and 1 kA/m. The results show that the preheating of the ferrofluid sample at the desired operating temperature has the advantage of using lower levels of H0 during over a shorter time period, as opposed to the case of an unheated sample. This concept of preheating a sample, has major significance for the treatment of cancer by magnetic hyperthermia in that the patient is subjected to lower values of H0, over the time period of the treatment.

Microwave propagation parameters in magnetic fluids.

Complex dielectric permittivity and complex magnetic permeability measurements of two magnetic fluids (as microwave propagation media), in the approximate range 0.2-5 GHz were performed. The two samples consisted of magnetite nanoparticles, dispersed in kerosene and in water, respectively. Based on the dielectric and magnetic measurements, the frequency (f ) dependence of the attenuation parameter, [Formula: see text], the phase constant, [Formula: see text], the propagation constant, [Formula: see text], the intrinsic impedance, Z(m), the refractive index, n, the reflection coefficient, R, the wavelength, [Formula: see text] and the skin depth, [Formula: see text], of the investigated samples were determined.

Microwave specific loss power of magnetic fluids subjected to a static magnetic field.

This paper reports on the frequency dependence of the magnetic and electric power dissipation in a magnetic fluid sample, in the microwave frequency range (0.5 to 8GHz), at various values of the static magnetic field (0 to 167.8kA/m). The computation of the power dissipation relies on the experimental values measured for the complex dielectric permittivity, varepsilon = varepsilon' - ivarepsilon'', and the complex magnetic permeability, mu = mu' - imu'', over the same frequency range. The results show that the magnetic power dissipation is much larger than the electric one for the investigated sample. At a specific frequency, f (Hz) , the power dissipation, p, depends on the external magnetic field, and exhibits a maximum. The result obtained suggests the possibility of controlling the energy absorption in the microwave range by means of the application of an external magnetic field.

On the significance of the area under the after-effect function curve of a magnetic fluid.

The after-effect function, b(t), describes how the magnetization of a dissipative magnetic fluid decreases with time when a polarizing field, H, is suddenly removed. It is shown that with increasing H, the rate of decay of b(t) increases and also that the area, [Formula: see text], under each decay curve decreases. Here we investigate the significance of this and by means of a simple model, show that the normalized function, B/b(0), is in fact equal to the Debye relaxation time τ(D). The results of applying the model to theoretically generated data and also to data obtained from a magnetic fluid sample are presented.