Measurements performed over the frequency range 700 MHz to 3 GHz, of the complex dielectric permittivity of (i) a kerosene-based magnetic fluid sample with magnetite particles stabilized with oleic acid, (ii) oleic acid and (iii) kerosene, are presented.The biphasic dielectric model describing the dielectric behaviour of magnetic fluids was redefined based on the experimental results of Rocchiccioli-Deltcheff et al. According to this proposed new model, the magnetic fluids can be considered as being biphasic systems in which one phase consists of surfacted colloidal particles electrically charged by adsorbed or chemisorbed ions and the other phase consists of the carrier liquid.Using the Sillars theory for heterogeneous dielectrics, in the biphasic approximation, we have determined the dielectric permittivity, electric conductivity and shape factor of the colloidal magnetite particles within the investigated magnetic fluid, in the microwave range. The values obtained for the colloidal magnetite particles are the shape factor n = 2.27, the real part of the dielectric permittivity ε 1 = 36.8, and the effective conductivity σ 1 eff = 1.5 −1 m −1 .The differences between the experimental results and the values of conductivity and permittivity of bulk magnetite are explained in terms of the proposed biphasic model.
Starting from the Landau–Lifshitz equation, with resonant frequency f0 = ω0/2π,
it is demonstrated that, in the case of a magnetic fluid, the measured resonant frequency,
fres is always
different from f0,
except for the case of pure resonance (i.e. zero damping
parameter of Landau–Lifshitz equation) where fres = f0. It is also
shown that fres
and the corresponding maximum absorption frequency,
fmax,
are different, thus supporting the deductions of Scaife, who arrived at
this conclusion using an alternative theoretical approach.Furthermore,
based on complex magnetic susceptibility measurements, over
the frequency range 100 MHz–6 GHz, the dependence of the ratio
fmax/fres
on an external polarizing magnetic field,
Hpol,
over the approximate range 0 and 1.3 kOe and on particle concentration has been
examined for different magnetic fluid samples. It is demonstrated how the ratio
fmax/fres
tends to unity both by (i) increasing the polarizing field and (ii) decreasing the
particle concentration of the samples.
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