On the resonant frequency of ferrofluids subjected to perpendicular and parallel polarizing fields

Abstract

In this paper the magnetic field dependence of the resonant frequency, fres, and frequency of maximum absorption, fmax, for a ferrofluid sample, for polarizing fields both perpendicular to and parallel with the axis of a coaxial cell, are presented for the first time. Measurements of the complex magnetic susceptibility, χ(ω) = χ′(ω)-iχ″(ω) as a function of frequency, f (Hz), over the range 100 MHz-6 GHz and external polarizing field, H (kA m-1), up to approximately 100 kA m-1, are obtained by means of the coaxial transmission line technique. The presence of ferromagnetic resonance in the ferrofluid, (at a frequency fres) is found for all values of the polarizing field and the influence of demagnetizing effects on fres is accounted for. The dependence of fres on H is found to be linear and an average value of internal anisotropy field, H̄A, and the corresponding magnetic anisotropy constant, K̄, are determined. The susceptibility component profiles obtained for both cases of polarization are similar to those obtained in an analogous theoretical study of dielectrics subjected to an external electric field.