An investigation of the microscopic and macroscopic properties of magnetic fluids

Measurements are presented of the frequency and field dependent complex magnetic susceptibility, χ(ω,H)=χ′(ω,H)-iχ″(ω,H) of three different magnetic fluids, over the frequency and polarizing field (H) ranges of 100 MHz to 6 GHz and 0-122 kA/m, respectively. The investigated samples were Isopar M-based magnetic fluids, with magnetite particles (sample S1), Ni0.5Zn0.5Fe2O4 particles (sample S2) and Mn0.66Zn0.34Fe2O4 particles (sample S3). In the investigated frequency range ferromagnetic resonance occurs at a frequency, fres, and a corresponding maximum absorption occurs at a frequency fmax. From a plot of fres against H, average values of the anisotropy constant, K¯, anisotropy field, H¯A, and gyromagnetic ratio, γ are determined. From a plot of fmax/fres against H the damping parameter, α, is obtained. Also, for each sample, an estimate is made of the particles magnetic viscosity, ηm, together with τ0, the pre-exponential factor of Brown's equation for the Néel relaxation time, τN, of the magnetic moment of superparamagnetic particles and τN itself.