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 ) - i χ ″ ( 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), Ni 0.5Zn 0.5Fe 2O 4 particles (sample S2) and Mn 0.66Zn 0.34Fe 2O 4 particles (sample S3). In the investigated frequency range ferromagnetic resonance occurs at a frequency, f res, and a corresponding maximum absorption occurs at a frequency f max. From a plot of f res against H, average values of the anisotropy constant, K ¯ , anisotropy field, H ¯ A , and gyromagnetic ratio, γ are determined. From a plot of f max /f res against H the damping parameter, α, is obtained. Also, for each sample, an estimate is made of the particles magnetic viscosity, η m , together with τ 0, the pre-exponential factor of Brown's equation for the Néel relaxation time, τ N , of the magnetic moment of superparamagnetic particles and τ N itself.