Study of magnetic fluids by means of magnetic spectroscopy

The dependence on frequency of the complex magnetic susceptibility χ=χ′-iχ″ over the range 10 Hz-100 MHz, of two magnetic fluids with magnetite and mixed ferrite particles of type Mn0.6Fe0.4Fe2O4 is analyzed. It allows Brownian and Néel relaxation processes to be identified. Based on these measurements we have determined the Brownian relaxation time, the hydrodynamic radius, the Néel relaxation time and the effective anisotropy constant. Also, the measurements of relaxation times give useful information about the colloidal stabilization of the magnetic particles within the investigated magnetic fluid sample in zero polarizing fields. Ferromagnetic resonance investigations of the above-mentioned samples are also presented. These results allow us to determine the effective anisotropy constant, the anisotropy field, as well as to obtain information about the colloidal stabilization of the magnetic particles within the investigated samples, in the presence of an external field. This brief review demonstrates how the magnetic spectroscopy can be employed to determine microscopic and macroscopic properties of the magnetic fluids.