Parametric resonance in magnetic fluids

A new resonance effect in the nonlinear behaviour of magnetically anisotropic objects in an alternating external magnetic field is proposed. Ferromagnetic particles with a "frozen" magnetic moment (due to a strong magnetic anisotropy), when located in an external alternating magnetic field, are able to rotate (or vibrate) and to transfer energy from the external field to the medium. The numerical solution of the appropriate parametrically driven nonlinear equation shows all types of nonlinear dynamic behaviour, including transition to chaos. The sensitivity of the proposed phenemenon could be used for an experimental analysis of the size distribution of the ferromagnetic particles in a ferrofluid or of the size of "magnetic holes".