Alternating magneto-birefringence of ionic ferrofluids in crossed fields

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value H _ani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: - a rigid dipole regime (large anisotropy energy with respect to k _B T) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity H _C(H _C < H _ani), - a soft dipole regime (anisotropy energy of the order of k _B T) for maghemite nanoparticles with a relaxation time independent of the field intensity H _C(H _C > H _ani). Received 5 June 2000 and Received in final form 8 January 2001