Alternating magneto-birefringence of ionic ferrofluids in crossed fields

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated hv another smaller field. normal and altern ating. The optical magneto-birefringence under these crossed magnetic field s is recorded as a function of the frequency for different field intensitie s and different sizes of the magnetic nanoparticles. A general reduced beha vior 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-an i of the anisotropy field of the nanoparticles, we can distinguish two diff erent high magnetic field regimes: - a rigid dipole regime (large anisotrop y energy with respect to k(B)T) for cobalt ferrite nanoparticles with a rel axation 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 magh emite nanoparticles with a relaxation time independent of the field intensi ty H-C(H-C > H-ani).