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 Hani 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 $k_{\rm B}T$) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity $H_{\rm C}(H_{\rm C} H_{\rm ani})$

Static magneto-optical birefringence of size-sorted γ-F

Magnetic birefringence experiments under static field are performed on ionic ferrofluid samples based on γ-Fe2O3 . A colloidal size-sorting of the particles allows to obtain narrow size distributions. The optical birefringence of the solutions is found positive and can be described by a Langevin formalism. It scales as H2 in the low field limit. In the high field limit the particles size dependence of the saturation birefringence is compatible with a surface anisotropy constant $K_S= 2.8\times 10^{-2}$ erg cm-2 associated with a particle elongation of 1.25 both coherent with Néel predictions of surface anisotropy in small grains