23 research outputs found
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 ) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity
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
erg cm-2 associated with a particle
elongation of 1.25 both coherent with Néel predictions of surface
anisotropy in small grains