Alternating magneto-birefringence of ionic ferrofluids in crossed fields
E.
Hasmonay, E. Dubois, S. Neveu, J.-C. Bacri, and R. Perzynski
Abstract
A dynamic probing of magnetic
liquids is performed experimentally, using a static magnetic field modulated by
another smaller
eld, 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 kBT) for cobalt ferrite nanoparticles
with a relaxation time inversely proportional to the eld intensity HC(HC < Hani), a softdipole regime (anisotropy
energy of the order of kBT) for maghemite nanoparticles with a relaxation time independent
of the field intensity HC(HC > Hani).