Static magneto-optical birefringence of size-sorted − Fe2O3
nanoparticles
E.
Hasmonay, E. Dubois, J.-C. Bacri, R. Perzynski, Yu.L. Raikher, and V.I.
Stepanov
Magnetic birefringence experiments under static field are performed on
ionic ferrofluid samples based on g−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 KS = 2.8x10−2 erg
cm−2 associated
with a particle elongation of 1.25 both coherent with Neel predictions of
surface anisotropy in small grains.