Static magneto-optical birefringence of size-sorted γ-Fe2O3 nanoparticles

Magnetic birefringence experiments under static field are performed on ionic ferrofluid samples based on gamma-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 H-2 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 x 10(-2) erg cm(-2) associated with a particle elongation of 1.25 both coherent with Neel predictions of surface anisotropy in small grains.