Magnetic and optical properties of ionic ferrofluids based on nickel ferrite nanoparticles

New ionic ferrofluids containing NiFe2O4 nanoparticles of size less than or equal to 10 nm are investigated. The crystalline structure of the particle s is probed by transmission electron microscopy and x-ray scattering. Stati c magnetization and field-induced birefringence measurements are performed on three samples differing by particle volume fraction. Cross analyzing of the results of those two types of macroscopic tests completely rejects a si mple single-domain particle model but readily supports the two-component sc heme of a particle as consisting of a core with a uniform magnetization and a surface layer of comparable thickness stowed with a spin-glass-like arra ngement. (C) 2000 American Institute of Physics. [S0021-8979(00)00618-6].