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

New ionic ferrofluids containing ${\mathrm{NiFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ nanoparticles of size ⩽10 nm are investigated. The crystalline structure of the particles is probed by transmission electron microscopy and x-ray scattering. Static 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 simple single-domain particle model but readily supports the two-component scheme 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 arrangement.