Magneto-induced anisotropy in magnetic colloids of superparamagnetic magnetite nanoparticles in an external magnetic field.

Abstract

Here we report a study of chain formation and the magnetic anisotropy induced by them in suspensions of slightly anisotropic Fe₃O₄ nanoparticles in water and in aqueous V₂O₅ suspensions. An investigation into the magnetization of the fluid and frozen suspensions, the application of dynamic light scattering techniques and the observation of the magnetic anisotropy in the frozen magnetically aligned samples allowed us to confirm the existence of chains of Fe₃O₄ in both suspensions. Our study shows that the magneto-induced anisotropy appearing in magnetic fields in colloids with Fe₃O₄ particles is mainly due to many particle (chain) magneto-induced anisotropy, but not due to single particle magneto-induced anisotropy connected with particle shape anisotropy. In other words, the single particle magneto-induced anisotropy is much smaller than the many particle (chain) anisotropy. The anisometry of the chains provides effective coupling with the nonmagnetic V₂O₅ component of the suspension and results in its strong sensitivity to the magnetic field.