Sustained magnetization oscillations in polyaniline-Fe3O4 nanocomposites.

We report experiments with polyaniline-Fe3O4 (PANI-Fe3O4) nanocomposites synthesized under several different conditions. With a reaction carried out at room temperature and assisted by intense ultra-violet (UV) irradiation, we observe sustained oscillations in the magnetization with a period of about 25 min. The oscillations are interpreted as the result of an oscillatory chemical reaction in which part of the Fe(+2) ions of magnetite, Fe3O4, are oxidized by the UV irradiation to form Fe(+3) so that a fraction of the magnetite content transforms into maghemite, γ-Fe2O3. Then, Fe(+3) ions at the nanoparticle surfaces are reduced and transformed back into Fe(+2), when acting as an oxidizing agent for polyaniline in the polymerization process. Since maghemite has smaller magnetization than magnetite, the oscillating chemical reaction results in the oscillatory magnetization. The observations are interpreted with the Lotka-Volterra nonlinear coupled equations with parameters that can be adjusted to fit very well the experimental data.