External magnetic field influence on magnetite and cobalt-ferrite nano-particles in ferrofluid

Using simple optical measurements, the selective spectral behavior of suspended nano-particles of magnetite (Fe3O4) and cobalt-ferrite (CoFe2O4) coated with starch, was observed through switching on/off of external magnetic field. The field strength was varied in the interval 30–300 mT. In these transient regimes, the unexpected and unusually large changes of transmitted light occur. An explanation is suggested, within the model based on ordering of magnetic moments of nano-particles along the lines of magnetic field into magnetic chains and organization of magnetic chains into spatial structure—a quasi-lattice. This fact indicates the possibility of more efficient local heating of nano-particles in the fast changing magnetic field; for instance when taking magnetic nano-particles for therapeutic purposes.