Iron oxide nanoparticles
Iron oxide nanoparticles are generally of three types: simple iron oxide <math>FeO</math>, magnetite <math>Fe_3O_4</math> and hermatite <math> Fe_2O_3</math>. These particles are generally used for their magnetic properties. Magnetite in particular is used in super-paramagnetic beads. It occurs naturally in certain magnetic bacteria such as Magnetospirillum magnetotacticum.
 Wikipadia "Iron Oxide"
 Science Daily article on potential risks