Role of Oxygen Coordination on the Ultrafast Demagnetization in Ferrite Nanoparticles

In this work, we have studied the ultrafast dynamics of charges and spins in assemblies of magnetite (Fe₃O₄) and maghemite (γ - Fe₂O₃) nanoparticles. We demonstrate that using time-resolved magneto-optics one is able to disentangle those very similar iron oxide structures. The Fe₃O₄ nanoparticles are elaborated by hydrothermal decomposition and deposited by drop on a glass substrate. γ - Fe₂O₃ nanoparticles assemblies have been obtained by annealing the Fe₃O₄ nanoparticles. Comparing time resolved transmission and Faraday rotation, our measurements show that in case of Fe₃O₄ the demagnetization occurs after the thermalization of the charges, as expected from previous works on ultrafast quenching of magnetization in ferromagnetic nanostructures. On the contrary, in the case of maghemite nanoparticles, an acceleration of the demagnetizing occurs, leading to a simultaneous charges and spins dynamics. We attribute this behavior to the rearrangement of vacancies and annealing of crystal defects in maghemite