Abstract
We present the first comparative study of ultrafast demagnetization between native magnetite (Fe3O4) nanoparticles, and maghemite (γ‒Fe2O3) nanoparticles issued from the same batch of nanoparticles and obtained after thermal annealing of the native ones. We demonstrate that the demagnetizing process is accelerated and becomes simultaneous to the electron thermalization time as the degree of oxidation is increased. Our interpretation is that thermal annealing, inducing a phase transition between the mixed valence state Fe3O4 and its oxidized phase γ‒Fe2O3, reinforces antiferromagnetic superexchange interactions which govern their magnetic properties. Consequently, this speeds up the ultrafast magnetization in the very short time scale. We show the role played by the exchange interaction in the ultrafast demagnetization following a femtosecond laser pulse excitation.
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