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Theses

Ultrafast Magnetization dynamics of magnetic nanostructures

Hasan Kesserwan
Abstract : Divided in two parts, this PhD thesis concerns the magnetization dynamics of magnetic nanoparticles. In the first part, we have described a detailed experimental study of the magnetization dynamics in core/shell CoPt nanoparticles. Towards that goal, we have performed Time Resolved Magneto-Optical Effect measurements using a femtosecond pump and probe set-up with pulse durations and wavelengths: pump 150 fs/400 nm and probe 150 fs/800 nm probe. We studied the different magneto-dynamical processes taking place on short time scales such as: the ultrafast demagnetization and the precession of the magnetization vector. The obtained results indicate a possibility of inducing a supra-crystalline ordering of the nanoparticles due to a mild laser annealing. We showed that there is an important influence of the thermal annealing on the magnetic properties of the nanoparticles. For example, it leads to a magnetic phase transition form super-paramagnetic to ferromagnetic above the room temperature. This ferromagnetism manifested itself as an increase in the magnetic anisotropy of the nanoparticles and in the precession of the magnetization vector induced by the pump pulses. The second part is devoted to the numerical simulations of the magnetization reversal in isolated and interacting nanoparticles. In isolated nanoparticles, the relaxation times follow the Arrhenius law provided by the Néel-Brown's model. To account for the magnetic dipolar interaction, we have introduced a simple and effective model based on the mean field approximation. In general, we have observed deviations from the Arrhenius law, and showed that the dipolar interaction accelerates the reversal process.
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Submitted on : Sunday, October 30, 2011 - 5:37:51 AM
Last modification on : Wednesday, November 2, 2011 - 10:39:56 AM
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  • HAL Id : tel-00637078, version 1

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Hasan Kesserwan. Ultrafast Magnetization dynamics of magnetic nanostructures. Condensed Matter [cond-mat]. Université de Strasbourg, 2011. English. ⟨tel-00637078⟩

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