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Philippe SCHEID Doctorant au sein de l'équipe "Spintronique et Nanomagnétisme" de l'Institut Jean Lamour, soutient sa thèse intitulée :
"Investigation of light–induced ultrafast magnetization dynamics using ab initio methods"
Date et lieu :
Mardi 22 septembre à 9h30
Institut Jean Lamour
Composition du jury :
Directeur de thèse :
- M. Sébastien Lebègue
Directeur de recherche, LPCT, Université de Lorraine
Co–directeur de thèse :
- M. Grégory Malinowski
Chargé de recherche, Institut Jean Lamour, Université de Lorraine
- Mme Sangeeta Sharma
Docteure, Max–Born Institute de Berlin
- Mme Émilie Gaudry
Professeure, Institut Jean Lamour, Université de Lorraine
- M. Xavier Blase
Directeur de recherche, Institut Néel
- M. Brice Arnaud
Professeur, Université du Mans
- M. Stéphane Mangin
Professeur, Institut Jean Lamour, Université de Lorraine
To improve the information storage technologies, faster and more energy efficient ways of manipulating the magnetization state of the matter are researched. Within this framework, the possibility of doing so by using solely femtosecond light pulses, as suggested by the successive discoveries of the light–induced ultrafast demagnetization by Beaurepaire et al. in 1996, and of the so–called all–optical helicity–dependent switching by Stanciu et al. in 2007, is particularly attrac-tive.
This thesis begins with a review of the current experimental and theoretical state of the art related to both of the aforementioned phenomena. This is followed by an overview of density functional theory, upon which relies most of the work reported thereafter.
The first set of results concerns the ab initio study of the effect of a rise in the electronic tempera-ture on the magnetized matter properties, and more specifically Fe, Co, Ni and FePt. Indeed, a the light primarily interacts with the electrons, and due to the fact that the duration of the pulse is shorter than the coupling of the electrons with the other degrees of freedom, one can simplyaccount for the absorbed energy by a rise of the electronic temperature. Doing so, we show that the magnetic moment carried by each atom disappears at the so–called Stoner temperature, and that this phenomenon impacts the electronic energy and specific heat, even at low electronic tem-perature. Then, we show that upon an increase in the electronic temperature, the interatomic Heisenberg exchange, which is responsible for the magnetic ordering, decreases. Using the atom-istic Langevin Landau–Lifshitz–Gilbert equation, we demonstrate that this decrease is enough to induce a large reduction of the average magnetization by creating transversal excitations.
The second set of results regards the origin of the helicity–dependent light–induced dynamics. While the literature attributes it mainly to the inverse Faraday effect, we argue that another and novel phenomenon, which occurs during the absorption of the light, may be more suited to ac- count for the experimental dynamics. Indeed, using the Fermi golden rule and ground state den- sity functional theory calculations in Fe, Co, Ni and FePt, we show that, as the light is absorbed and electrons are excited, concurrently to the increase of the electronic energy, the spin–state is also changed in presence of spin–orbit coupling. This results in a difference in the value of the atomic magnetic moments, persisting even after the light is gone, as opposed to the inverse Faraday effect.
Then, using real–time time–dependent density functional theory, we compute the magne-tization dynamics induced by real optical and XUV femtosecond circularly polarized pulses. We show that, in both cases the dynamics is helicity–dependent and that this characteristic is largely amplified in the XUV regime involving the semi–core 3p states. Finally, we compare the relative role of the inverse Faraday effect and the magnetization induced during the absorption of the light and show that the latter plays a prominent role, especially after the light has gone, and in the XUV regime.