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Theses

Transfert de spin et dynamique de l'aimantation

Abstract : In this text we present a detailed analysis of the magnetization dynamics in nano-structures when subjected to spin torque, using numerical simulations. We focused on current-induced precession states in spin-valve nano-pillars, where spectral signature can be directly compared to experimental results. Two types of calculations have been done, one based on the macro-spin approximation and the others done in the micro-magnetic framework. In the first case, simulations predict correctly excited states when spin polarisation and applied field directions are aligned with the nano-pillar long axis. We find the two regimes described in the literature: hysteretic switching and precession states stabilized above a current threshold. We give an analytic form for critical current that is in accordance with experimental data. The motion of the spatially average magnetisation calculated in micro-magnetic simulations is found to be similar. We reveal that excitation distribution in the free layer is inhomogeneous at low current, and this state corresponds to the eigen-modes of the system. In plane precession regime is quite uniform, whereas out-of-plane precession states are highly disordered. We have also examined systems where electron spin-polarisation and applied field directions are not aligned. Micro-magnetic simulations show that frequency jumps as a function of current correspond to abrupt changes in excitation distribution. We note that these modes are markedly more localised than in the axial case and the resonance peaks are much narrower. Long-term coherence of these oscillations can not be understood in the single-domain model. We extend this analysis by computing peak line-widths near the critical current for sustained precession, results being in this case in good agreement with recent literature. We present a simple model that explains both the lorentzian peak profile and the linear dependence of width as a function of temperature observed in calculations. But quantitative comparison with experiments still remains delicate.
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https://tel.archives-ouvertes.fr/tel-00192958
Contributor : Benoit Montigny <>
Submitted on : Saturday, December 1, 2007 - 3:02:32 PM
Last modification on : Wednesday, September 16, 2020 - 4:32:35 PM
Long-term archiving on: : Tuesday, September 21, 2010 - 2:54:45 PM

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  • HAL Id : tel-00192958, version 2

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Benoit Montigny. Transfert de spin et dynamique de l'aimantation. Matière Condensée [cond-mat]. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00192958v2⟩

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