Influence du couple de transfert de spin sur les fluctuations magnétiques thermiquement activées dans les jonctions tunnel magnétiques

Abstract : The size reduction of spintronic devices causes the decrease of the signal to noise ratio, especially due to the increase of magnetic fluctuations near the super-paramagnetic limit. Simultaneously, the size reduction of these devices brought about the emergence of a new phenomenon: the spin transfer. A spin polarized current leads to a torque on the magnetization of a ferromagnetic layer. When the current density exceeds a threshold value called the critical current, the magnetization of the ferromagnetic layer can be reversed or led into a steady-state precession. Many studies have been conducted on these two effects, from a fundamental or an applied point of view. But few have been done at currents below the critical threshold, that is to say in a regime where the spin transfer torque is expected to have no macroscopic effects on the magnetization dynamic.

In this context, we showed that the spin transfer torque strongly impacts the spectrum of magnetic fluctuations at the ferromagnetic resonance in magnetic tunnel junctions, even for currents far below the critical current. This result could be obtained since we realized a broadband experimental set-up (DC − 26 GHz) to measure the noise emitted by a magnetoresistive device. The detection limit of the bench is less than 0.5 nV/pHz. Moreover, we developed a model based on the fluctuation-dissipation theorem that explains the changes in the magnetic fluctuation spectrum due to the spin polarized current. Thus, we could evidence the coexistence of two terms of spin transfer torque in magnetic tunnel junctions.
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Sébastien Petit. Influence du couple de transfert de spin sur les fluctuations magnétiques thermiquement activées dans les jonctions tunnel magnétiques. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2007. Français. ⟨tel-00293055⟩

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