Etude de la dynamique des oscillateurs à vortex par synchronisation et modulation de fréquence

Abstract : Since 2004, research on radiofrequency (RF) spintronic devices has been very active, both from a fundamental point of view as well as for their potential applications as RF oscillators or spin-diodes. These devices are based on spin transfer torque (STT). In this context, I studied vortex oscillators based on ultra-low resistance magnetic tunnel junctions in which vortex dynamics is driven into a periodic motion by STT. The vortex oscillations are observed when the junction is subjected to a large dc bias current and a low in-plane field. The dc current produces both a large Oersted field which contributes to the vortex nucleation and a STT that starts the vortex oscillation. This oscillation leads to a large output power up to 20nW with a fundamental frequency around 450MHz and many harmonics.Synchronization with an external signal was then tested by adding a RF current to the dc bias current. With a large enough input power, the oscillator locks on the external source: the noise is then drastically reduced and the spectral purity of the signal significantly increases. These observations are explained by describing the system as a parametric oscillator. This model predicts, as experimentally observed, that, for a small amplitude of the RF excitation, a dynamical instability can be more easily reached when its frequency is twice the natural frequency of the oscillator than for any other frequencies.Then, I performed frequency modulation measurements by exciting the dc-biased sample with a low frequency ac-current. The power spectral density was measured as I swept the modulation frequency for various modulation powers. It appears that the description previously used to describe modulation experiments does not apply when the modulation frequency is a significant fraction of the natural frequency. The vortex response time appears to play a significant role, so that the concept of deviation sensitivity has to be introduced to explain the observations: it corresponds to the dynamical dependence of the oscillator frequency with an applied current that varies with time.
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Sylvain Yoann Martin. Etude de la dynamique des oscillateurs à vortex par synchronisation et modulation de fréquence. Matière Condensée [cond-mat]. Université de Grenoble, 2013. Français. ⟨NNT : 2013GRENY073⟩. ⟨tel-01367954⟩

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