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Séminaire « Magnétisme et Physique du Spin » de l’INSP

Coupled vortex dynamics in spin-torque oscillators : From resonant excitation to mutual synchronization - Romain Lebrun - Mardi 9 février 2016 à 11 h

INSP - UPMC - 4 place Jussieu - 75005 Paris
Barre 22-12, 4e étage, salle 426

Romain Lebrun - Unité Mixte CNRS-Thales

Abstract

The discovery of the giant magnetoresistance in 1988 is considered as the birth date of a new and dynamic research field called spintronics. The rich physics associated with spin transport has created a breakthrough for the future of nano-electronics. In the magnetism roadmap, spin-torque oscillators (STOs) are candidates for future generation of spintronic based rf-devices [1]. However one major issue of spin-torque oscillators is their poor spectral coherence given their highly nonlinear behavior. To overcome this issue, I have investigated different approaches during my thesis : (i) the development of magnetic materials with a low damping and large spin-polarization, (ii) the study of collective mode dynamics in hybridized magnetic systems (iii) the stabilization of the STO dynamics with a reference external signal (iv) the synchronization of multiple STOs to enhance both their power and spectral coherence. My work particularly focus on vortex based STOs which present higher spectral coherences than other kinds of STOs.

In a first part, I will present the different mechanisms that can drive and stabilize the dynamics of a vortex based STO in the autonomous and non-autonomous regimes. I will first highlight that the excitation of collective modes allows the harnessing the rf-properties of a single and isolated in a double vortex based STO [2]. Then I will demonstrate that we can phase-lock a STO with an external rf-current without any phase slips, i.e desynchronization events, when the locking process is driven by a Field-like in-plane torque. Thanks to two spin-transfer locking torques (one Slonczewski and one Field-torque) in our magnetic tunnel junction, we can control with precision the characteristics of the locking process [3]. Such a degree of control, unexpected for a nano-scale oscillator, is particularly promising for the development of STO based nanodevices such as rf-threshold detector [4].

In a second part, I will show how we recently succeeded to mutually synchronize two STOs through their self-emitted current [5]. In the synchronized state, the spectral coherence and the output power are respectively enhanced by a factor N (2) and N² (4). The full control of the synchronized state is then achieved by tuning the phase shift between the two oscillators, either externally with an electrical delay or internally through the different spin-transfer torques. These first promising results marks an important milestone towards the observation of large variety of nanoscale collective dynamics of nonlinear oscillators, and opens among others the perspective of STO networks mimicking some of the basic brain functionalities.

[1] N. Locatelli, V. Cros, and J. Grollier, Nat Mater 13, 11 (2014). [2] R. Lebrun, N. Locatelli, S. Tsunegi, J. Grollier, V. Cros, F. Abreu Araujo, H. Kubota, K. Yakushiji, A. Fukushima, and S. Yuasa, Physical Review Applied 2, (2014). [3] R. Lebrun, A. Jenkins, A. Dussaux, N. Locatelli, S. Tsunegi, E. Grimaldi, H. Kubota, P. Bortolotti, K. Yakushiji, J. Grollier, A. Fukushima, S. Yuasa, and V. Cros, Physical Review Letters 115, (2015). [4] A. S. Jenkins, R. Lebrun, E. Grimaldi, S. Tsunegi, P. Bortolotti, H. Kubota, K. Yakushiji, A. Fukushima, G. de Loubens, O. Klein, S. Yuasa, and V. Cros, Nature Nanotechnology (2016). [5] R. Lebrun, S. Tsunegi, P. Bortolotti, H. Kubota, A. S. Jenkins, M. Romera, K. Yakushiji, A. Fukushima, J. Grollier, S. Yuasa, and V. Cros, arXiv:1601.01247 [cond-mat] (2016).