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Séminaire de groupe

Magnetic Vortex dynamics in nano-point contact devices
Sébastien Petit-Watelot
Institut d’Electronique Fondamentale, UMR CNRS/Université Paris-Sud 11
jeudi 08 décembre 2011 , 11h00
Salle de séminaire du groupe de Physique Statistique

Nano-spin-torque oscillators (STNO) have drawn a lot of interest during the past decade, resulting in the study of numerous systems. One of the first developed systems was nano-point contact (nPC) on top of a large spin-valve structure. Uniform magnetization dynamic driven by a large DC current density through spin transfer [1] was first observed in 2003 [2] in such devices. More recently it has been demonstrated that, under specific conditions, the uniform magnetization dynamics below the point contact can be replaced by the nucleation and the gyromotion of a magnetic vortex in the free magnetic layer of the spin valve [3]. In a first part, I will present the main features of the nucleation process. The Large Ampere field generated close to the point contact can trigger the nucleation of several vortex pairs to minimize the Zeeman energy. The process implies a thermally activated step involving the nucleation and annihilation of at least two vortex/antivortex pairs. In a second part, I will focus on the description of the gyromotion of the vortex in the &#8220;linear regime&#8221;, which results from the compensation of the damping by the in-plane spin transfer torque. In this regime the voltage generated by the current perpendicular to the plan giant magnetorsistance (CPP-GMR) effect, can be tuned almost linearly with the DC applied current between 100 MHz and 250 MHz [ 4 ]. Finally I will show how it is possible to reach a new regime, transforming the STNO into a relaxation oscillator above a current threshold. In this regime the instantaneous velocity reaches the critical velocity where core reversal occurs [5], leading to synchronization between the gyration frequency and the core switching frequency.<br> [1] J. C. Slonczewski, Journal of Magnetism and Magnetic Materials 159, L1 (1996)<br> [2] W. H. Rippard et al., Appl. Phys. Lett. 82, 1260 (2003)<br> [3] Q. Mistral et al., Phys. Rev. Lett. 100, 257201 (2008).<br> [4] J.-V. Kim and T. Devolder, arXiv:1007.3859v1 [cond-mat.mtrl-sci] (unpublished).<br> [5] K. Guslienko et al., Phys. Rev. Lett. 100, 027203 (2008).<br>



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