Spin-torque excited spin waves revealed by micro-focused Brillouin light scattering*

Since the discovery of the spin transfer torque (STT) effect [1,2] a great effort has been devoted to the realization and study of spin torque oscillators (STOs) because of their potential applications as spin waves injectors in magnonic devices or current-tunable broad-band microwave sources. More recently the possibility to synchronize multiple STOs [3,4] via the emitted spin waves, propagating in the magnetic ``free'' layer, envisioned a way to overcome their main limitation in the output power. For these reasons it's now crucial to obtain a detailed knowledge and understanding of the emitted spin waves properties like: their spatial distribution, their propagating or localized character, their decay length, wavelength and group velocity. In the last two years micro-focused Brillouin light scattering ($\mu $-BLS) revealed to be a powerful tool in order to investigate several of this properties [5,6]. In this presentation we discuss the potentialities of $\mu $-BLS to the study of emitted spin waves in STOs systems with particular focus on the results of our latest work [6]. Here we took advantage of our $\mu $-BLS setup in order to study spin waves emitted by an out-of-plane magnetized nano-contact STO. Performing a ``wave-vector resolved'' $\mu $-BLS experiment we provided the first direct experimental evidence of the \textit{propagating} nature of SWs emitted from an out-of-plane magnetized STO. The decay of the propagating SW intensity up to several microns away from the nano-contact position showed great potential for STT based magnonic devices. We also investigated the STO tunability measuring the emitted SW frequency as a function of both the applied direct current and external field intensities. Micromagnetic simulations provided the theoretical support to quantitatively reproduce the results. \\[4pt] [1] Slonczewski, J. C. J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [2] Berger, L. Phys. Rev. B 54, 9353 (1996).\\[0pt] [3] Kaka, S. et al. Nature 437, 389 (2005).\\[0pt] [4] Mancoff, F. B., Rizzo, N. D., Engel, B. N., Tehrani, S. Nature 437, 393 (2005).\\[0pt] [5] Demidov, V. E., Urazhdin, S., and Demokritov, S. O. Nature materials, 9(11), (2010).\\[0pt] [6] Madami, M., Bonetti, S. et al. Nature Nanotechnology, 6, 635 (2011).