Direct observation of a propagating spin wave induced by spin-transfer torque M. Madami 1† * , S. Bonetti 2† * , G. Consolo 3,4 , S. Tacchi 1 , G. Carlotti 1,5 , G. Gubbiotti 1,6 , F. B. Mancoff 7 , M. A. Yar 8 and J. Åkerman 2,9 Spin torque oscillators with nanoscale electrical contacts 1–4 are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation 5,6 , fast spin-wave frequency modulation 7–9 , and the possibility of synchronizing multiple spin-wave injection sites 10,11 . However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave- vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results. Much effort has recently been devoted to a better understanding of the details of the spin waves excited in magnetic films by nano- contact-based spin torque oscillators (STOs) 12,13 . In particular, it has been predicted that the spatial characteristics of spin-wave exci- tations have a critical dependence on the direction of the magnetiza- tion angle in the out-of-the plane film direction (and therefore on the externally applied field angle) 14–16 . Only very recently has it been demonstrated experimentally (by means of electrical microwave detection) that above a certain critical angle a propagating spin- wave mode can be excited, and both localized and propagating spin waves can be excited alternately below this critical angle 17 . Although it was possible to elucidate a number of important charac- teristics of both propagating and localized spin-wave modes using only electrical detection (for example, the current and field depen- dencies of the frequency, the linewidth and the output power), direct evidence of their propagating nature is still lacking. Micro-focused Brillouin light scattering (m-BLS) 18 is a powerful technique for resolving the spatial profile of spin waves in magnetic nanostructures, and has recently been used in pioneering studies 19,20 to experimentally observe spin waves caused by spin transfer torque (STT) in in-plane magnetized nanocontact STOs. However, the pro- pagating character of the radiated spin waves has not been proven experimentally. Here, we use m-BLS to study spin waves emitted in an out-of-plane magnetized nanocontact STO and provide experimental proof that propagating spin waves are radially emitted from the nanocontact region into the continuous ferromagnetic thin film up to several micrometres away from the nanocontact. The sample under investigation comprises a pseudo spin valve stack with the layer structure Co 81 Fe 19 (20 nm)/Cu(6 nm)/Ni 80 Fe 20 (4.5 nm), patterned into an 8 × 26 mm 2 mesa. The thicker CoFe layer is considered the ‘fixed’ magnetic layer, and the thinner NiFe plays the role of the low-dissipation ‘free’ magnetic layer in which a steady STT-driven spin wave can be sustained. The thickness of the copper spacer (6 nm) ensures that there is negligible interlayer exchange coupling between the two magnetic layers. A circular contact of diameter d ¼ 200 nm is patterned in the middle of the spin valve mesa, and a thick (400 nm) aluminium ground–signal– ground waveguide is deposited on top of the mesa, allowing for the injection of a high, spin-polarized, current density 21 and the subsequent extraction of the generated microwave voltage. Optical m-BLS access to the active region of the free layer was achieved through a combination of focused ion-beam (FIB) and H ext Cu spacer Ni 80 Fe 20 free layer Co 81 Fe 19 fixed layer Optical window Pd-Cu top electrode Pd-Cu bottom electrode Al coplanar waveguide SiO 2 insulator d.c. source Probing laser light - + + / Nanocontact Figure 1 | Schematic sample layout. Cross-section of the sample, revealing the layers of the spin valve mesa and the active area of the STO device. An aluminium coplanar waveguide is deposited onto the spin valve mesa, and an optical window is etched into the central conductor of the waveguide close to the nanocontact. 1 CNISM, Unita ` di Perugia and Dipartimento di Fisica, Universita ` di Perugia, Via A. Pascoli, I-06123 Perugia, Italy, 2 Materials Physics, School of Information Communication Technology, KTH – Royal Institute of Technology, Electrum 229, 164 40, Kista, Sweden, 3 Dipartimento di Scienze per l’Ingegneria e l’Architettura, Universita ` di Messina C.da di Dio, I-98166 Messina, Italy, 4 CNISM, Unita ` di Ferrara, Via G. Saragat 1, I-44100 Ferrara, Italy, 5 Centro S3, CNR-Istituto di Nanoscienze, Via Campi 213A, I-41125 Modena, Italy, 6 Istituto Officina dei Materiali del CNR (CNR-IOM), Unita ` di Perugia, c/o Dipartimento di Fisica, Via A. Pascoli, I-06123 Perugia, Italy, 7 Everspin Technologies, Inc., 1347 N. Alma School Road, Suite 220, Chandler, Arizona 85224, USA, 8 Functional Materials Division, School of Information Communication Technology, KTH – Royal Institute of Technology, Electrum 229, 164 40, Kista, Sweden, 9 Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden; † These authors contributed equally to this work. *e-mail: marco.madami@fisica.unipg.it; bonetti@kth.se LETTERS PUBLISHED ONLINE: 28 AUGUST 2011 | DOI: 10.1038/NNANO.2011.140 NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 1 © 2011 Macmillan Publishers Limited. All rights reserved.