Direct observation of a propagating spin waveinduced by spin-transfer torqueM. Madami1†*, S. Bonetti2†*, G. Consolo3,4, S. Tacchi1, G. Carlotti1,5, G. Gubbiotti1,6, F. B. Mancoff7,

M. A. Yar8 and J. Åkerman2,9

Spin torque oscillators with nanoscale electrical contacts1–4 areable to produce coherent spin waves in extended magneticfilms, and offer an attractive combination of electrical andmagnetic field control, broadband operation5,6, fast spin-wavefrequency modulation7–9, and the possibility of synchronizingmultiple spin-wave injection sites10,11. However, many potentialapplications rely on propagating (as opposed to localized) spinwaves, and direct evidence for propagation has been lacking.Here, we directly observe a propagating spin wave launchedfrom a spin torque oscillator with a nanoscale electricalcontact into an extended Permalloy (nickel iron) film throughthe spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, showthat spin waves with tunable frequencies can propagate forseveral micrometres. Micromagnetic simulations provide thetheoretical support to quantitatively reproduce the results.

Much effort has recently been devoted to a better understandingof the details of the spin waves excited in magnetic films by nano-contact-based spin torque oscillators (STOs)12,13. In particular, ithas 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 theexternally applied field angle)14–16. Only very recently has it beendemonstrated experimentally (by means of electrical microwavedetection) that above a certain critical angle a propagating spin-wave mode can be excited, and both localized and propagating spinwaves can be excited alternately below this critical angle17.Although it was possible to elucidate a number of important charac-teristics of both propagating and localized spin-wave modes usingonly 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 powerfultechnique for resolving the spatial profile of spin waves in magneticnanostructures, and has recently been used in pioneering studies19,20

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 beenproven experimentally.

Here, we usem-BLS to study spin waves emitted in an out-of-planemagnetized nanocontact STO and provide experimental proof thatpropagating spin waves are radially emitted from the nanocontact

region into the continuous ferromagnetic thin film up to severalmicrometres away from the nanocontact.

The sample under investigation comprises a pseudo spin valvestack with the layer structure Co81Fe19(20 nm)/Cu(6 nm)/Ni80Fe20(4.5 nm), patterned into an 8 × 26 mm2 mesa. The thicker CoFelayer is considered the ‘fixed’ magnetic layer, and the thinner NiFeplays the role of the low-dissipation ‘free’ magnetic layer in which asteady STT-driven spin wave can be sustained. The thickness of thecopper spacer (6 nm) ensures that there is negligible interlayerexchange coupling between the two magnetic layers. A circularcontact of diameter d¼ 200 nm is patterned in the middle of thespin valve mesa, and a thick (400 nm) aluminium ground–signal–ground waveguide is deposited on top of the mesa, allowing forthe injection of a high, spin-polarized, current density21 and thesubsequent extraction of the generated microwave voltage.

Optical m-BLS access to the active region of the free layer wasachieved through a combination of focused ion-beam (FIB) and

Hext

Cu spacerNi80Fe20 free layer

Co81Fe19 fixed layer

Optical window

Pd-Cutop electrode

Pd-Cubottom electrode

Al coplanarwaveguide

SiO2insulator

d.c. sourceProbing

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.

1CNISM, Unita di Perugia and Dipartimento di Fisica, Universita di Perugia, Via A. Pascoli, I-06123 Perugia, Italy, 2Materials Physics, School of InformationCommunication Technology, KTH – Royal Institute of Technology, Electrum 229, 164 40, Kista, Sweden, 3Dipartimento di Scienze per l’Ingegneriae l’Architettura, Universita di Messina C.da di Dio, I-98166 Messina, Italy, 4CNISM, Unita di Ferrara, Via G. Saragat 1, I-44100 Ferrara, Italy, 5Centro S3,CNR-Istituto di Nanoscienze, Via Campi 213A, I-41125 Modena, Italy, 6Istituto Officina dei Materiali del CNR (CNR-IOM), Unita di Perugia,c/o Dipartimento di Fisica, Via A. Pascoli, I-06123 Perugia, Italy, 7Everspin Technologies, Inc., 1347 N. Alma School Road, Suite 220, Chandler, Arizona85224, USA, 8Functional Materials Division, School of Information Communication Technology, KTH – Royal Institute of Technology, Electrum 229, 164 40,Kista, Sweden, 9Department 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

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selective wet-etching processes to open an optical window (dimen-sions, 4 × 6 mm2) in the aluminium waveguide at a distance of�500 nm from the nanocontact (Fig. 1). An example of such amodified sample is shown in Fig. 2a. To validate the effectivenessof this process we performed two separate checks, includingenergy dispersive spectroscopy (EDS) and m-BLS measurementsof thermal spin waves in two regions of the sample. The entireprocedure is described in the Methods, and the results prove thatwe are able to access a portion of the free magnetic layer close tothe nanocontact region.

In our first set of measurements, we focused the laser spot in themiddle of the optical window. We detected a strong spin-wave signalonly for one current polarity, which corresponds to electrons drift-ing from the free to the fixed magnetic layer. No signal was detectedfor the opposite current polarity (shown in the inset of Fig. 3).Remarkably, not only was the intensity of the emitted spin wavemuch larger than that of the thermal spin waves, but the frequencyf of the excited mode was also considerably higher than that of the

ferromagnetic resonance (FMR) mode. This is consistent with theexpected blueshift of propagating spin waves excited by STT in ananocontact geometry when the free layer is magnetized out ofthe film plane14,15. Figure 3 shows f as a function of the injecteddirect current I for two different values of the perpendicular fieldm0Hext. In both cases, f exhibits an almost linear increase with I,resulting in current and field tunabilities consistent with all-electri-cal results obtained in previous works on the same samples6,21.These behaviours have been reproduced by micromagnetic simu-lations, the results of which are also shown in Fig. 3. Simulationsalso reveal that, as I is increased, the wavelength of the excitedspin waves decreases from �300 nm at 40 mA to �200 nm at80 mA (Fig. 5, inset).

As a second step of the dynamic characterization of our sample,we demonstrated that the emitted spin waves have indeed a propa-gating character. To this aim, we performed wave-vector-resolvedm-BLS measurements by using the procedure described in ref. 22and illustrated in Fig. 4. Because the detected spin wave is excitedby STT within the nanocontact region and propagates away fromit, its wave vector (KSW) has a well-defined direction in the planeof the free layer. As a consequence, photons that undergo a Stokes(anti-Stokes) process (in other words that create (destroy) a spin-wave quantum, or magnon) will be scattered at opposite angleswith respect to the sample normal. This is analogous to the positiveor negative Doppler shift, which affects light beams diffracted inopposite directions, after interaction with propagating acousticwaves in Raman-Nath acousto-optic modulators23. To understandwhether such a process occurs in our system, we use a beamshutter by setting, alternately, its full or half aperture towards theinterferometer. When the full beam is sent to the interferometer(Fig. 4, lower spectrum), two spin-wave peaks are present, one onthe Stokes side of the spectrum and the other on the anti-Stokesside. Alternatively, by selecting half of the beam, one of the twopeaks disappears, depending on which half of the beam is selected(Fig. 4, middle and upper spectra). This result clearly demonstratesthat the emitted mode propagates with a uniquely defined wave-vector direction. In fact, in the presence of a stationary or localized(not propagating) wave, counts are expected on both the Stokes andanti-Stokes regions of the spectrum, whatever the selected half of the

10 μm

r

Nanocontact

Opticalwindow

O Si Cu Co Pd Ni Fe AlElement

Rela

tive

atom

ic c

onte

nt (%

)

Inte

nsity

(a.u

.)

Outside

Inside

Outside

Inside

Frequency (GHz)−12 −10 −8 −6 −4

35

30

25

20

15

10

5

0

a

b

Figure 2 | Characterization of the optical window. a, Scanning electron

microscope image of a processed device, showing the optical window in the

central conductor of the aluminium waveguide, the nanocontact approximate

position and the line (dotted arrow) across which the m-BLS laser spot was

scanned. b, EDS data acquired in regions outside and inside the etched

window (indicated by dashed and solid squares in a, respectively). Inset:

experimental m-BLS spectra (Stokes side), measured outside and inside the

etched optical window in the absence of any injected current and within an

applied perpendicular field of 2.0 kOe, revealing the presence of thermal

spin waves.

20

10

0FMR

−20 −15 −10

I = −70 mA

I = +70 mA

−5Frequency (GHz)

Inte

nsity

(a.u

.)μ0Hext = 0.6 T

20

FMR

10

040

μ0Hext = 0.7 T

50Modulus of the applied d.c. |I| (mA)

Spin

-wav

efre

quen

cy, f

(G

Hz)

Spin

-wav

efre

quen

cy, f

(G

Hz)

60 70 80 90 100

40 50Modulus of the applied d.c. |I| (mA)

60 70 80 90 100

Figure 3 | Spin-wave frequencies as a function of the injected d.c.

intensity. Measured (filled symbols) and simulated (open symbols) spin-

wave frequency dependence on d.c. intensity for two different values of the

magnetic field. Dashed lines represent the calculated FMR frequency.

Inset: m-BLS spectra (Stokes side) recorded at m0Hext¼0.6 T and for

different signs of the current |I|¼ 70 mA.

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collected light beam. Note, however, that in the present case ofm-BLS, one also has to carefully consider the effect of uncertaintyin the wave-vector conservation due to the sub-micrometricdimension of the illuminated area. This is quantitatively discussedin the Methods.

At the final stage of our study, evolution of the emitted spin-waveintensity as a function of distance from the nanocontact (r) wasmeasured by performing a scan of the laser spot across the openoptical window (dotted arrow in Fig. 2a). At a constant current ofI¼ 70 mA and a bias magnetic field of m0Hext¼ 0.6 T, we measureda detectable spin-wave signal in the entire optical window, that is, up

to a maximum distance from the contact of �4 mm. The results areshown in Fig. 5, where a marked reduction of the spin-wave inten-sity (J(r)) with distance from the nanocontact (r) is clearly observed.This decreasing behaviour can be accounted for if one considersboth the cylindrical symmetry of the emitted spin wave and thefinite propagation length in the free layer due to the intrinsicdamping of Permalloy. The former can be easily described by a1/r dependence, and the latter can be accounted for by an exponen-tial term exp(–r/lr) (ref. 19). Here, lr¼ vg/(2av) representsthe decay length related to the spin-wave group velocity (vg), a isthe Gilbert damping parameter and v¼ 2pf is the spin-waveangular frequency ( f¼ 15.3 GHz). By representing the spin-waveintensity as J(r)¼ ((J0/r)e2r/lr), in the range of values correspond-ing to the open optical window, and by extracting the characteristicparameters (vg¼ 3.3 mm ns21, a¼ 0.008) from the micromagneticsimulations already exploited to fit the frequencies in Fig. 3, weobtained a very good agreement with the experimental data. Thedecay length value lr¼ 2.1 mm thus obtained is very similar toliterature values for Permalloy24.

We have provided experimental evidence that spin waves emittedfrom an out-of-plane magnetized nanocontact STO into a continu-ous NiFe film propagate unidirectionally several micrometres awayfrom the nanocontact. Our findings show that STOs represent avery attractive nanoscale current- and field-controlled broadbandspin-wave generator for future use in magnonic devices. Thisstudy is also relevant for the ongoing attempts to synchronizelarge arrays of spin torque oscillators by means of the emission ofpropagating spin waves10,11,25, with the aim of achieving theminimum emitted power required for moving these devices out ofthe laboratory and into actual microwave applications.

MethodsExperimental. Giant magnetoresistance (GMR) films were sputter-deposited inultrahigh vacuum on silicon wafers coated with SiO2. The GMR film stack consistedof a seed layer of 5 nm palladium, a 25 nm copper bottom electrode, a 20 nmCo81Fe19 fixed layer, a 6 nm copper spacer layer, a 4.5 nm Ni80Fe20 free layer, a 2 nmcopper top electrode and a 3.5 nm palladium cap. The GMR films were patterned onan 8 × 26 mm2 mesa using optical lithography and then coated with a SiO2 interlayerdielectric deposited by chemical vapour deposition. The point contact area wasdefined using electron-beam lithography and reactive ion etching through the SiO2.Finally, a 400 nm aluminium top electrode was patterned by optical lithography,sputter deposition and lift-off.

For FIB processing of the optical window, we used a dual-beam FEI Quanta-3Dfield-emission gun (FEG) FIB-scanning electron microscope system, where bothelectron (5 kV) and Gaþ (30 kV, 50 pA) beams were operated simultaneously tomonitor the etching process in real time. EDS was used to compare the chemicalcomposition of two regions of the sample: inside the optical window (black square inFig. 2a) and between the signal and ground lines (dashed square in Fig. 2a). The resultsof this analysis, shown in Fig. 2b, demonstrate that aluminium has been completelyremoved, but the SiO2 insulating layer (transparent to visible light) is still present. Wealso performed m-BLS (ref. 26) measurements of thermal spin waves in the same tworegions of the sample. Results are shown in the inset in Fig. 2b. We detected thermalspin waves with the same f and virtually the same amplitude both inside the opticalwindow and between the signal and ground aluminium lines. Once all the preliminaryanalyses were performed, a projected field electromagnet was placed below the sample,in close proximity to it, with the aim of providing a tunable source of a perpendicular-to-plane bias magnetic field (m0Hext) up to 0.7 T. In addition, by means of a low-noiseelectric d.c. source, an electric direct current (I ) was allowed to flow through thenanocontact. To stabilize the sample properties against irreversible heating effectsduring the set of measurements, these were performed, for each value of the appliedfield, by decreasing the applied current from an initial higher value (80 mA).

Another important issue is the uncertainty in the in-plane component of thewave vector of scattered photons resulting from the limited spatial extent of the laserspot on the sample. In a previous work27 we estimated the full-width at half-maximum of our laser spot to be 235 nm. Using this value as DX in the waveuncertainty relation DX.DK ≈ 2p, one finds the uncertainty in the values of thein-plane component of the photon wave vector involved in the scattering process tobe DK ≈ 27 × 104 cm21. This uncertainty is almost equal to the modulus of thespin-wave wave vector KSW¼ 2p/lSW because, from the simulated wavelength(Fig. 5, inset) it varies in the range 22 × 104 to 31 × 104 cm21. It follows that there isa spread in the scattering angle: u¼ arctan[(KSW+DK )/Klight], where Klight ≈ 12 ×104 cm21 (the scattering angle, measured against the normal to the sample, isindicated in Fig. 4). For instance, at I¼ 70 mA, u varies from �08 to 758. In the

Stokesprocess

Anti-Stokesprocess

θ−20

Frequency shift (GHz)−10 0 10 20

KL

KSW

a b

Figure 4 | Proof of spin-wave propagation. a, Schematic of the experimental

procedure used to prove the propagating character of the detected spin

wave. KL and KSW represent the wave vectors of the incoming light and of

the emitted spin wave, respectively. b, Measured m-BLS spectra (I¼ 70 mA

and m0H¼0.6 T) corresponding to the case of fully opened (bottom

spectrum) and partially closed (upper spectra) collected beam.

2.05

10

15

20

25

2.5

Inte

grat

ed in

tens

ity, J

(r) (

a.u.

)

3.0 3.5Distance from the contact r (μm)

40 60

300

200

I (mA)

λ SW

(nm

)

80

μ-BLS measurements

Calculated intensity

4.0

Figure 5 | Spin-wave attenuation as a function of distance from the STO.

Integrated intensity (symbols) of the spin-wave excitations detected using

m-BLS as a function of distance from the centre of the point contact (r).

Analytical calculation (line) of the decay obtained using the function

described in the text. Inset: simulated spin-wave wavelength as a function

of applied d.c. intensity.

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presence of a collecting angle of our objective of about+508 (NA¼ 0.75), thisdemonstrates the feasibility of our wave-vector selection method (illustrated inFig. 4), because the uncertainty does not cause the Stokes and anti-Stokes conesof emission to overlap spatially.

Using the same argument, one finds that it is important to use a relatively largecontact size because our m-BLS setup can detect spin waves, with sizable intensity,down to wavelengths of �200 nm.

Numerical. Micromagnetic simulations were performed using a three-dimensionalcode that integrates the Landau–Lifshitz–Gilbert–Slonczewski equation of motion bymeans of a finite-difference time-domain (FDTD) approach15,28,29. The effective fieldincludes all the main standard micromagnetic contributions arising fromdemagnetizing, exchange, Zeeman and Oersted fields. We neglect thermalfluctuations (which only affect the linewidths of the output signal) andmagnetocrystalline anisotropy (as usual for Permalloy-based materials). A uniformlydistributed current density within the current-carrying region, which exhibitsan abrupt cutoff outside that area, has been considered. We also assume that thespin-transfer torque perturbation acts only on the thinner free layer. Because ofobvious constraints on computational time and memory allocation, a reducedcomputational region with sides of 1 mm and with the nanocontact in the middlewas simulated. To reduce the spurious effect of spin-wave reflection from thecomputational boundaries, absorbing boundary conditions have beenimplemented28. The computational domain was discretized in prismatic cells of4 × 4 × 5 nm3. The material parameters used in our setup are: a saturationmagnetization m0MS¼ 0.5 T, Gilbert damping constant a¼ 0.008, spin-torqueefficiency¼ 0.25, exchange constant A¼ 1.0 × 10211 J m21 and nanocontact radius(RC¼ 120 nm). We believe that the slightly reduced values of the exchange constantand saturation magnetization with respect to their nominal values are a consequenceof the limited NiFe thickness and possibly also due to the effect of local heating in thearea of the nanocontact, leading to local oxidation and copper interdiffusion. Similarvalues were recently used in micromagnetic simulations of spin torque excitations inthin NiFe nanowires30. The increased value of the nanocontact radius takes intoaccount the local current spreading inside the extended free layer20. The magneticfield implemented in the simulations was allowed to be 10% higher than the nominalexperimental value and applied at 858 with respect to the sample plane, reflecting theuncertainty in the intensity and direction of the field projected by the magnet.

Received 20 June 2011; accepted 22 July 2011;published online 28 August 2011

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AcknowledgementsThis work was supported by CNISM under the m-BLS INNESCO project. Authorsacknowledge the European Community’s Seventh Framework Programme (FP7/2007-2013, grant agreement no. 228673, MAGNONICS). Support from the Swedish Foundationfor Strategic Research (SSF), the Swedish Research Council (VR) and the Knut and AliceWallenberg Foundation is gratefully acknowledged. J.Å. is a Royal Swedish Academy ofSciences Research Fellow supported by a grant from the Knut and Alice WallenbergFoundation. The authors gratefully acknowledge S. Redjai Sani at the Royal Institute ofTechnology for help with the wet etching process, F. Magnusson and W. Michelsen atNanOsc AB for their help in designing the printed circuit boards, and S. Gunnarsson,S. Sandelin and K. Penkkila at Sivers IMA AB for performing the wire bonding.S.B. gratefully acknowledges support from the C.M. Lerici foundation.

Author contributionsM.M., G.G., S.T. and G.Ca. performed m-BLS measurements. S.B., M.A.Y. and J.Å. realizedthe procedure to open the optical access to the sample and performed EDS measurements.F.B.M. fabricated the original samples. G.Co. performed numerical simulations. All authorsco-wrote the manuscript.

Additional informationThe authors declare no competing financial interests. Reprints and permission information isavailable online at http://www.nature.com/reprints. Correspondence and requests for materialsshould be addressed to M.M. and S.B.

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