news & views the magnetization aligned to the spin–orbit torque4 (Fig. 1b), that is, to replace the perpendicular ferromagnet with an in-plane one. However, in-plane ferromagnets are not suited to data storage applications because of their reduced thermal stability. Yu and colleagues propose instead to line up the spin–orbit torque with the magnetization, by introducing a lateral breaking of the structural symmetry 2. Their idea is to sandwich the ferromagnet between a heavy metal (Ta) and an oxide with a laterally varying stoichiometry (TaOx). An electric current orthogonal to the oxidation gradient induces a perpendicular spin–orbit torque that switches the magnetization (Fig. 1c). By studying the dependence of the switching efficiency on the degree of oxidation, the researchers propose that the symmetry breaks due to the lateral variations of the interfacial magnetic anisotropy that are associated with the varying oxygen content. Although this correlation can point to an effect originating from the spin–orbit interaction (which also creates the interfacial anisotropy), it may also be the sign of an artefact: by altering the micromagnetic properties of the structures, the anisotropy gradient may create a complex switching mechanism that complies with the experimental observations. The researchers ruled out this possibility by performing measurements of the strength of the torque using a recently developed technique, which is now the standard for quantitative torque measurements5–7. The

results support the researchers’ hypothesis of a novel torque mechanism. Although the researchers could observe the macroscopic switching of the magnetization, its microscopic mechanisms remain to be determined. In general, the origin of the spin–orbit interaction can be traced back to relativistic time–distance transformations. The crystalline electric field appears as a magnetic field to the spin that is sitting in the moving frame of the electron. Just like any other magnetic field, the spin–orbit field may act on the magnetic moment in two ways. First, if the crystalline field does have inversion asymmetry (for example, at an interface), the spin–orbit field will generate a torque that rotates the spin8 — the Rashba effect. Second, if the field is spatially inhomogeneous, its gradient can exert a force that deflects the electron trajectory. The resulting spin segregation is the origin of the spin Hall effect 9. Both these mechanisms (rotation and deflection) can induce spin polarization in metals and create torques in ferromagnets3,4. The spin rotation has been associated with interfacial effects because in the ferromagnetic transition metals the broken symmetry only occurs at interfaces, whereas the spin deflection, which does not require broken symmetry, is considered to arise mainly from the bulk. Because the torque direction and magnitude are controlled by the oxide/ ferromagnet interface, Yu and colleagues speculate that the mechanism responsible for their observations originates from a

horizontal Rashba effect. This picture is somewhat counterintuitive. As the oxidation varies very smoothly in-plane and abruptly out-of-plane, the lateral symmetry breaking is weaker than the vertical one. Nevertheless, the torques resulting from the two broken symmetries have similar magnitudes. The failure to explain the strength of the torques by simple arguments indicates that the microscopic mechanisms for the in-plane and out-of plane torques are likely to differ significantly. Independently of the detailed microscopic mechanisms, the spin–orbit torque described by Yu and colleagues could, in principle, be useful for magnetic random access memories. From a practical perspective this is both appealing and challenging: the structure employs materials already in use by industry, but manufacturing graded tunnel barriers with industrial quality will be extremely difficult. ❐ Ioan Mihai Miron is at the Centre national de la recherche scientifique, SPINTEC, Grenoble, France. e-mail: [email protected] References 1. Myers, E. B., Ralph, D. C., Katine, J. A., Louie, R. N. & Buhrman, R. A. Science 285, 867–870 (1999). 2. Yu, G. et al. Nature Nanotech. 9, 548–554 (2014). 3. Miron, I. M. et al. Nature 476, 189–193 (2011). 4. Liu, L. et al. Science 336, 555–558 (2012). 5. Pi, U. H. Appl. Phys. Lett. 97, 162507 (2010). 6. Kim, J. et al. Nature Mater. 12, 240–245 (2013). 7. Garello, K. et al. Nature Nanotech. 8, 587–593 (2013). 8. Edelstein, V. M. Solid State Commun. 73, 233–235 (1990). 9. Hirsch, J. E. Phys. Rev. Lett. 83, 1834–1837 (1999).


Channelling spin waves

Spin waves generated by a spin-torque nano-oscillator can be propagated in a magnonic nanowaveguide fabricated next to the oscillator.

R. K. Dumas and J. Åkerman


pin waves, which can be thought of as a magnetic analogue of sound or light waves, can exhibit both classical and quantum-mechanical properties including quantization, localization, interference, reflection, Doppler effects, and even tunnelling. In magnonics1,2, the aim is to use spin waves, or their quantized particlelike counterparts called magnons, for information transmission and processing. The wavelengths of magnons are orders of magnitude shorter than photons of the same frequency, which makes them suitable for integration into nanoscale

magnonic devices. They can also offer new functionalities and better prospects for miniaturization compared with what is currently available in photonic or electronic devices. A complete magnonic device requires a spin-wave source, a mechanism to channel these spin waves to a functional medium, which is susceptible to some sort of external control, and finally, a spin-wave detector. Conventional sources of spin waves rely on inductive methods that require micrometre-sized antenna structures and are not suitable for integration into nanostructures.


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Writing in Nature Nanotechnology, Vladislav Demidov and co-workers at the University of Muenster, Emory University, and the Institute of Metal Physics, Russia now report the integration of a localized and nanoscale spin-torque-driven spinwave source with a magnonic waveguide, providing a practical route to integrated magnonic circuits3. As a spin-wave source, the researchers use a spin-torque oscillator 4, which has an intrinsic nanoscale footprint and potential applications in a variety of communications technologies. Importantly, 503

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Spin-torque oscillator spin-wave source H0 Cu bottom electrode

Dipole field

Figure 1 | A nanomagnonic waveguide architecture. The CoFe nanowire (blue) generates a dipolar magnetic field (red oval) that acts on the NiFe (grey) film on top of it. The dipolar field opposes the externally applied field, H0, thus locally reducing the magnetic field, which allows for an efficient matching of the frequencies of the spin-wave source to the propagating spin waves in the waveguide.

for their approach, nanocontact 5,6 spintorque devices can act as nanoscale sources of localized7 and propagating 8 spin waves. The typical architecture of a spin-torque oscillator relies on a trilayer thin-film structure where two magnetic layers, designated as the ‘fixed’ and ‘free’ layers, are separated by a non-magnetic spacer that acts to decouple the magnetic layers. The fixed layer behaves as a spin filter, polarizing the stream of electrons. As a result, the electrons carry a significant amount of angular momentum that can then supply a torque onto the magnetization of the free layer, after they have traversed the non-magnetic spacer. This spin-transfer torque9,10 can induce oscillations, typically in the gigahertz frequency range, or even switch the equilibrium orientation of the magnetization of the free layer. These dynamical processes locally generate spin waves11. The main challenge faced by researchers looking to integrate a spin-torque-driven spin-wave source with a nanoscale


waveguide is finding a waveguide structure that is matched with the source. Demidov and colleagues’ solution is to fabricate a CoFe nanowire beneath a continuous NiFe film, as shown in Fig. 1. The nanowire is 200 nm wide and 5 nm thick, and the nanocontact spin-torque oscillator is situated at one of its ends. The nanowire acts as a magnonic waveguide thanks to its dipolar magnetic field, which decreases the internal magnetic field inside the NiFe directly on top. This field reduction then lowers the band of frequencies of the allowed propagating spin wave, resulting in an overlap with the frequencies generated by the nano-oscillator. This spectral matching allows for efficient channelling of the generated spin waves down the length of the nanowire waveguide. The researchers are then able to use micro-focused Brillouin light scattering to spatially probe the directionality of the spin-wave propagation, with a resolution of approximately 250 nm. They find a characteristic propagation length of 1.3 μm down the length of the waveguide. Together with micromagnetic

simulations, they also revealed that the modes responsible for the observed propagation length are propagating edge modes, characterized by a particularly large group velocity. These results are an important first step towards the realization of magnonic devices, and demonstrate that spin waves can be locally generated and then transported at will. It seems likely that the role played by nano-contact spin-torque oscillators will soon be expanded beyond spin-wave generation to include spinwave manipulation and detection, thus realizing all the fundamental magnonic building blocks12 necessary for a complete and functional magnonic device. That being said, magnonics is still in its infancy and much work remains to be done. It is important to find methods that decrease the spin-wave damping, which, although relatively low in materials like NiFe and CoFe, is still prohibitively large for most applications. Future efforts should be more devoted to developing conducting and insulating materials with ultralow damping, such as Heusler alloys and yttrium iron garnet thin films. Perhaps, clever utilization of pure spin currents using, for example, the spin Hall effect may also prove fruitful in extending the propagation length of spin waves generated by spin-transfer torque. ❐ R. K. Dumas and J. Åkerman are in the Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden. e-mail: [email protected] References 1. Neusser, S. & Grundler, D. Adv. Mater. 21, 2927–2932 (2009). 2. Kruglyak, V. V., Demokritov, S. O. & Grundler, D. J. Phys. D 43, 264001 (2010). 3. Urazhdin, S. et al. Nature Nanotech. 9, 509–513 (2014). 4. Kiselev, S. I. et al. Nature 425, 380–383 (2003). 5. Mancoff, F. B., Rizzo, N. D., Engel, B. N. & Tehrani, S. Nature 437, 393–395 (2005). 6. Rippard, W. H. & Silva, T. J. J. Magn. Magn. Mater. 320, 1260–1271 (2008). 7. Demidov, V. E., Urazhdin, S. & Demokritov, S. O. Nature Mater. 9, 984–988 (2010). 8. Madami, M. et al. Nature Nanotech. 6, 635–638 (2011). 9. Berger, L. Phys. Rev. B 54, 9353–9358 (1996). 10. Slonczewski, J. C. J. Magn. Magn. Mater. 159, L1–L7 (1996). 11. Slonczewski, J. C. J. Magn. Magn. Mater. 195, L261–L268 (1999). 12. Bonetti, S. & Åkerman, J. Topics Appl. Phys. 125, 177–186 (2013).


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Spintronics: channelling spin waves.

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