Realization of a spin-wave multiplexer

ARTICLE Received 30 Jan 2014 | Accepted 26 Mar 2014 | Published 23 Apr 2014 DOI: 10.1038/ncomms4727 Realization of a spin-wave multiplexer K. Vogt1,2, F.Y. Fradin3, J.E. Pearson3, T. Sebastian1,4, S.D. Bader3,5, B. Hillebrands1, A. Hoffmann3 & H. Schultheiss3,4 Recent developments in the field of spin dynamics—like the interaction of charge and heat currents with magnons, the quasi-particles of spin waves—opens the perspective for novel information processing concepts and potential applications purely based on magnons without the need of charge transport. The challenges related to the realization of advanced concepts are the spin-wave transport in two-dimensional structures and the transfer of existing demonstrators to the micro- or even nanoscale. Here we present the experimental realization of a microstructured spin-wave multiplexer as a fundamental building block of a magnon-based logic. Our concept relies on the generation of local Oersted fields to control the magnetization configuration as well as the spin-wave dispersion relation to steer the spin-wave propagation in a Y-shaped structure. Thus, the present work illustrates unique features of magnonic transport as well as their possible utilization for potential technical applications. 1 Fachbereich Physik and Forschungszentrum OPTIMAS, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany. 2 Graduate School of Excellence ‘‘MAterials science IN mainZ’’, Gottlieb-Daimler-Strasse 47, D-67663 Kaiserslautern, Germany. 3 Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. 4 Institut für Ionenstrahlphysik und Materialforschung, Helmholtz-Zentrum Dresden-Rossendorf, D-01328 Dresden, Germany. 5 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA. Correspondence and requests for materials should be addressed to H.S. (email: ). NATURE COMMUNICATIONS | 5:3727 | DOI: 10.1038/ncomms4727 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4727 R ecently, several new concepts were presented that associated with a novel magnonic approach for information transport and processing1–7. Magnons are the spin-wave excitation quanta of magnetic materials. They can be understood classically as the collective precession of the electrons’ spins. The collective nature is mediated by the short-range quantum– mechanical exchange interaction, as well as the non-local magnetic dipole interaction. Spin waves carry spin angular momentum, as do electrons in spintronic applications8,9, and enable the coherent transport of information encoded in its phase and amplitude. However, in contrast to spin-polarized electronbased spintronics, spin waves do not require dissipative charge transport that is subject to unavoidable scattering of individual electrons. Spin waves are collective excitations of the electronic system and propagate over significantly larger distances compared with typical electron spin diffusion lengths in metals or semiconductors, even at room temperature. Typical loss channels for spin waves, for example, spin–orbit interaction, can be minimized by the choice of suitable materials like yttrium iron garnets10 or Heusler compounds11,12. Furthermore, spin waves have the advantage that their frequencies increase faster for decreasing feature sizes in comparison to electromagnetic waves. Thus, even at 0.1–1 THz, spin-wave wavelengths can be as short as only a few nanometres, which is orders of magnitudes smaller compared with electromagnetic waves. This facilitates higher integration density in spin-wave logic devices. Spin waves exhibit a peculiarity that differentiates them from light or sound waves: their dispersion relation is highly anisotropic13,14. This means that their energy significantly depends on the relative angle between their propagation direction and the magnetization orientation. As a result, if the magnetic material is exposed to a uniform external magnetic field, the spin waves will travel in the preferred direction with respect to the magnetization orientation. The need for such external magnetic fields has hampered the development of spin-wave devices that are based on combining spin waves in two dimensions. Thus, even though simulations of nanometre-sized Mach–Zehnder-type interferometers demonstrate the feasibility to build spin-wave logic gates15,16, their realization to date has only been accomplished on the millimetre scale17. Herein we demonstrate a simple implementation of a microstructured spin-wave multiplexer, which serves as an initial step towards the development of a viable spin-wave-based processor. In our approach, locally generated magnetic fields, rather than uniform external fields, alter the magnetization direction solely in designated regions of the spin-wave multiplexer. This enables us to tailor the dispersion relation in different parts of the structure to our advantage18. Thereby, we exploit the unique feature of the anisotropic dispersion relation of spin waves, and, thus, are able to switch spin-wave propagation directions. Results Sample characteristics and concept. Figure 1a shows an optical microscope image of the device. Using electron-beam lithography and lift-off techniques, a 30-nm-thick Ni81Fe19 (permalloy, Py) layer was patterned into a Y-shaped spin-wave waveguide having a constant width of 2 mm. To evaluate a possible angle dependence of the spin-wave propagation, structures with the opening angles 30°, 60° and 90° were fabricated and tested experimentally. In the following we will concentrate on the 60° structure where the best results were obtained. A comparison of the different structures will be presented later in the article. To control the magnetization direction in the Y structure with local Oersted fields, a 50-nm thick and 3-mm wide Au conduit 2 was fabricated below the spin-wave waveguide. Leads at the three ends of the Au conduit allow for connecting either its base and left arm (switch in Fig. 1a in position S1) or its base and right arm (switch in position S2) to an electric current source, respectively. To keep the electric current from flowing in the magnetic material, a 50-nm thick MgO layer serves as an insulator from the Au conduit. A microwave current in the shorted coplanar waveguide generates oscillating magnetic fields Hrf that excite spin waves in the Py waveguide. For in-plane magnetized films, the anisotropic dispersion relation favours propagation perpendicular to the magnetization direction M. As depicted by the dispersion relations19 in Fig. 1b (calculated with the material parameters for Ni81Fe19: saturation magnetization: MS ¼ 800 kA m  1, gyromagnetic ratio: g/2p ¼ 28 GHz T  1 and exchange constant: A ¼ 1.6  10  11J m  1), a perpendicular orientation between the spin-wave wave vector k and the magnetization direction M (dashed, blue line, geometry II) results (...truncated)