Fiber optics for spin waves

OPEN NPG Asia Materials (2016) 8, e246; doi:10.1038/am.2016.25 www.nature.com/am ORIGINAL ARTICLE Fiber optics for spin waves Xiangjun Xing1 and Yan Zhou2 Magnetic skyrmions in chiral magnets with the Dzyaloshinskii–Moriya interaction have received intensive attention because of their potential in prospective applications. Here, we theoretically demonstrate that another novel spin texture in chiral magnets— the chiral strip-domain wall (SDW)—can generate a deep one-dimensional potential well of magnetic origin. We show by micromagnetic simulations that the potential well caused by a SDW can serve as an internal channel to guide spin-wave (SW) propagation, which makes the ultrathin chiral magnet including the SDW become a reconfigurable self-cladding optic-fiber-like magnonic waveguide with a graded refractive index. Furthermore, we design logical NOT and NAND gates based on the statemodulated transmission property of the magnonic waveguide. We also reveal that a SDW can be reliably written into the gate arms using the Slonczewski spin torque. Finally, prospective applications of the observed potential well in other fields are envisioned. This work is expected to open new possibilities for SW guiding and manipulation in ultrathin magnetic nanostructures as well as to help shape the field of beam magnonics. NPG Asia Materials (2016) 8, e246; doi:10.1038/am.2016.25; published online 18 March 2016 INTRODUCTION Magnonic circuits are estimated to be capable of providing substantial throughput enhancement compared with those circuits currently based on complementary metal-oxide-semiconductor technology; special tasks such as advanced image processing and speech recognition that benefit from parallel processing of information will also be possible.1,2 To achieve multifunctionality of magnonic circuits, controlled propagation and manipulation of spin waves (SWs) in a rich variety of magnetic nanostructures are required. With the continued downscaling of magnetic films, the antisymmetric Dzyaloshinskii–Moriya interaction (DMI)3,4 arising from spinorbit scattering of itinerant electrons has manifested its roles in ultrathin samples with inversion-asymmetric interfaces,5,6 where various spin textures such as spin spirals,7 chiral domain walls8,9 and skyrmions10 were observed recently. As already shown in (refs 11–15), inhomogeneous spin textures may offer efficient means for tailoring spin-wave (SW) propagation in magnetic nanostructures. Consequently, ultrathin nanostructures with interface-induced DMI will become promising candidates for the construction of magnonic devices with versatile functionalities.1,16–18 So far, guided propagation and manipulation of SWs in chiral nanostructures containing unique spin textures have not yet been addressed, despite their strong relevance to potential applications of chiral magnets in magnonics as well as to understanding the strength of relevant interactions.19,20 In the widely used Damon–Eshbach (DE) propagation geometry,21 SWs in a strip-type waveguide—regardless of the center or edge modes22— have nonzero precession amplitude at the boundary;23 therefore, they might suffer from undesired scattering caused by boundary irregularities that have been found to result in reduced attenuation length.22 The self-cladding waveguide with well-defined internal channels suggested by Duerr et al.13 can resolve the edge-scattering problem, but the channels must be maintained by an applied field, which is not preferred in real devices. In this work, we demonstrate the realization of a novel type of graded-refractive-index magnonic waveguides with self-cladding internal nanochannels, in which SWs are trapped transversely and propagate an ultra-narrow beam less than 10 nm in beamwidth. The channeling effect stems from the strong inhomogeneity of the internal field in the direction normal to the strip-domain wall (SDW) stretching direction. The drastic decrease in this field inside the SDWs creates a narrow potential well that is deep enough to allow precession of magnetization at ultralow frequencies. In addition, multichannel SW propagation in a single waveguide is accomplished by introducing multiple SDWs into the waveguide (such a multi-SDW spin texture should exist ubiquitously in magnetic films with interfacial DMI, as has been observed in a few samples made of different materials and/or structures).9,24,25 Furthermore, we show that when the waveguide is toggled to a quasiuniform single-domain state, no SWs can enter and pass through the waveguide unless the frequency is increased to tens of gigahertzes. Finally, we implement logic-NOT and -NAND gates by virtue of the observed state-modulated SW transmission, and we demonstrate how a SDW can be reliably written into the gate’s arm to ensure the feasibility of this logic architecture. The advantage lies in its complete compatibility with racetrack memory,26 that is, it can be 1 Institute of Micro-Nano Structures and Optoelectronics, College of Physics and Electronic Information Engineering, Wenzhou University, Wenzhou, China and 2School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, China Correspondence: Professor Y Zhou, School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. E-mail: Received 6 July 2015; revised 20 November 2015; accepted 17 December 2015 Spin-wave channeling X Xing and Y Zhou 2 directly interconnected with the latter circuits because the unit (magnonic waveguide) used in these gates can, after proper reconfiguration, work as a nanotrack for skyrmion motion in skyrmion-based racetrack memories.27,28 MATERIALS AND METHODS Micromagnetic models The layout of the present study is shown in Figure 1a. The magnonic waveguide, in which SWs will be excited and guided, is composed of a rectangular magnetic plate elongated spatially in x direction. It should be patterned from an ultrathin magnetic film with inversion-symmetry-breaking interfaces to ensure the occurrence of an induced DMI5 required to stabilize chiral SDWs in the waveguide. In the following sections, we will demonstrate, as a first step, the fundamental principle of utilizing a SDW to channel SW propagation in a magnonic waveguide. Here, we chose a 1200-nm-long and 60-nm-wide waveguide, in which a single SDW is included (Figures 1,2,3 and Supplementary Figures 1). For comparison, however, we also checked SW propagation in the same waveguide with a distinct spin configuration. Subsequently, we will show that SW propagation along separate channels in parallel is attainable in a single waveguide. For this, the additional widths of 120 and 160 nm were adopted to accommodate more than one SDW (Figure 4). Finally, we will illustrate how to perform logic operations using the statemodulated SW output. To this end, a waveguide of 300 nm long and 60 nm wide was e (...truncated)