Composite topological structure of domain walls in synthetic antiferromagnets

www.nature.com/scientificreports OPEN Received: 29 June 2018 Accepted: 7 October 2018 Published: xx xx xxxx Composite topological structure of domain walls in synthetic antiferromagnets A. G. Kolesnikov1, V. S. Plotnikov1, E. V. Pustovalov1, A. S. Samardak L. A. Chebotkevich1, A. V. Ognev1 & Oleg A. Tretiakov 4,1 1,2,3 , We experimentally study the structure and dynamics of magnetic domains in synthetic antiferromagnets based on Co/Ru/Co films. Dramatic effects arise from the interaction among the topological defects comprising the dual domain walls in these structures. Under applied magnetic fields, the dual domain walls propagate following the dynamics of bi-meronic (bi-vortex/bi-antivortex) topological defects built in the walls. Application of an external field triggers a rich dynamical response: The propagation depends on mutual orientation and chirality of bi-vortices and bi-antivortices in the domain walls. For certain configurations, we observe sudden jumps of composite domain walls in increasing field, which are associated with the decay of composite skyrmions. These features allow for the enhanced control of domain-wall motion in synthetic antiferromagnets with the potential of employing them as information carriers in future logic and storage devices. In recent years there was an enormous interest in complex magnetic nanostructures due to a wide variety of novel effects arising from their magnetic properties and the corresponding prospects for their applications for future spintronic memory, logic, and sensors1–3. In particular, significant efforts have been focused on studying statics and dynamics of domain walls (DWs) in ferromagnetic wires4–22 and thin films with perpendicular magnetic anisotropy23–27. In ferromagnet/heavy-metal multilayer systems, very high velocities have been observed for chiral DWs24,28,29. Topology of the magnetic textures has been shown to play an essential role for the potential applications, leading to new ideas of employing compact topological spin textures – skyrmions – for the racetrack memory prototypes30. This boosted various observations of skyrmions in multilayer materials, e.g. using a constriction in a trilayer system to create skyrmions at room temperatures31 and demonstrate their current-driven dynamics32. The observation of room-temperature magnetic skyrmions has been also established in ultrathin metallic ferromagnetic (FM) multilayers, such as Pt/Co/Ta and Pt/CoFeB/MgO33, and the skyrmion Hall effect has been revealed by time-resolved X-ray microscopy34. More recently magnetic bilayer-skyrmions experiencing no skyrmion Hall effect have been predicted in antiferromagnetically coupled FM bilayers35, similarly to real antiferromagnetic (AFM) systems36,37. The absence of skyrmion Hall effect allows for better skyrmion motion control in future nanodevices, and antiferromagnetically coupled FM bilayer systems – synthetic antiferromagnets (SAFs)38,39 – offer a particular advantage, since this effect is easier to observe in them and employ for spintronic applications. An antiferromagnetic interlayer exchange coupling occurs as a result of oscillatory RKKY interaction for a particular thickness of the intermediate layer between the two ferromagnets40. Especially strong AFM coupling is reached in multilayers with the interlayer of Ru41,42, which is also favorable for coupling the ferromagnetic layers with perpendicular magnetic anisotropy43,44. SAFs were already demonstrated to have serious advantages for applications: high speeds of the DWs have been observed in them43 and the magnetic microscopy has shown the topological stability of homochiral Néel DWs45. As we show below, SAFs possess much richer diversity of magnetic topological defects, especially for materials with in-plane anisotropy, however this fact began to be explored experimentally only very recently. Earlier most of theoretical and experimental studies have been devoted to the films with the indirect ferromagnetic coupling46, where e.g. asymmetric domain nucleation and unusual magnetization reversal has been observed in ultra-thin Co films with perpendicular anisotropy47. 1 School of Natural Sciences, Far Eastern Federal University, Vladivostok, 690950, Russia. 2Center for Spin-Orbitronic Materials, Korea University, Seoul, 02841, Republic of Korea. 3National Research South Ural State University, Chelyabinsk, 454080, Russia. 4Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan. Correspondence and requests for materials should be addressed to O.A.T. (email: ) SCientifiC REPOrTS | (2018) 8:15794 | DOI:10.1038/s41598-018-33780-6 1 www.nature.com/scientificreports/ Figure 1. Domain wall building blocks: (a) Vortex, (b) Antivortex, (c) Transverse domain wall, (d) Néel domain wall. (e–h) Vortex (antivortex) pairs with polarities p = + 1 (−1) in the top and bottom layers. (e) Vortex pair with p = + 1, p = − 1; (f) Vortex pair with p = + 1, p = + 1; (g) Antivortex pair with p = + 1, p = − 1; and (h) Antivortex pair with p = − 1, p = + 1. In this article we reveal the composite structure of magnetic domains in SAFs with in-plane anisotropy based on Co/Ru/Co films. We find that in this system both dual Néel (NDW) and transverse (TDW) domain walls are present and connected by dual topological defects – bi-merons, which are pairs of coupled vortices or antivortices in the upper and lower layers of SAF, see Fig. 1(e–h). Generally, merons are nontrivial topological objects with half of the topological charge of a skyrmion48,49, and in the case of ferromagnetic films are magnetic vortices (with +1/2 topological charge) or antivortices (with −1/2 topological charge). However, in SAFs because of the SCientifiC REPOrTS | (2018) 8:15794 | DOI:10.1038/s41598-018-33780-6 2 www.nature.com/scientificreports/ Figure 2. (a–d) The Lorentz microscopy images of the domain structure for the films Co/Ru(0.9 nm)/Co are shown for the fields from 0 to 0.8 kOe. (e–h) The MFM images of the domain structure of a different part of the same film. presence of two ferromagnetic layers these objects are bi-merons, e.g. a pair of magnetic vortices (or antivortices). Furthermore, we show that under the influence of magnetic field the magnetization reversal processes in the SAF domains are determined by the dynamics of these composite DWs coupled in the two layers. The controllable transformations of the DW types we observe in the process of magnetization reversal are demonstrated to be governed by the motion and annihilation of magnetic bi-merons. Our experimental results are supported by both micromagnetic simulations and general topological arguments. Results Magnetization measurements. To investigate the magnetic properties of Co/Ru/Co films with AFM exchange coupling we have measured the hysteresis loops by longitudinal magneto-optical Kerr effect with fields applied parallel to the plane of the film. The shape of the hysteresis loop at the first AFM max (...truncated)