Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L7: Focus Session: Dynamics and Symmetries of Magnetic Domain Walls |
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Sponsoring Units: GMAG DMP Chair: Haifeng Ding, Nanjing University, China Room: 106 |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L7.00001: Chiral Magnetic Domain Wall Structure in Epitaxial Multilayers Invited Speaker: Yizheng Wu In magnetic ultrathin films, the common textbook picture distinguishes two canonical types of DWs: Bloch walls for perpendicularly magnetized films and N\'eel walls for in-plane magnetized films. It still remains an open question whether the Bloch wall should be necessarily the only type of DWs in perpendicularly magnetized ultrathin films although it has been the textbook example for a long time. In ultrathin film, the inversion symmetry broken at interface will induce Dzyaloshinskii-Moriya interaction (DMI). In this talk, we will show that the DMI at interface will induce the chiral N\'eel type domain wall in perpendicularly magnetized films. The spin structure in magnetic domain wall was identified in real-space at room temperature by spin-polarized low energy electron microscopy (SPLEEM). The chiral N\'eel-type domain wall was identified in the magnetic stripe domain phase in Fe/Ni/Cu(001), and the chirality can switch from the right-hand cycloid in Fe/Ni/Cu(001) to the left-hand cycloid in Ni/Fe/Cu(001), which indicates that the chirality is caused by the DMI mainly located at the Fe/Ni interface [1]. The chiral domain wall structure can also be observed in [Co/Ni]$_{n}$ multilayer grown on Pt(111) and Ir(111)[2]. We found that Pt(111) substrate can induce right-handed chirality, whereas Ir(111) substrate can induce left-handed chirality, moreover, the chirality of the DW evolves from right-handed to left-handed in [Co/Ni]$_{n}$ grown on Ir/Pt(111) by changing Ir thickness, and the DW near the transition point shows non-chiral Bloch-type. Our results prove that domain wall chirality together with the sign and strength of the DMI can be tuned through the interface engineering, which may enable more possibility for designing of new spintronics devices. This work was collaborated with G. Chen, J. Zhu, A. T. N'Diaye, T. P. Ma, H.Y. Kwon, C. Won, Y. Huo, J. Li, A. K. Schmid. \\[4pt] [1] G. Chen, et al., Phys. Rev. Lett. 110, 177204 (2013).\\[0pt] [2] G. Chen, et al., Nature Communication, 4,2671(2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L7.00002: Spiral Spin Texture at Domain-Wall driven by Dzyaloshinskii-Moriya Interaction Minjun Lee, Jeonghoon Kwon, Sungmin Kim, Seong Joon Lim, Young Kuk Controlling an electron spin and its measurement play a crucial role in spintronics. Especially in nano-scale devices local probing capability is important. From this point of view, spin-polarized scanning tunneling microscopy is a suitable local probing method for revealing surface spin texture. Here, we report the observation of spiral spin texture at a domain-wall. Spiral spin texture is driven by Dzyaloshinskii-Moriya (DM) interaction, which is realized by spin-orbit coupling of electrons in an inversion-asymmetric crystal field. We chose Co/Pt(111) system to study the DM interaction. Because Pt is a substrate with strong spin-orbit coupling, and Co is a ferromagnetic material with out-of-plane spin direction. We grew Co islands on Pt(111) single crystal, and found the spiral spin texture at the magnetic domain wall using a spin-polarized scanning tunneling microscope. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L7.00003: Interaction of magnon current with a domain wall in an antiferromagnet Se Kwon Kim, Oleg Tchernyshyov, Yaroslav Tserkovnyak We study the dynamics of magnons in an easy-axis antiferromagnet in the presence of a domain wall (DW). As in a ferromagnet [1], magnons pass through a DW with a $\tanh{}$ profile with no back scattering. An important difference is that in an antiferromagnet a magnon can have spin $+1$ or $-1$ along the easy axis, whereas in a ferromagnet a magnon's spin is always opposite to the direction of magnetization. We find that magnons in an antiferromagnet pass through a $\tanh{}$ domain wall with their spin reversed and transfer two units of angular momentum to the DW. A magnon spin current can be generated by breaking the degeneracy of the two branches of spin waves in one domain, e.g., by irradiating it with circularly polarized microwaves. Uniform magnetization accumulated on the DW as a result of magnon spin inversion will cause the staggered magnetization to precess. We present a quantitative model incorporating magnon spin current in the equations of motion written in terms of the wall's collective coordinates [2]. The analytical results are confirmed by numerical simulations. \\ \\ \noindent $[$1$]$ P. Yan, X. S. Wang, and X. R. Wang, Phys. Rev. Lett. \textbf{107}, 177207 (2011). \\ \noindent $[$2$]$ E. G. Tveten \textit{et al.}, Phys. Rev. Lett. \textbf{110}, 127208 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L7.00004: Resonance in Magnetostatically Coupled Transverse Domain Walls Andrew Galkiewicz, Liam O'Brien, Paul Keatley, Russel Cowburn, Paul Crowell In a system of adjacent ferromagnetic nanowires, stray magnetic fields from a transverse domain wall (TDW) in one wire can give rise to an attractive interaction with a TDW in a separate wire. This has previously been shown to lead to an increase in the depinning fields for TDW propagation, and has also been predicted to lead to oscillatory motion should the two TDWs be separated laterally from equilibrium. Using time-resolved Kerr microscopy, we have observed the resonance associated with the TDW interaction. In addition, another resonance has been observed that we find is due to the pinning of the TDWs by the intrinsic edge roughness of the nanowires. The overall system dynamics are well described by a 1-D analytical model that incorporates both effects. Micromagnetics show that the energy scales of the intrinsic pinning and the inter-TDW coupling are similar and suggest that roughness should be accounted for in future dynamical investigations. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L7.00005: Domain Wall Trajectory Determined by its Fractional Topological Edge Defects Invited Speaker: Aakash Pushp The theory of topological defects has had a significant influence on the understanding of various physical phenomena ranging from superfluid Helium-3 to liquid crystals. Topological defects are general features in systems with broken symmetries such as head-to-head (HH) and tail-to-tail (TT) domain walls (DWs) in soft ferromagnetic nanowires (NWs). Such DWs are further composed of elementary topological bulk and edge defects with integer and fractional winding numbers, respectively; whose relative spatial arrangement determines the chirality of the DW. Understanding the influence of the DW structure on its motion is critical for both fundamental and technological reasons. In this talk, I will show how one can understand and control the trajectory of DWs in magnetic branched networks, composed of connected NWs, by a consideration of their fractional elementary topological defects and how they interact with those innate to the network. I will describe a simple and yet a highly reliable mechanism that we have developed for the injection of a DW of a given chirality into a NW and exploit it to show that it is the DW's chirality that determines which branch the DW follows at a symmetric Y-shaped magnetic junction, the fundamental building block of the network. Using these concepts, I'll unravel the microscopic origin of the one-dimensional (1D) nature of magnetization reversal of artificial spin ice systems that have been observed in the form of Dirac strings. This understanding will allow for the formation of more complex chiral magnetic orders by controllably generating and propagating several domain walls of specific chiralities into artificial spin ice structures to form defined lattices of Dirac strings. \\[4pt] Reference:\\[0pt] A. Pushp*, T. Phung*, C. Rettner, B. P. Hughes, S.-H. Yang, L. Thomas, S. S. P. Parkin, Nature Phys. 9, 505-511 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L7.00006: Single domain wall manipulation in curved nanowires using a mobile, local, circular field Madeline Shortt, Jessica Bickel, Mina Khan, Mark Tuominen, Katherine Aidala Ferromagnetic nanostructures present exciting physics with a range of potential applications in data storage devices, such as magnetoresistive random access memory (MRAM). These proposals require precise control and understanding of domain wall (DW) movement and interactions. We developed a technique that generates a local circular Oersted field at a precise location by applying current through the tip of the atomic force microscope (AFM). We previously used this technique to control DW motion in nanorings [1]. We extend this method to control individual DW movement in curved nanowires by placing the tip near a 180 DW at the vertex of a curved wire and generating a local field. In this way, we can examine the motion of domain walls through regions with different curvature and the effects of pinning. [1] T. Yang, N. R Pradhan, A Goldman, A. Licht, Y. Li, M T. Tuominen and K. E. Aidala, Applied Physics Letter, http://apl.aip.org/resource/1/applab/v98/i24/p242505$\backslash $\textunderscore s1 98, 242505, (2011) [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L7.00007: Observation of resonant modes of coupled domain walls Timothy Phung, Aakash Pushp, Charles Rettner, Brian Hughes, See-Hun Yang, Stuart S.P. Parkin Domain walls (DWs) in permalloy nanowires can be coupled together to form bound states (360$^{\circ}$ DWs) due to the repulsion arising from the interaction of the elementary topological defects of the DWs. Such repulsion prevents the annihilation that would otherwise occur due to magnetostatic interaction between the DWs. Here we demonstrate that the adjacent DWs mimic the response of coupled oscillators when driven by spin-polarized currents, and that their coupling can be tuned by applying a magnetic field to either push them closer or pull them apart. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L7.00008: Stabilization of Magnetic Antivortices and the role of Shape Anisotropy Martin Asmat-Uceda, Lin Li, Brian Shaw, Arabinda Haldar, Kristen Buchanan Magnetic vortices have attracted a great deal of interest in recent years due to their potential for applications such as data storage, microwave resonators, magnonic crystals, etc. Magnetic antivortices (AV) are expected to possess similarly interesting physical attributes; however, they have not been explored with the same intensity. The AV spin configuration may present some advantages over vortices, especially for channeling spin waves emitted from the dynamic core reversal and for de-coupling spin-transfer torque effects from parasitic Oersted fields. Currently only a few geometries have been identified that reliably promote the formation of an AV, thus limiting the study of their properties. We recently demonstrated a method to form AV's in pound-key-like structures made of Permalloy (Haldar et al. APL \textbf{102}, 112401, 2013). Here we investigate the dependence of the reliability of the AV formation on the details of the geometry of these structures. Magneto-optical Kerr effect (MOKE) hysteresis and magnetic force microscopy measurements show that the coercive field is also the nucleation field for the AV's. Micromagnetic simulations agree well with the experiments and highlight the role of shape anisotropy in the AV formation. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L7.00009: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L7.00010: Bloch points are sticky Oleg Tchernyshyov, Se Kwon Kim Bloch points are zero-dimensional topological defects in three-dimensional ferromagnets. A representative magnetic configuration is a hedgehog with magnetization pointing away from a center. The singular nature of a Bloch point's core leads to interesting and observable consequences [1]. A simple argument based on dimensional analysis shows that a magnetic lattice creates a periodic potential that can pin a Bloch point even if the lattice has no defects. The pinning force is of the order of the micromagnetic exchange constant, a few piconewtons in a typical ferromagnet. A domain wall in a cylindrical ferromagnetic wire with the diameter of a few tens of nanometers may contain a Bloch point. Such a domain wall will have a sizable depinning field, tens of oersteds. A Bloch point moving through an atomic lattice should emit electromagnetic waves at the frequency of a few hundred gigahertz. \\[4pt] [1] S. K. Kim and O. Tchernyshyov, Phys. Rev. B \textbf{88}, 174402 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L7.00011: In-situ magnetizing interlayer exchange coupled ferromagnetic discs with pinning exchange bias Sheng Zhang, Charudatta Phatak, Amanda Petford-Long, Olle Heinonen Multilayer structures consisting of both interlayer exchange coupling between two ferromagnetic layers separated by a nonmagnetic spacer and exchange bias from an antiferromagnetic layer on top were patterned into 1 micron diameter discs using focused ion beam lithography. The initial domains of the top ferromagnetic film set a linear exchange bias in the adjacent antiferromagnetic layer, causing a bias at the nucleation field of vortex structures in the in-situ magnetization experiments using Lorentz TEM when applying magnetic field in the positive and negative directions. We also observe unexpected vortex core shifting at some specific field during the in-situ magnetization experiment, possibly due to local pinning site and magnetization reversal of the pinned ferromagnetic disc layer. Micromagnetic simulations were performed to understand the magnetization reversal behavior on both ferromagnetic disc layers. [Preview Abstract] |
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