Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F30: Focus Session: Skyrmions II 
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Sponsoring Units: GMAG DMP FIAP Chair: Peter Fischer, University of California, Santa Cruz Room: 206B 
Tuesday, March 3, 2015 8:00AM  8:12AM 
F30.00001: Firstprinciples analysis of a DzyaloshinskiiMoriya driven magnetic structure in a monolayer Cr on W(110) Stefan Bl\"ugel, Bernd Zimmermann, Timo Schena, Marcus Heide, Yuriy Mokrousov, Gustav Bihlmayer The observation of a chiral magnetic spin spiral in a magnetic Mn monolayer on a W(110) substrate has opened a new vista in lowdimensional magnetism [1] with possible applications in spintronics. The origin was explained by the occurrence of the DzyaloshinskiiMoriya interaction (DMI), which occurs due to the presence of a structure inversionasymmetry. In this talk, we turn our attention to a monolayer Cr on W(110) [2]. We show by DFT calculations, that the DMI is so strong that it creates a noncollinear spinspiral ground state, with excellent agreement to spinpolarized STM experiments [3]. We determine a considerable inhomogeneity of the spin spiral by means of a micromagnetic model. We compare our results to the systems Mn and Fe on W(110), where the direction of the spiral and period length are different. We present a minimal tight binding model [4] and show that indeed the sign and strength of the DMI reveals a nontrivial dependence on the electronic structure. \\[4pt] [1] M. Bode \textit{et al.}, Nature \textbf{447}, 190 (2007).\\[0pt] [2] B. Zimmermann \textit{et al.}, Phys. Rev. B \textbf{90}, 115427 (2014).\\[0pt] [3] B. Santos \textit{et al.}, New J. Phys. \textbf{10}, 013005 (2008).\\[0pt] [4] V. Kashid \textit{et al.}, Phys. Rev. B \textbf{90}, 054412 (2014). [Preview Abstract] 
Tuesday, March 3, 2015 8:12AM  8:24AM 
F30.00002: Majorana bound state in a magnetic biskyrmion Guang Yang, Daniel Loss Magnetic biskyrmion, recently discovered [1] in thin film dipolar ferromagnet with uniaxial anisotropy, is a highly mobile nanoscale topological spin texture. We show that a magnetic biskyrmion in proximity to an swave superconductor supports a zeroenergy Majorana bound state in its core. The Majorana bound state can be manipulated through driving the motion of the magnetic biskyrmion with electric current. We discuss the realization of nonAbelian statistics of such Majorana bound states. \\[4pt] [1] X. Z. Yu, Y. Tokunaga, Y. Kaneko, W. Z. Zhang, K. Kimoto, Y. Matsui, Y. Taguchi, and Y. Tokura, Nature Communications 5, 3198 (2014). [Preview Abstract] 

F30.00003: ABSTRACT WITHDRAWN 
Tuesday, March 3, 2015 8:36AM  9:12AM 
F30.00004: Unraveling skyrmion spin texture using resonant soft xray scattering Invited Speaker: Sujoy Roy The recent discovery of skyrmions, that were originally predicted in context of high energy physics, in magnetic materials has sparked tremendous interest in the research community due to its rich physics and potential in spintronics applications. Skyrmions have an unusual spin texture that manifests as magnetic knot and can be easily moved around. Understanding the fundamental physics and mechanisms for controlling their dynamical properties presents important scientific challenges. So far experimental verifications of the skyrmions in magnetic systems have come from neutron scattering and Lorentz transmission electron microscopy (TEM) measurements. In this talk we report the first observation of the skyrmions using resonant soft xray scattering. We have used soft xrays tuned to the Cu L$_{3}$ edge to diffract off the skyrmion lattice in a multiferroic Cu$_{2}$OSeO$_{3}$ compound. We show that in Cu$_{2}$OSeO$_{3}$ there exist two skyrmion lattices arising due to the two inequivalent CuO sublattices that have two different magnetically active $d$orbitals. The two skyrmion sublattices are mutually rotated with respect to each other. The angle of rotation could be changed by an external magnetic field, thereby indicating possible existence of a new phase. We have also studied skyrmion spin texture in an ultrathin Fe/Gd multilayer that shows perpendicular anisotropy. The Fe/Gd sample exhibits a near perfect aligned stripe phase. Within a small range of temperature and magnetic field we observe a hexagonal scattering pattern due to skyrmion bubbles. Analysis of the scattering pattern suggests that the skyrmion lattice unit cell contains two skyrmions. The biskyrmion state is also revealed by Lorentz TEM images. The near room temperature discovery of skyrmion in a technology relevant material is a significant step towards using skyrmions in magnetic devices. [Preview Abstract] 
Tuesday, March 3, 2015 9:12AM  9:24AM 
F30.00005: Xray microscope Imaginging of skyrmions in Ultrathin Films with Strong DzyaloshinskiiMoriya Interaction Seonghoon Woo, Benjamin Kruger, Mathias Kl\"aui, Peter Fischer, Geoffrey Beach Spin textures stabilized by the DzyaloshinskiiMoriya interaction (DMI) have been of considerable recent interest due to extraordinary static and dynamic behaviors derived from their topological nature. It has recently been shown that DMI can also manifest in buried ultrathin sputtered film stacks. Here we examine magnetic bubble domains in submicron patterned dots with strong DMI. We use magnetic transmission Xray microscopy to image the evolution of the magnetization configuration as a function externally applied fields. We imaged a series of [Pt(3nm)/Co(0.9nm)/GdOx(3nm)]x15, where the DMI is strong, and [Pt(3nm)/Co(0.9nm)/Pt(3nm)] x15 stacks, where DMI is small enough due to symmetric structure, and 15 repeats were used to enhance XMCD contrast. We observed that the size of domain can be significantly narrower for the case of strong DMI and micromagnetic modelling confirmed the observation. We also imaged that magnetic bubbles can be easily nucleated and controlled using external fields in micronsizedisk patterns. The static stability of bubbles for two cases were tested using external bias field, showing skyrmionic bubble has larger bubblecollapse field by the factor of two. Other qualitative and quantitative measurements will also be presented. [Preview Abstract] 
Tuesday, March 3, 2015 9:24AM  9:36AM 
F30.00006: Skyrmions in quasi2D chiral magnets with broken bulk and surface inversion symmetry James Rowland, Sumilan Banerjee, Mohit Randeria Most theoretical studies of skyrmions have focused on chiral magnets with broken bulk inversion symmetry, stabilized by easyaxis anisotropy [1]. Recently, we considered 2D systems with broken surface inversion [2] and showed that skyrmion crystals are more stable than in 3D, pointing out the importance of easyplane anisotropy. In the present work we investigate quasi2D systems which break both bulk and surface inversion symmetry. The LandauGinzburg free energy functional thus contains two DzyloshinskiiMoriya terms of strength $D_D$ and $D_R$ arising from Dresselhaus and Rashba spinorbit coupling respectively. We trace the evolution of the phase diagram as $D_D/D_R$ is varied, and find that skyrmions are increasingly destabilized with respect to the cone phase as $D_D$ increases relative to $D_R$. We find an evolution from vortexlike skyrmions in the pure Dresselhaus limit to hedgehoglike skyrmions in the pure Rashba limit. We discuss the relevance of these results to existing experiments and the prospects of tuning the ratio of Dresselhaus and Rashba spinorbit coupling via film thickness and strain. [1] M. Wilson et al., PRB 89, 094411 (2014). [2] S. Banerjee, J. Rowland, O. Erten, and M. Randeria, PRX 4, 031045 (2014). [Preview Abstract] 
Tuesday, March 3, 2015 9:36AM  9:48AM 
F30.00007: Anisotropic Magnetoresistance in the Layered Chiral Helimagnet Cr$_{1/3}$NbS$_{2}$ Alexander Bornstein, Nirmal Ghimire, David Mandrus, David Parker, Minhyea Lee We study the magnetotransport properties and the low temperature specific heat in a highly anisotropic helimagnet Cr$_{1/3}$NbS$_{2}$. We compare the dependence of both measurements on field orientation: within the crystallographic plane and normal to it. In transport measurements, the current always remains within the crystallographic abplane. We find a three times larger reduction in the resistivity when the magnetization is saturated normal to the plane compared to within the plane. This discrepancy occurs below 50 K $<$ T$_{C}$=130 K. In the same temperature range, an unusual field dependence of the Hall Effect is also observed. From first principles calculations, we connect the changes in resistivity to modification of the density of states dependent on the direction of spin polarization. The Sommerfeld constant for both spin orientations is discussed in relation to the transport behavior. [Preview Abstract] 
Tuesday, March 3, 2015 9:48AM  10:00AM 
F30.00008: Fermi Surface Induced Scalar Chiral Stripes in the Kondo Lattice Model on a Square Lattice Ryo Ozawa, Kipton Barros, GiaWei Chern, ShiZeng Lin, Masafumi Udagawa, Yukitoshi Motome, Cristian Batista The stability and controllability of emergent nano/mesoscale spin structures are one of the central issues for spintronics. The possibility of stabilizing noncoplanar spin structures in chiral magnets (e.g. skyrmion crystals) is opening a new avenue for controlling transport properties with small magnetic fields. An electron moving in a loop picks up a Berry phase proportional to the net scalar spin chirality of the underlying spin configuration that is enclosed by the loop. In other words, the scalar spin chirality acts as an effective magnetic field that couples to the electronic orbital motion. By solving the Kondo lattice model on a square lattice, we will show that mesoscale chiral stripes emerge in the vicinity of a Lifshitz transition of the Fermi surface. Our unbiased results are obtained by applying a novel algorithm which allows for very efficient simulations based on Langevin dynamics. [Preview Abstract] 
Tuesday, March 3, 2015 10:00AM  10:12AM 
F30.00009: Magnetoresistance measurements in a chiral magnet CrNb$_{3}$S$_{6}$ subject to locally applied magnetic field Yu Matsumoto, Yusuke Kousaka, Jun Akimitsu, Sadafumi Nishihara, Katsuya Inoue, Robert Stamps, Alexander S. Ovchinikov, Junichiro Kishine, Yoshihiko Togawa CrNb$_{3}$S$_{6}$ is one of interesting magnetic materials with structural chirality, wherein chiral magnetic orders are formed as a consequence of the competition between Heisenberg exchange and antisymmetric DzyaloshinskyMoriya (DM) interactions. Interestingly, a chiral helimagnetic order appears as the ground state at zero magnetic field, while it transforms into a chiral spin soliton lattice; a spin superlattice of forced ferromagnetic regions partitioned by a soliton with $2\pi $ rotation of spin magnetic moments, in the presence of magnetic field applied perpendicular to the chiral structural axis [1]. In this work, we have investigated the magnetoresistance (MR) in a microfabricated single crystal of CrNb$_{3}$S$_{6}$ by means of the standard ac transport measurement. We have found that the MR exhibits a hysteresis behavior during the magnetic field cycle, although a continuous negative MR is observed in a bulk single crystal [2]. In the presentation, we will show the MR changes induced by locally applied magnetic field and discuss their properties in terms of the macroscopic coherence of the chiral spin soliton lattice.\\[4pt] [1] Y. Togawa et al., Phys. Rev. Lett. 108, 107202 (2012).\\[0pt] [2] Y. Togawa et al., Phys. Rev. Lett. 111, 197204 (2013). [Preview Abstract] 
Tuesday, March 3, 2015 10:12AM  10:24AM 
F30.00010: Meron crystals and skyrmion fractionalization in chiral magnets Avadh Saxena, ShiZeng Lin, Cristian D. Batista The recent discovery of skyrmions in chiral magnets, e.g. MnSi, has triggered enormous interest due to their huge potential for spintronics. Unlike magnetic domain walls, skyrmions can be manipulated with very small electric currents, thus rendering them as prime candidates for novel information storage devices with much lower power consumption. Here we study the equilibrium phase diagram of ultrathin chiral magnets with an easyplane anisotropy $A$. The triangular skyrmion lattice phase that is obtained for $A=0$ evolves through different structural phase transitions upon increasing $A$, which are related to the compact packings of disks with two different radii. Meanwhile, the topological charge of a skyrmion decreases continuously and we call this process skyrmion fractionalization. For a strong easyplane anisotropy, a meronantimeron crystal is stabilized. Akin to the case of skyrmions, the resulting merons can be manipulated with external current, and they behave like particles. Meron charge can be measured in transport experiments or by direct imaging of meron motion. Our work demonstrates that symmetric magnetic anisotropy can be used as a knob for tuning the topological character of the emergent mesoscale particles as well as the nature of the crystal that they form. [Preview Abstract] 
Tuesday, March 3, 2015 10:24AM  10:36AM 
F30.00011: Theory of electromagnons in CuO Kun Cao, Feliciano Giustino, Paolo Radaelli Recently, an electromagnon excitation was found in the multiferroic phase of CuO for electric field parallel to the [101] direction, with excitation energy $ \sim 3$ meV. We performed symmetry analysis to show that the experimentally measured electromagnon cannot be explained by the exchangestriction mechanism. We then studied the CuO electromagnons using abinitio calculations and effective model simulations. The experimentally measured electromagnon was found to originate from the DzyaloshinskiiMoriya interaction, with the magnon part corresponding to a phason mode. We further predict that a new highenergy electromagnon with selection rule E $\parallel$ [010] should also exist in the multiferroic phase of CuO, due to the exchangestriction mechanism. [Preview Abstract] 
Tuesday, March 3, 2015 10:36AM  10:48AM 
F30.00012: Wave Equation of ThreeDimensional Skyrmion Line Junichi Iwasaki, Christoph Sch\"{u}tte, Naoto Nagaosa Magnetic skyrmion is a particle in magnets, which is now regarded as one of the most promising candidate for information career in future memory devices. In the bulk of chiral magnets, skyrmions form lines along the applied magnetic field. Previous studies report that the emergent magnetic monopoles are assocoated with the creation and annihilation of these lines. Here, we raise more fundamental question: how does the wave propagates in these onedimensional ``strings"? Surprisingly, the numerical simulation reveals that the waves propagating in positive and negative directions are different. This asymmetric feature is described by magnon contribution, which has, in general, $k$linear term in its dispersion relation under the DzyaloshinskiiMoriya interaction. Starting from the action of the spin system in chiral magnets, we derive the wave equation of the skyrmion line. [Preview Abstract] 
Tuesday, March 3, 2015 10:48AM  11:00AM 
F30.00013: Thermalization and dynamic phase transition of quantum spins Mehrtash Babadi, Eugene Demler, Michael Knap We develop a controlled field theoretic technique for studying farfromequilibrium dynamics of interacting quantum spins. This is achieved by combining the Majorana fermion representation of spins and 1/N expansion of the twoparticle irreducible effective action (2PIEA). We use the technique to study the relaxation dynamics of quantum spin spirals in the Heisenberg model. The nonequilibrium magnetization and spin correlations are found by solving the KadanoffBaym and BetheSalpeter equations resulting from the 1/N expansion of the 2PIEA to the nexttoleading order. In three dimensions, we identify a dynamic phase transition in the steady state magnetization for spiral states near the Ne\'el order. We further find a dynamical stabilization of the initial outofplane ordering instability in the course of the relaxation dynamics, in contrast to the linear response analysis. [Preview Abstract] 
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