Topological defects in ferroelectric or ferromagnetic materials : domain wall, vortices, skyrmions and beyond
Topology has gain in stature in condensed matter physics over the past years, as a fundamental basis to describe numerous phenomena such as topological defects, quantum hall effect and topological insulators. This symposium will be focused on the very rich physics related to the excitations of topological defects in ferroic materials.
Topological defects in ferromagnetic and ferroelectric materials have attracted much attention both as a playground of unique physical phenomena but also for potential applications in electronic devices such as reconfigurable memories or logic based devices.
This symposium aims at covering the different topics related to this new branch of condensed matter physics going from (i) the exploration of new material phases both in ferromagnetic or ferroelectric thin films or low symmetry materials (growth, structural characterization, imaging techniques etc..), (ii) the investigation of the magnetoelectric properties associated to chiral spin textures (iii) to the dynamics of magnetic solitons for potential applications in memory or logic devices. The goal is to bring together material scientists, chemists, physicists from different disciplines (magnetism, ferroelectricity, spintronics, synchrotron techniques etc..) to share the recent progress and identify the new directions for the physics of topological defects.
Some examples of more focused topics include:
- Experimental investigation and control of vortex patterns in low-dimensional ferroelectrics
- Electronic transport in ferroelectric thin films with ferroelectric vortices
- Dynamics of magnetic domain wall and vortices induced by spin transfer torques
- Non collinear spin textures stabilized by chiral interactions
Hot topics to be covered by the symposium:
- New magnetic phases in low dimensional magnetic materials
- Control of the dynamical properties of magnetic solitons: domain wall, vortices and/or skyrmions
- Emergent electromagnetic properties associated to topological defects
- S. Blügel “Theory of spin orbit related phenomena” from Forschung Zentrum Jülich, Germany
- G. Meier “Magnetic vortices - From single oscillators to magnonic vortex crytals” from Max-Planck Institute for the Structure and Dynamics of Matter CFEL/DESY, Hamburg, Germany
- C.H. Marrows “Bulk and interface Dzyaloshinkii-Moriya interactions in magnetic thin films” from University of Leeds, UK
- R. Wiesendanger “Tailoring of nano-scale skyrmions by interface and strain engineering” from University of Hamburg, Germany
- H.J. Swagten “Experiments of DW motion through spin orbit torques” from TU Eindhoven, The Netherlands
- S. W. Cheong “Topology of domain wall junctions in improper ferroelectrics” from Rutgers University, USA
- P. Zubko “Dynamics of ferroelectric stripe domains” from London Center for Nanotechnology, UK
- V.P. Kravchuk “Topological and curvilinear effects in low-dimensional ferromagnetic systems” from Bogolyubov Inst. Theoretical Physics, Kiev, Ukraine
- S. Tagantsev “Charged walls in ferroelectrics” from EPFL, Lausanne, Switzerland
Scientific committee members:
- S. Heinze, Kiel University (Germany)
- R. Stamps, Glasgow University (UK)
- B. Noheda, Groningen University (The Netherlands)
- U. Roessler, IFW-Dresden (Germany)
- G. Catalan, ICN2-Barcelona (Spain)
Papers will be published in Physica Status Solidi (Wiley)
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Topological defects in ferromagnetic systems : Chairman : Pr. C.H. Marrows
Authors : Roland Wiesendanger
Affiliations : Interdisciplinary Nanoscience Center Hamburg University of Hamburg, D-20355 Hamburg, Germany
Resume : Based on the development of atomic-resolution spin-polarized scanning tunneling microscopy (SP-STM) , we have discovered nanoskyrmion lattices with a periodicity of only one nanometer in single atomic layers of Fe on Ir(111) [2,3], where the interfacial Dzyaloshinskii-Moriya interaction plays a crucial role for their stability. More recently, we have made use of multiple interface engineering in bilayer and multilayer systems in order to demonstrate the direct observation and manipulation of individual skyrmions of single-digit nanometer-scale size . By locally injecting spin-polarized electrons from an atomically sharp SP-STM tip, we were able to write and delete individual skyrmions one-by-one, making use of spin-transfer torque exerted by the injected high-energy spin-polarized electrons. The creation and annihilation of individual magnetic skyrmions demonstrates their great potential for future nanospintronic devices making use of individual topological charges as information carriers . Moreover, magnetic skyrmion lattices with their special topological properties can efficiently mediate transport of spin information between hybrid nano-objects, as recently demonstrated for organic molecules adsorbed on Fe/Ir(111) [6,7]. Finally, we will demonstrate how strain engineering in ultrathin transition metal films can be employed for guiding skyrmions in future spintronic devices, such as skyrmion-based racetrack memories or hard disk drives.  R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).  S. Heinze et al., Nature Physics 7, 713 (2011).  A. Sonntag et al., Phys. Rev. Lett. 113, 077202 (2014).  N. Romming et al., Science 341, 6146 (2013).  A. Fert et al., Nature Nanotechnology 8, 152 (2013).  J. Brede et al., Nature Nanotechnology 9, 1018 (2014).  M. Cinchetti, Nature Nanotechnology 9, 965 (2014).
Authors : Sung-Jun Oh, Chung-Hyo Lee
Affiliations : Department of Advanced Materials Science and Engineering, Mokpo National University
Resume : Magnetic nanocomposites have received increased attention because of their unique electrical, mechanical and magnetic properties. In particular, magnetic nanocomposite powders with fine microstructure by mechanical alloying are of interest because of possible applications as permanent magnets and soft magnetic material for high frequency. Mechanical alloying (MA) has been widely used for preparing numerous advanced engineering materials with unique properties and structures. Also, the solid-state reduction of an iron oxide by a reactive metal element during MA has occurred through an in-situ reaction, forming several magnetic nanocomposite phases. In this work, we report the results of a study of the structure and magnetic properties of MgO dispersed nanocomposite fabricated by MA and subsequently spark plasma sintering (SPS). An optimal ball milling and heat treatment conditions to obtain a magnetic nanocomposite with nano-sized grains were investigated. Magnetic measurement supports the characterization of the solid-state reduction between Mg and iron oxide due to MA process. Acknowledgements; This work is financially supported by the Ministry of Knowledge Economy (MKE) of Korea.
Authors : Charles Paillard [1,6], Bertrand Dupé [2,3], Ritwik Mondal , Marco Berritta , Zhigang Gui [5,6], Surendra Singh ,Brahim Dkhil , Peter M. Oppeneer  and Laurent Bellaiche [5,6]
Affiliations :  Laboratoire SPMS, UMR 8580 CNRS/Ecole Centrale Paris, Grande Voie des Vignes 92 295 Châtenay-Malabry CEDEX, France  Institute of Theoretical Physics, Christian-Albrechts Universität zu Kiel, Germany  Peter Grünberg Institute and Institute for Advanced Simulation, Forschungzentrum Jülich and JARA, D-52425 Jülich, Germany  Dept. of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-75120 Uppsala, Sweden  Physics Department, University of Arkansas, 72701 Fayetteville, AR, USA  Institute for Nanoscience and Engineering, University of Arkansas, 72701 Fayetteville, AR, USA
Resume : Topological solitons such as skyrmions hold great promise to build ultra-high density magnetic racetrack memory because they can be reduced down to the nanoscale and be moved with low-density current. Achieving such a device can only be realized by a deep understanding of the interaction between the skyrmion and electrons. So far these interactions, such as the topological Hall effect (THE), have only been described in the complex framework of the Berry phase theory. Recently, a new energy term coupling the angular momentum of light with a magnetic moment has been shown to be able to re-derive complex magneto-electric effects such as the spin current model in multiferroics, the anomalous Hall effect, inverse Rashba-Edelstein effect, anisotropic magnetoresistance and planar Hall effects. However, no rigourous proof of this Angular MagnetoElectric (AME) coupling was given. In this work, we demonstrate the existence of this coupling starting from the Dirac equation, and are able to re-derive the THE. In addition, we are able to show that, under the application of electric fields, the force applied on the skyrmion could be finely tuned. Furthermore, the direct relationship between the transverse Hall conductivity and the skyrmion radius could allow the study of dynamical excitation of the skyrmion such as e.g., breathing modes. The AME coupling also allows for the prediction of novel magnetoelectric effects in metals hosting skyrmions.
Authors : L. Lorenzelli (1), A. Dussaux (1), K. Chang (1), P. Schoenherr (2), C. Degen (1), D. Meier (2)
Affiliations : (1) Department of Physics, ETH Zürich, Otto Stern Weg 1, 8093 Zürich, Switzerland; (2) Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 4, 8093 Zürich, Switzerland
Resume : The interest in magnetically ordered B20 compounds not only resides in their A-phase, but also in their helical and conical ordered phases and the related precursor phenomena. Their complex nature is outlined by a sequence of thermally driven phase transitions and crossovers in the vicinity of the critical temperature Tc, with anomalous magnetic ordering processes and the occurrence of intermediate mesophases between the paramagnetic and the helical states. NV center magnetometry may provide a means to image complex spin textures and other antiferromagnetic materials, detecting the local dynamics owing to its high sensitivity magnetic field sensing and high spatial resolution. Here we report temperature dependent investigations of the helimagnetic order in FeGe at zero external magnetic field, using a single-spin defect in diamond with magnetic field-dependent fluorescent properties as a probe: the nitrogen-vacancy (NV) center. Our results outline stochastic jumps and thermally driven phase transitions close to Tc.
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Authors : D. Braun, M. Schmidbauer, A. Kwasniewski, P. Müller, Jan Sellmann, Michael Hanke, Hubert Renevier, J. Schwarzkopf
Affiliations : (1) Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin; (2) Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin; (3) Laboratoire des Materiaux et du Genie Physique, Grenoble INP - Minatec, Grenoble, France;
Resume : The domain structure of (Na,K)NbO3 as a lead-free alternative is rather well investigated in bulk form, but little is known for thin films. Particularly the growth on lattice mismatched substrates has a decisive impact on the formation of ferroelectric phases and can yield new properties not observed in corresponding bulk crystals. In this study, 20-30 nm thick (K,Na)NbO3 films with pure ac-surface orientation were epitaxially grown on (110) NdScO3 crystalline substrates by MOCVD. Hence, they experience anisotropic lattice stress resulting in average in a slight compressive strain. X-ray diffraction data demonstrates that the (K,Na)NbO3 unit cells are monoclinically distorted along the NSO in-plane direction. Piezoresponse force micrographs (PFM) reveal both a lateral and a vertical component of the electric polarization. The lateral PFM shows irregularly arranged 180° domains along NSO superimposed by well-ordered ferroelastic subdomains with a lateral periodicity of 30-40 nm along [1-10]NSO. These ferroelastic domain walls cannot be described by low index crystallographic net planes. They rather incline a variable angle to the NSO direction depending on chemical composition and in-plane monoclinic distortion of the film unit cells. Summarizing, the occurrence of such a domain structure is compatible only with monoclinic domains separated by S walls, which does not exist in tetragonal, rhombohedral or orthorhombic structures.
Topological defects in ferroelectric systems : Chairman : Pr. E. Salje
Authors : P. Zubko (1), M. Hadjimichael (1), S. Fernandez-Pena (2) and J.-M. Triscone (2)
Affiliations : (1) London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London WC1H 0HA, United Kingdom (2) Département de Physique de la Matière Quantique (DQMC), University of Geneva, 24 Quai Ernest-Ansermet, 1211 Genève 4, Switzerland
Resume : Over the last decade, there have been a number of significant breakthroughs in our understanding of ferroelectric domains and domain walls. The traditional view that ferroelectric domain walls are purely Ising-like has been challenged by recent high-resolution TEM data and theoretical calculations that predict a plethora of complex internal polarisation structures and even separate phase transitions within domain walls.Perhaps even more exciting have been the numerous discoveries of unexpected emergent behaviour at domains walls, not intrinsic to the bulk of the material.Enhanced domain wall conductivity, large photovoltaic responses, domain wall magnetism and the appearance ofpolarisation in otherwise centrosymmetric materials are just a few of the phenomena that bring hope of new technologies and fuel the ever-growing interest in the newly emerged field of domain-wall nanoelectronics. Collective responses of domain-walls are also being re-examined, particularly as research on nanoscale ferroelectrics has revealed that extremely dense, regular domain structures can be engineered in ultrathin films and heterostructures and are predicted to have interesting collective domain-wall dynamics. We have investigated the dielectric properties PbTiO3-based superlatticeswith such ultradense, nanoscale ferroelectric stripe domains. Dielectric impedance spectroscopy measurements in the 100 Hz-1MHz frequency range have been performedover a wide temperature range, from the cryogenic regime where domain walls are pinned to well above the Curie point where they no longer contribute to the dielectric response. The effect of domain structures and domain-wall motion on the dielectric response and the ferroelectric phase transition will be discussed.
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Topological defects in ferroelectric systems : Chairman : Pr. P. Zubko
Authors : Ekhard Salje
Affiliations : Department of Earth Sciences University of Cambridge UK
Resume : Polar vortex structures and domain walls : Bloch line switching Switchable Bloch lines in SrTiO3 originate from polar dipoles inside twin walls. These vectors correlate as one-dimensional strings of switchable vectors in planar walls. These strings are called 'Bloch lines' in analogy to magentic excitations with a similar topology. They were first predicte by Houchmanzadeh et al (1991) and cofirmed theoretically by Conti et al. (2011). Detailed computer simulations by Zykova-Timan and Salje (2014) show that Bloch lines are not just polar but their polarity can be reversed by an applied field. Additional vortices between twin walls occur in virtually all ferroelectric/ferroelastic materials at finite temperatures, the vortices are found to flicker in time and space. Widely spaced ferroelastic twin boundaries nucleate vortices while dense twin boundaries suppress them. The time averaged number of vortices at any site decays exponentially, indicating the highly mobile dynamics of the vortex lattice. Applied electric fields break the rotational symmetry of vortices and finally destroy them. The observed vortex structures are akin to those observed in magnetic and superconducting disordered vortex lattices (Zhao et al. 2014). Zykova-Timan, T.,; Salje, E.K. H. APL 104,082907 (2014) Conti et al. JPCM 23, 142203 (2011) Houchmanzadeh et al. JPCM 3, 5163 (1991) Zhao et al. APL 105, 112906 (2014)
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