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Dongwook Go

Spin current is one of the central concepts in spintronics. While early studies of giant magnetoresistance and spin-transfer torque have shown good agreement between the theory and experiment, recent experiments of current-induced torques in spin-orbit coupled systems imply that we need a theory which goes beyond “spin current picture”. In general, angular momentum can be carried by other degrees of freedom as well as the spin. For electrons, the angular momentum is encoded in not only the spin but also orbital part of the wave function, thus one can think of transport of orbital angular momentum carried by electrons in analogy to the spin transport.
In this talk, I will explain how to electrically generate orbital current and utilize it to exert a torque on the magnetization. As a way to generate the orbital current, I introduce a mechanism of orbital Hall effect, which is defined as orbital current response along transverse directions to an external electric field [1]. Then I show that injection of the orbital current to a ferromagnet can excite magnetization dynamics, which we call orbital torque [2]. One advantage of utilizing the orbital current is that it does not require spin-orbit coupling for electrical generation, which is in contrast to spin current generation, e.g., by spin Hall effect. Thus, the orbital torque mechanism predicts sizable current-induced torque even for weakly spin-orbit coupled materials. However, since the spin and orbital angular momenta transform in the same way upon symmetry operations, it is challenging to disentangle the orbital transport effect from the spin transport effect in experiments. For this purpose, we recently developed a general theory which can track angular momentum transfer between different angular-momentum-carrying degrees of freedom, which include not only the spin and orbital of the electron but also crystal lattice and local magnetic moment [3]. From a first-principles implementation of the formalism, we show that the orbital torque mechanism behaves qualitatively different from the “conventional” contribution caused by the spin Hall effect. This provides microscopic understanding of the orbital torque in terms of the electronic structure. Finally, I discuss further experimental implications and conceptual difference between the orbital transport and spin transport.
We acknowledge funding under SPP 2137 “Skyrmionics” (project MO 1731/7-1) and TRR 173 − 268565370 (project A11) of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation).

[1] D. Go, D. Jo, C. Kim, and H.-W. Lee, Intrinsic Spin and Orbital Hall Effects from Orbital Texture, Phys. Rev. Lett. 121, 086602 (2018).
[2] D. Go and H.-W. Lee, Orbital Torque: Torque Generation by Orbital Current Injection, Phys. Rev. Res. 2, 013177 (2020).
[3] D. Go, F. Freimuth, J.-P. Hanke, F. Xue, O. Gomonay, K.-J. Lee, S. Blügel, P. M. Haney, H.-W. Lee, and Y. Mokrousov, Theory of Current-Induced Angular Momentum Transfer Dynamics in Spin-Orbit Coupled Systems, arXiv:2004.05945.

Bo Li

In the progress of spintronics, magnon, the spin wave quanta, undertakes an important role due to its nature of low dissipative angular momentum carrier. The magnon band structure in ferromagnetic and antiferromagnetic insulating systems can host nontrivial topology, which draws fundamental interest and provides superiority to the transport property therein. In this talk, I will
discuss the magnon band topology and magnon mediated spin generation and transportation in insulating magnetic systems. First, I will review some topological system of magnons, such as magnon Chern insulator, magnon Weyl semimetal, and concentrate on a 3D topological insulator model where a surface Dirac cone exists due to lattice chiral symmetry [1]. Second, I will discuss spin Nernst effect and temperature gradient induced spin accumulation in noncollinear antiferromagnetic insulators [2,3]. A linear response theory of thermal driving force will be discussed and specific examples of kagome and pyrochlore antiferromagnet will be given. Finally, I will talk about the spin Nernst effect in ferromagnetic and antiferromagnetic skyrmions, where magnon Landau levels and some relevant interesting results will be presented.

[1] B. Li and A. A. Kovalev, Phys. Rev. B 97, 174413 (2018).
[2] B. Li, S. Sandhoefner, and A. A. Kovalev, Phys. Rev. Research 2,
013079 (2020).
[3] B. Li, A. Mook, A. Raeliarijaona, and A. A. Kovalev, Phys. Rev. B
101, 024427 (2020).

Rafael L. Seeger

The paradigm shift consisting of using the spin-dependent transport properties of antiferromagnets in electronics led to many exciting challenges.1),2)In this talk, we will first discuss the nature of a spin current flowing through fluctuating antiferromagnets and distinguish between electronic and magnonic spin transport. The method used to inject the spin currents involved ferromagnetic resonance and spin pumpingin ferromagnetic-spin-injector/(non-magnetic-spin-conductor)/antiferromagnetic-spin-sink multilayers. Three typical cases will be presented, magnonic spin flow in the insulating antiferromagnets NiO and NiFeOx, electronic spin flow in the metallic antiferromagnet IrMn, and electronic and magnonic parallel spin flows in IrMn when the latter is directly exchange coupled to the ferromagnetic-spin-injector. In this latter case, how it is possible to unravel the spin injection efficiency of the two types of spin flows will be demonstrated. We will also demonstrate how linear spin fluctuations enhance spin injection in spin-sinks(Fig. 1)and show why this is pertinent for studies ofcritical phenomenon like magnetic phase transitions in ultra-thin films. To show the far-reaching practical relevance of the method, extension to various phase transitions will be presented.3)-6)In search for spin fluctuations in several antiferromagnetic spin-sinks, we will also discuss how we found experimental evidence the impact of eddy-currents7)and of self-induced spin-charge conversion in the spin-injector, corroborating the results of first-principle calculations.8),9)Beyond spin currents, we will finally present a stimulating example of how antiferromagnets and superconductors may envision a common future by showing how to infer essential information about domain walls using Cooper pairs through antiferromagnets.10),11)

[1] T. Jungwirth et al, Nat. Nanotechnol. 11, 231 (2016)
[2] V. Baltz et al, Rev. Mod. Phys. 90, 015005 (2018)
[3] Y. Ohnumaet al, Phys. Rev. B 89, 174417 (2014)
[4] L. Frangou et al, Phys. Rev. Lett. 116, 077203 (2016)
[5] Z. Qiu et al, Nat. Commun. 7, 12670 (2016)
[6] O. Gladii et al, Phys. Rev. B 98, 094422 (2018) ; Appl. Phys. Express 12, 023001 (2019)
[7] R. L. Seeger et al, Appl. Phys. Lett. 115, 032403 (2019)
[8] A. Tsukahara et al, Phys. Rev. B 89, 235317 (2014)
[9] O. Gladii et al, Phys. Rev. B 100, 1174409 (2019)
[10] A. I. Buzdin, Rev. Mod. Phys. 77, 935 (2005)
[11] R. L. Seeger et al, in preparation(2020)

Richard Schlitz

The impact of non-trivial magnetic topology on the magnetoelectric and magnetothermaltransport response is actively studied at the moment. In particular, a large anomalous Halland Nernst effect can be observed in non-collinear antiferromagnets despite their vanishingnet magnetization [1,2]. Additionally, topological transport signals like the topological Hall andtopological Nernst effect can arise in the presence of non-trivial magnetic topology [3]. Suchtransport signatures allow accessing the microscopic properties of topological materials andwill be essential for exploiting the full potential of topological and antiferromagneticspintronics.I will first report on the observation of a large topological Hall and Nernst effect inmicropatterned thin films of Mn1.8PtSn below the spin reorientation temperature TSR ≈ 190 K.Our data can be used as a model system, allowing to calculate a so-called topologicalquantity. With this topological quantity, the detection of topological transport effects withoutthe need for independent magnetometry data is possible. Our approach opens the door forstudies of topological transport effects also in nanopatterned materials [4].In the second half of my talk, I will demonstrate the access to the local magnetic structure inthin films of the non-collinear antiferromagnet Mn3Sn by scanning thermal gradientmicroscopy (STGM). This technique is based on scanning a focused laser spot over thesample's surface and recording the ensuing thermo-voltage [5]. In addition to imaging theantiferromagnetic domain structure, STGM can also be used to prepare a definedantiferromagnetic magnetic domain state using a heat-assisted magnetic recording scheme,where local laser heating an magnetic fields are combined. Finally, I will address the impact ofthe hexagonal crystal symmetry of Mn3Sn on the STGM images of the local anomalousNernst effect.

[1] Nakatsuji et al., Nature 527, 212-215 (2015)
[2] Ikhlas et al., Nature Physics13, 1085-1090 (2017)
[3] Nagaosa et al., Reviews of Modern Physics 82, 1539 (2010)
[4] Schlitz et al., Nano Letters 19, 4, 2366-2370 (2019)
[5] Reichlova et al., Nature Communications 10, 5459 (2019)

Nina Meyer

Topological Insulators (TI) open up a new route to influence the transport of charge and spin via spin-momentum locking [1,2]. It has been demonstrated experimentally [2] that spin-polarized surface currents can be generated and controlled by illuminating a TIwithcircularly polarizedlight.In this talk,we will present the experimental results onphotocurrent measurements on(Bi,Sb)2Te3thin filmHall bar devicesand on Bi2Se3and Bi2Te3nanowire devices. We generate and distinguish the different photocurrent contributionsby controlling the polarization of the driving light wave, focusing on the polarization independent term whichis related to theSeebeck effect and the helicity dependentterm whichwe relate to the circular photogalvanic effect.Moving the laser spot across the sample surface and analyzing the measured photocurrentspatially resolved at every laser spot position enables us to display and discuss the thermoelectric and spin-polarized current as two-dimensionalmaps. For the (Bi,Sb)2Te3Hall bar deviceswe see a lateral accumulation of spin-polarizedcurrent at the TI’s edgeswhichin combination with the thermalgradient along the Hall bar can be explainedby the spin Nernst effect [3]. For the nanowire devices,the findings depend on the region of the sample.When the laser spot illuminates the layer stack of the contact and the nanowire the thermoelectric and the spin-polarized current are enhancedand the sign of spin-polarized current differs at the contact edges. Where the gold contacts of the nanowire are negligible we detect a constant spin polarized current along the nanowirewhich shows their promising potential for optospintronic applications [4].
We acknowledge funding through DFG priority program SPP "Topological Insulators" and DAAD PPP Czech Republic "FemtomagTopo".

[1] S.D. Ganichev et al., J. Phys.: Condens. Matter 15 (2003) R935-R983
[2] J.W. McIver et al., Nature Nanotechnology 7, 96-100 (2012)
[3] T. Schumann et al., arXiv:1810.12799
[4] N. Meyer et al., Appl. Phys. Lett. 116, 172402 (2020)

Paul Alexander McClarty

In this talk I'll discuss experimental signatures of the pseudospin texture in momentum space inthe vicinity of linear magnon touching points. I go on to describe the effects of magnoninteractions and argue that non-Hermitian topology underlies a generic anisotropy in themagnon lineshape around linear touching points.

Dominik Kriegner

In my talk I will present our systematic study of the variation of magnetization and anomalous Hall effect in Mn3−xPtxGa thin films as function of the Pt content and temperature. Since Mn and Pt sharea lattice site and Mn occupies two distinct sites with ferrimagnetic arrangement of the magnetic moments, the system can be driven to a magnetic compensation point [1]. At this point Mn3−xPtxGa can behave similar to the well known antiferromagnetic half Heusler CuMnAs with which is shares its symmetry. CuMnAs was found to have a switchable antiferromagnetic order by a charge current induced staggered Neel spin orbit torque [2]. Recent works also highlighted that thermal effects can complicate the data analysis of aforementioned switching experiments [3].We therefore propose to use Mn3−xPtxGa near the compensation point as a model system to study this spin orbit torque. Here the non-zero magnetization can be used to orient the ferrimagnetic orderby an external magnetic field to prepare the system in a defined state. The current induced torque, (i.e. the variation of the magnetization,) is obtained by homodyne detection and its effect is mapped out for various magnetization orientations. Using this model system we try to contribute to the ongoing discussion about spin orbit torques in antiferromagnets.

[1] R. Sahoo et al., Adv. Mater. 28, 8499 (2016)
[2] P. Wadley et al., Science 351, 587 (2016)
[3] C. C. Chiang, et al. Phys. Rev. Lett. 123, 227203 (2019)

Jungwoo Koo

Geometrically frustrated antiferromagnets have been the subject of extensivetheoretical and experimental works, mainly due to its disordered classical andquantum ground states whose short- or long-range order is determined by adelicate balance between (symmetric and antisymmetric) exchange interactionsand magnetic anisotropy. In addition, large anomalous Hall and Nernst effectsat room temperature have been evidenced from Mn3Sn[1] and Mn3Ge[2,3]. Theunexpected anomalous transport properties from these antiferromagnets areattributed to the inverse triangular spin structure in the basal plane. Moreover,the brokenPTsymmetry of the ground state allows the existence of gaplesselectronic excitations, i.e., the Weyl fermions, in the electronic band structuregiving rise to large anomalous Hall and Nernst conductivities. These materialsare currently of great interest to the topological magnetics community, dueboth to their fascinating Berry curvature driven magnetotransport propertiesand their potential applications in spintronic devices.Despite being an appealing system for the AFM spintronics research, all theintriguing transport properties of Mn3Sn and Mn3Ge were examined withsingle-crystal bulk specimens or polycrystalline thin films. Especially, owingto an ill-defined crystallographic property of polycrystalline thin films, therelationship of anomalous transport properties and the noncollinear AFM spinstructure has yet to be scrutinized thoroughly.In this talk, anomalous Hall and Nernst effects associated with the inversetriangular AFM spin structure of Mn3Sn thin films will be presented. Neutrondiffraction measurement of Mn3Sn thin film confirmed that the spin structureof our thin film is exactly the same type as the bulk specimen. Magnetic phasetransition from the inverse triangular spin structure to the helical order wasnot detected. Anomalous Hall and Nernst effects of Mn3Sn thin films weremeasured along the same crystallographic orientation as the single crystal bulkspecimens, e.g., AHE :I||[01 ̄10] andH||[2 ̄1 ̄10] and ANEQ||[0001] andH||[01 ̄10].Finally, noncollinear AFM domain wall switching and SMR experiments willbe discussed.

[1]S. Nakatsuji, N. Kiyohara, and T. Higo, Large anomalous hall effect in a non-collinear antiferromagnet at room temperature,Nature527, 212 (2015)
[2]N. Kiyohara, T. Tomita, and S. Nakatsuji, Giant anomalous hall effect in the chiral antiferromagnet mn3ge, Physical ReviewApplied5, 10.1103/physrevapplied.5.064009 (2016)
[3]A. K. Nayak, J. E. Fischer, Y. Sun, B. Yan, J. Karel, A. C. Komarek, C. Shekhar, N. Kumar, W. Schnelle, J. K ̈ubler,C. Felser, and S. S. P. Parkin, Large anomalous hall effect driven by a nonvanishing berry curvature in the noncolinearantiferromagnet mn3ge, Science Advances2, e1501870 (2016)

Börge Göbel

Magnetic skyrmions have attracted enormous research interest since their discovery a decade ago. Especially the non-trivial real-space topology of these nano-whirls leads to fundamentally interestingand technologically relevant effects –the skyrmion Hall effect of the texture and the topological Hall effect of the electrons. Furthermore, it grants skyrmions in a ferromagnetic surrounding great stability even at small sizes, making skyrmions aspirants to become the carriers of information in the future. Still, the utilization of skyrmions in spintronicdevices has not been achieved yet, among others, due to shortcomings in theircurrent-driven motion. In this talk, we present recent trends in the field of topological spin textures that go beyond skyrmions. The majority of these objects can be considered the combination of multiple skyrmions or the skyrmion analogues in different magnetic surroundings, as well as three-dimensional generalizations. Weclassify the alternative magnetic quasiparticles –some of them observed experimentally, others theoretical predictions –and present the most relevant and auspicious advantages of this emerging field.A special focus is on magnetic antiskyrmions[1], bimerons[2], antiferromagnetic skyrmions[3]and hopfions[4].These objects (shown in Fig. 1)exhibitadvantageous emergent electrodynamiceffectscompared to skyrmions, either due to their lower symmetry or due to a compensated topological charge.

[1] J. Jena*, B. Göbel*, T. Ma, V. Kumar,R. Saha, I. Mertig, C. Felser, S. Parkin. “Elliptical Bloch skyrmion chiral twins in an antiskyrmion system”Nature Communications11, 1115 (2020)
[2] B. Göbel, A. Mook, J. Henk, I. Mertig, O. Tretiakov.“Magnetic bimerons as skyrmion analogues in in-plane magnets”Phys. Rev. B. 99, 060407(R)(2019)
[3]B. Göbel, A. Mook, J. Henk, I. Mertig,“Antiferromagneticskyrmion crystals: Generation,topological Hall, and topological spin Hall effect”Phys. Rev. B.96, 060406(R)(2017)
[4] B. Göbel, C. Akosa, G. Tatara, I. Mertig.“Topological Hall signatures of magnetic hopfions”Phys. Rev. Research2, 013315 (2020)

Amilcar Bedoya-Pinto

Weyl Semimetals(WSMs), materials with three-dimensional topologically protected electronic states, show highly interesting physical properties including surface Fermi-arcs, the chiral magneto-transport anomalyand extremely high electron mobilities. Still, its potential for device applications needs to be exploitedthrough the preparation of thin films, which would enable the design of functional heterostructures. Onepromising application field of WSMs is spin-orbitronics, as the Fermi-surface is expected to play an importantrole in the spin-to-charge conversion efficiency, according to theoretical investigations[1,2]. In this work, we report the growth of epitaxial, single-crystallineNbP and TaP Weyl Semimetal thin films[3]by means of molecular beam epitaxy, and their successful integration in spin-torquedevices. We have assessed the structural quality of the films(Fig.1a)featuring an atomically flat, ordered surface, essential for the observation of topological bandsby photoemission (Fig. 1b). Furthermore, werely on the preparation of high-quality in-situ TaP/Permalloy interfacesto investigatethe spin-orbit torques produced by the topological WSM by means of spin-torque ferromagnetic resonance (ST-FMR). First resultsof TaP/Py/MgO device structures at room temperature show readily signatures of largespin-orbit torques induced by the Weyl Semimetal: (i) a very strong symmetric component of the voltagelineshape across the resonancerelated to damping-like torques(Fig.1c), much different than the FMR responseof a reference ferromagnet, and(ii) a clear scalingof the resonance linewidth by applying an external DC bias through the bilayer(Fig. 1d). The connectionbetween Fermi-surface topology and spin-to-charge conversionis addressedby performing angle-resolved photoemission measurements on the TaP thin film surfaces prior to the in-situ depositionof the magnetic layers, and probing the spin-torque efficiencyalong the high-symmetry directions of theWSM, where the surface states areexpected to have asubstantialcontribution

[1] Sun, Y, et.al. Strong Intrinsic SpinHall Effect in the TaAs Family of Weyl Semimetals. Phys. Rev. Lett. 117, 146403 (2016)
[2] Johannson, A. et.al. Edelstein effect in Weyl semimetals. Phys. Rev. B 97, 085417 (2018)
[3] Bedoya-Pinto,A.et.al. Realization of NbP and TaP Weyl Semimetal Thin Films, ACS Nano, 14, 4, 4405 (2020)