News and posts

Kouta Kondou

Spin Hall effect (SHE) provides the spin-charge interconversion in non-magnetic materials, which has drawn much attention because of its potential application for efficient magnetization switching via the spin torque [1].
Here we focus on the topological antiferromagnet Mn3Sn to realize the new functionality in spin-charge conversion. Mn3Sn exhibits the large anomalous Hall effect comparable with ferromagnet at room temperature [2]. Figure 1 shows a devise structure for spin accumulation detection. By applying the charge current on a Mn3Sn strip, spin accumulation can be detected electrically by the ferromagnetic electrode. This technique enables us to observe the SHE in Mn3Sn, exhibiting a sign change when its small magnetic moment switches orientation. Additionally, we succeeded in observation of the sign change in the inverse effect by means of spin pumping method [3]. These new SHEs were named Magnetic spin Hall effect (MSHE) and Magnetic inverse spin Hall effect (MISHE). Recently we fabricated the ferromagnetic layer/Mn3Sn bilayer to investigate the spin torque due to MSHE in Mn3Sn, which can be expected to realize the unconventional spin torque generation.

[1] L. Liu, C-F. Pai, Y. Li, H. W. Tseng, D. C. Ralph and R. A. Buhrman, Science 336, 555 (2012).
[2] S. Nakatsuji, N. Kiyohara and T. Higo, Nature 527, 212 (2015).
[3] M. Kimata, H. Chen, K. Kondou et al., Nature 565, 627 (2019).

Annika Johansson

The Edelstein effect, also known as inverse spin-galvanic effect, is a magnetoelectric phenomenon providing charge-current-to-spin conversion in systems with broken inversion symmetry. In pristine nonmagnetic materials, a finite macroscopic spin polarization can be induced purely electrically by the application of an electric field [1,2]. Originally, the Edelstein effect has been discussed for two-dimensional Rashba systems at surfaces or interfaces. Whereas for isotropic Rashba systems a large spin-orbit splitting is crucial for efficient charge-to-spin conversion, current research aims at finding novel materials beyond conventional Rashba systems providing a large direct or inverse Edelstein effect, for example three-dimensional topological insulators and oxide interfaces.
In this talk the Edelstein effect in Rashba systems and topological materials is discussed within the semiclassical Boltzmann transport theory [3,4]. Here, one focus is on finding materials in which a large Edelstein effect can be realized. Considering geometrical and topological properties, Weyl semimetals are identified as candidates for a highly efficient charge-to-spin conversion [4].
Further, SrTiO_3-based two-dimensional electron gases (2DEGs) have been found to provide a large inverse Edelstein effect [5], in particular the 2DEG emerging at the interface between SrTiO_3 and AlO [6]. The application of a gate voltage leads to a strong variation and even sign changes of the spin-to-charge conversion efficiency. This unconventional gate dependence is explained by a band-resolved analysis of the Edelstein signal. The experimentally observed spin-to-charge conversion is related to the band structure as well as the topological character and the spin texture of the 2DEG [6]. In addition the orbital Edelstein effect, originating from the orbital moments, is analyzed, which can exceed the conventionally discussed spin Edelstein effect by one order of magnitude.

[1] A. G. Aronov, Y. B. Lyanda-Geller, JETP Lett. 50, 431 (1989)
[2] V. M. Edelstein, Solid State Commun. 73, 233 (1990)
[3] A. Johansson et al., Phys. Rev.B 93, 195440 (2016)
[4] A. Johansson et al., Phys. Rev.B 97, 085417 (2018)
[5] E. Lesne et al., Nat. Mater. 15, 1261 (2016)
[6] D. Vaz et al., Nat. Mater. 18, 1187 (2019)

Max Hirschberger

Recently, we have explored a variety of metallic materials where non-coplanar magnetic order occurs on characteristic length scales λ comparable to the size of a single crystallographic unit cell. In this limit, the standard (real-space) model of the ‘topological’ Hall effect, the key transport signature of Berry curvature due to canted magnetism, is expected to fail. In metallic magnets where the carrier mean free path exceeds λ, a momentum-space picture of the THE is expected to be more appropriate for an adequate description.
We aim to approach the momentum space regime using four model compounds, listed here in order of decreasing λ: (1) In rare-earth magnets with inversion center such as Gd2PdSi3 and Gd3Ru4Al12, we observed nanometer-sized skyrmion textures in highly symmetric lattices and in absence of Dzyaloshinskii-Moriya interactions (λ~2-3 nm). (2) The (breathing) Kagome system Dy3Ru4Al12 realizes a peculiar arrangement of antiferromagnetically stacked canted spin-trimers (λ~1-2 nm). (3) The metallic pyrochlore oxide Nd2Mo2O7 is a canted ferromagnet where non-coplanarity occurs within a single unit cell (λ~1 nm).
The topological Hall and Nernst effects of these materials were studied while tuning a variety of ‘external knobs’. We changed the Fermi energy via substitutional doping and also modified the lattice spacing via hydrostatic pressure. Thus, we aim to develop new phenomenology of transport signatures characteristic in the limit of entangled real-space canted magnetism and momentum space Berry curvature.

Sergey Grytsiuk

Three-dimensional magnetic textures with particle-like properties [1-2] have been recently growing in popularity due to a great potential for innovative spintronic applications [3-5] and brain-inspired computing. However, only little is known about solids where such 3D magnetic solitons may exist and a complete theoretical model for the underlying magnetic interactions is remarkably elusive until now. While, for instance, the basic magnetic properties of the 2D skyrmions are determined by an intricate competition involving the Heisenberg exchange and the chiral relativistic Dzyaloshinskii–Moriya interaction (DMI), such models fail to explain the 3D-magnetic texture of few-nm size observed in MnGe.
Recently, we have discovered a conceptually new class of the magnetic interactions rooted in the so-called topological orbital moment, a Berry-phase effect that results from the orbital motion of the electron in a complex magnetic background [6]. We refer to these interactions as topological–chiral interactions, favouring the emergence of non-coplanar magnetic structures with scalar spin chirality of specific sign even without an external magnetic field. The novel interactions offer fundamentally different opportunities for imprinting chiral magnetism, as they manifest in the scalar chirality of spin arrangements on triangular plaquettes, as opposed to the vector chirality between pairs of spins in the case of DMI. By means of density functional calculations we demonstrate that these interactions are not small but can dominate over the celebrated DMI in selecting the chiral ground state, providing possibly a key for solving the open question of the recently observed complex 3D magnetic structures in B20-type chiral magnet MnGe. In addition, we show that in the continuum limit, the spin-chirality relates to the curvature of the magnetization field and one flavour of the topological–chiral interaction reverts to the Faddeev model [1] with solutions for the 3D magnetic solitons, known as hopfions.
In this talk, I will give brief introduction to the topological–chiral interactions focusing on rigorous derivation based on Multiple scattering theory. Also, I will discuss the role of the novel interactions in stabilising the nontrivial magnetic orders in bulk and thin films of different materials.

[1] Faddeev, L. D. Preprint IAS Print-75-QS70 (Inst. Advanced Study, Princeton, NJ, 1975), 32 pp. 31.
[2] N. Manton and P. Sutcliffe, Topological Solitons (Cambridge University Press, Cambridge, England, 2004).
[3] A. M. Kosevich et al., Phys. Rep. 194, 117 (1990).
[4] X. S. Wang et al., PRL 123, 147203 (2019)
[5] Yizhou Liu et al., 124, 127204 (2020)
[6] S. Grytsiuk et al., Nature Comm., 11, 511 (2020)

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.