News and posts

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)

Can Onur Avci

Perpendicularly magnetized ferrimagnetic insulators (FMI) have been drawing increasing attention in spintronics research. Recent achievements of efficient current-induced switching and domain wall motion [1-5] in perpendicular FMIs, combined with theirhighly tunable propertiesopen novel avenues for practical applications. Despite theFMIs’electrically insulating nature, the spin Hall magnetoresistance (together with its anomalous Hall component) [6] provides us with the relevant tools to detect theirmagnetization vector ina local geometry using a Hall bar device. Later on, it was discovered that thein-plane magnetization vectorofan FMIcan be detectedalsoin a nonlocal geometry by long distance magnon transport[7,8]. However, the detection of the perpendicular magnetization vector of an FMI in a nonlocal geometry remained a challenge for a long time. In this work[9],wedemonstratethat,by usingan engineered temperature gradient,one can detect theout-of-plane magnetization of anFMIby simply measuring the transverse voltage drop across the Ptstrip placed on top. This is due to a conceptually new mechanism that combines the spin currents driven by an out-of-plane(∇#$)and in-plane(∇%$)temperature gradientsin a Pt/FMI bilayer generated by a single nonlocal heat source. When the magnetization has a component oriented perpendicular tothe plane, ∇#$drives a spin current into Pt with out-of-plane polarization due to the spin Seebeck effect. ∇%$then drags the resulting spin-polarized electrons in Pt parallel to the plane against the gradient direction. This finally produces an inverse spin Hall effect voltage in Pt, transverse to ∇%$and proportional to the out-of-plane component of theFMI’smagnetization(see Fig.1). This simple method enables the detection of the perpendicular magnetization component in anFMIin a nonlocal geometryand opens up new routes towardsengineeringtemperature gradients togenerateandmanipulate thermal magnons and pure spin currents.

[1]Avci et al.Nat. Mater.16, 309 (2017)
[2]Shao et al.Nat. Commun. 9, 3612 (2018)
[3]Avci etal.Nat. Nanotech. 14, 561 (2019)
[4]Velez et al.Nat. Commun. 10, 4750 (2019)
[5]Ding et al.PRB 100, 100406(R)(2019)
[6]Nakayama et al.PRL 110, 206601 (2013)
[7]Cornelissen et al.Nat. Phys. 11, 1022 (2015)
[8]Goennenwein etal.107, 172405 (2015)
[9]Avci et al.PRL124, 027701 (2020)

Vivek Amin

Ferromagnets generate spin currents under an applied electric field. For example, charge currents in ferromagnets are spin-polarized because majority and minority carriers have different conductivities. However, in the presence of spin-orbit coupling, electrons can carry a substantial spin current flowing perpendicularly to the electric field with spin directions both longitudinal and transverse to the magnetization.
In this talk, we discuss several mechanisms to electrically generate spin currents in ferromagnets. These mechanisms are closely related to the anomalous and planar Hall effects but yield spin currents with spin directions transverse to the magnetization. Such spin currents can be detected through the torques they exert at layer boundaries [1]. We present first principles transport calculations giving the strength and magnetization dependence of the electrically generated spin currents allowed by symmetry via both intrinsic [2] and extrinsic [3] mechanisms. We find that in transition metal ferromagnets, the spin currents with spin direction transverse to the magnetization can have an associated conductivity comparable to the spin Hall conductivity in Pt.

[1] W. Wang, T. Wang, V. P. Amin, Y. Wang, A. Radhakrishnan, A. Davidson, S. R. Allen, T. J. Silva, H. Ohldag, D. Balzar, B. L. Zink, P. M. Haney, J. Q. Xiao, D. G. Cahill, V. O. Lorenz, and X. Fan, Anomalous Spin-Orbit Torques in Magnetic Single-Layer Films, Nature Nanotechnology, 14, 819-824, 2019
[2] V. P. Amin, J. Li, M. D. Stiles, and P. M. Haney, Intrinsic Spin Currents in Ferromagnets, Phys. Rev. B 99, 220405(R), 2019
[3] V. P. Amin, J. Zemen, and M. D. Stiles, Interface generated spin currents, Phys. Rev. Lett. 121, 136805 (2018)