Antiferromagnets (AFMs) are strong candidates for future spintronic applications largely because of their fast dynamics and lack of stray fields. For the required switching of the Néel vector (staggered magnetization), the predicted current-induced bulk Néel spin-orbit torque (NSOT) (1) is most promising. Only two compounds with the required symmetry are known: for both CuMnAs (2,3) and Mn2Au (4,5) experimental evidence for current-induced NSOT was shown based on a small modification of the AFM domain pattern. However, the significance of NSOT for current-induced manipulation of Neel vector orientation stays controversial because competing heating-related mechanisms result in similar effects (6-8).
Here, we investigate the effect of current pulsing with different lengths along the different crystallographic directions on the AFM domain pattern of epitaxial Mn2Au (001) thin films. In-situ imaging of AFM domains before and after the current pulses was performed using x-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM). Reversible and repeated 90° Neel vector rotation of essentially the complete active area of the pattern cross structures was observed (Fig. 1).
Switching was only observed for current parallel to the easy crystallographic axis resulting in a Néel vector rotation perpendicular to the pulse direction, which is consistent with the NSOT. The required current density for switching is essentially independent of the pulse length (10 µs to 1 ms) indicating that thermal activation or other thermal effects are not significant. For current pulses applied parallel to a <100> hard axis, inversion of the current pulse polarity led to a partially reversible motion of domain walls, which is consistent with the NSOT acting on them. Our results confirm the fundamental role of NSOT for current-induced Néel vector switching in Mn2Au, providing an effective mechanism for spintronics applications. 

**References:** 
(1) J. Zelezný H. Gao, K. Výborný et al., Phys. Rev. Lett. 113, 157201 (2014). 
(2) P. Wadley, B. Howells, J. Zelezný et al., Science 351, 587 (2016). 
(3) P. Wadley, S. Reimers, M. J. Grzybowski et al., Nat. Nanotechnol. 13, 632 (2018). 
(4) S. Bodnar L. Smejkal, I. Turek et al., Nat. Commun. 9, 348 (2018). 
(5) S. Y. Bodnar, M. Filianina, S. P. Bommanaboyena et al., Phys. Rev. B 99, 140409(R) (2019). 
(6) T. Matalla-Wagner, J.-M. Schmalhorst, G. Reiss et al., Phys. Rev. Res. 2, 033077 (2020). 
(7) H. Meer, O. Gomonay, C. Schmitt et al., arXiv:2205.02983v1 [cond-mat.mtrl-sci]. 
(8) H. Meer, F. Schreiber, C. Schmitt et al., Nano Lett.21, 114-119 (2020). 

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Néel spin orbit torque induced remnant switching of the Néel vector in antiferromagnetic Mn2Au

Magnetic skyrmions [1] are topologically stabilized, magnetic quasi-particles, which exhibit interesting physical phenomena and are also potential candidates for efficient neuromorphic computation schemes. On the micro-meter scale, they are detectable with Kerr-microscopy, using the magneto-optical Kerr-effect. However, depending on the material stack, temperature, the growth procedure and other influences, those measurement might suffer from noise, low-contrast, intensity gradients or defects, therefore manual data treatment is normally necessary to generate data that can be evaluated easily.
Our approach is to use a convolutional neural network, based on the U-Net [2], to detect the position and shape of the skyrmions in our measurement data. This work was motivated by reports that machine learning has been successfully applied to (micro-)magnetic problems [3]. We are tuning the U-Net with various techniques to optimize the predictions and identify skyrmions and defects fast and with high reliability. A well-trained neural network is shown to minimize manual treatment of data. The approach is also easily extendable to other magnetic structures. 

**References:** 

[1] Everschor-Sitte et al., Journal of Applied Physics 124, 240901 (2018) 
[2] Ronneberger et al., MICCAI 2015. Lecture Notes in Computer Science, vol 9351. Springer, Cham. (2015) 
[3] Alexander Kovacs et al., Journal of Magnetism and Magnetic Materials 491, 165548 (2019) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b905ba9abf4990b3c3c3e3a184f5069f.png)

Skyrmion detection with U Net

Magnetic skyrmions [1] are magnetic quasi-particles with interesting properties for possible future applications in efficient low-power neuromorphic computing concepts. Thermally excited skyrmions in confined geometries, as necessary for device applications, have been shown to arrange based on commensurability effects [2]. Here, we investigate the enhanced dynamics and altered equilibrium state of a system in which the intrinsic skyrmion-skyrmion and skyrmion-boundary interaction compete with spin-orbit torques (SOTs) due applied currents [3]. In particular, we employ particle-based simulations to study four skyrmions in a triangular confinement, while injecting spin-polarized currents between two corners of the structure [4]. To analyze the observed positions of the skyrmion ensemble, we coarse-grain the skyrmion states in the system. In the context of neuromorphic computing, such coarse-graining may be key to identify the most suitable positions for potential readouts as well as to understand the collective skyrmion dynamics in systems with competing interactions at different scales. 
**References** 
[1] K. Everschor-Sitte et al., Journal of Applied Physics 124, 240901 (2018) 
[2] C. Song et al., Adv. Funct. Mater. 31, 2010739 (2021) 
[3] K. Raab et al., arxiv:2204.14720 (2022) 
[4] T. Winkler et al., in preparation (2022)

Analyzing collective thermal Skyrmion Dynamics by coarse graining

The discovery of two-dimensional van der Waals ferromagnets [1,2] has been long anticipated since the effects of spatial dimensionality on criticality were recorded [3,4]. The potential applications of this new class of materials seem vast within the field of spintronics. Fe3GeTe2 has already proved highly efficient for spin-orbit torque switching [5], yet rich skyrmionic textures [6,7] and semi-metallicity [8] are also found within its roster of ever-growing phenomena. Fe5GeTe2, a related compound with a higher Tc [9], presents its own unique set of magnetic order that can be tuned with doping [10] and gate voltage [11]. In a recent collaboration [12] we found spin-orbit torques within bulk Fe3GeTe2 due to a site preference that breaks the crystal’s centrosymmetry [13]. Meanwhile, in Fe5GeTe2, we found skyrmionic textures present even up to room temperature [14]. To understand the origins of these effects one must delve into the underlying electronic structure to provide a comprehensive understanding of the associated spin dynamics.

Using a variety of first-principles methods [12,14,15] we evaluate the exotic phenomena observed in both Fe3GeTe2 and Fe5GeTe2. In Fe3GeTe2 strong ‘hidden’ current-induced torques are harvested by each of the two-dimensional Fe3GeTe2 layers separately. Analysis of the electronic structure elucidates an interplay of spin and orbital degrees of freedom which have a profound impact on spin-orbit torques. A drastic difference in the behavior of the individual components of the torque results in non-trivial switching. In Fe5GeTe2 we use first principles methods in combination with atomistic spin-dynamics simulations to characterize the domain wall and subsequent spin textures. These distinct features from both compounds present an opportunity to tailor the spin and orbital dynamics with Fe concentration, opening a new era of van der Waals spin orbitronics. 
**References** [1] C. Gong et al., Nature 546, 265 (2017).
[2] B. Huang et al., Nature 546, 270 (2017).
[3] R. B. Griffiths, Physical Review 136, 437 (1964).
[4] N. D. Mermin et al., Physical Review Letters 17, 1133 (1966).
[5] M. Alghamdi et al., Nano Letters 19, 4400 (2019).
[6] B. Ding et al., Nano Letters 20, 868 (2020).
[7] H.-H. Yang et al., 2D Materials 9, 025022 (2022).
[8] K. Kim et al., Nature Materials 17, 794 (2018).
[9] A. F. May et al., ACS Nano 13, 4436 (2019).
[10] A. F. May et al., Physical Review Materials 4, 074008 (2020).
[11] C. Tan et al., Nano Letters 21, 5599 (2021).
[12] F. Martin et al., arXiv:2107.09420.
[13] A. Chakraborty et al., Advanced Materials 34, 2108637 (2022).
[14] M. Schmitt et al., arXiv:2204.11348.
[15] T. G. Saunderson at al., arXiv:2204.13052.

Harnessing the spin and orbital dynamics of iron based 2D van der Waals ferromagnets

Reservoir computing (RC) has been considered as one of the key computational principles beyond von-Neumann computing. Magnetic skyrmions, topological particle-like spin textures in magnetic thin films, are particularly promising for implementing RC [1] since they respond strongly nonlinear to external stimuli and feature inherent multiscale dynamics. We propose and demonstrate experimentally a conceptually new approach to skyrmion RC that exploits the thermally activated diffusive motion of skyrmions [2]. By confining the thermal and low-energy, electrically gated skyrmion motion, together with employing spatially resolved input and readout, we find that already a single skyrmion in a confined, triangular geometry (Fig. 1) suffices to realize non-linearly separable functions, which we demonstrate for the XOR gate along with all other Boolean logic gate operations [3] (Fig. 2). Our proposed concept can be readily extended by linking multiple confined geometries and/or by including more skyrmions in the reservoir, suggesting high potential for scalable and low-energy reservoir computing.

Brownian Reservoir Computing Realized Using Geometrically Confined Skyrmions

We report the experimental demonstration of long-range orbital torques generated by Nb and Ru and detected by spin-torque ferromagnetic resonance (ST-FMR) measurements in Nb/Ni and Ru/Ni bilayers. We identify the orbital torques from the sign-reversal and a strong enhancement in the damping-like torque observed in Nb (Ru)/Ni bilayers as compared to Nb (Ru)/FeCoB bilayers as theoretically predicted1. The long-range nature of orbital transport in the ferromagnet was revealed by varying the thickness of Ni in Nb (Ru)/Ni bilayers which is markedly different compared to the regular spin absorption in the ferromagnet that takes place in the first few layers confirming the recent prediction2. Finally, we show that the external injection of orbital current in a heavy metal such as Pt can convert the orbital current into a spin-current leading to higher switching efficiency and the enhanced spin-current generation that could both be potentially attractive for the application3,4. 
**References** 1. Go, D. & Lee, H.-W. Orbital torque: Torque generation by orbital current injection. Phys. Rev. Res. 2, 013177 (2020).
2. Go, D. et al. Long-Range Orbital Magnetoelectric Torque in Ferromagnets. arXiv 2106.07928, 1–6 (2021).
3. Ding, S. et al. Harnessing Orbital-to-Spin Conversion of Interfacial Orbital Currents for Efficient Spin-Orbit Torques. Phys. Rev. Lett. 125, 177201 (2020).
4. Go, D., Jo, D., Lee, H.-W., Kläui, M. & Mokrousov, Y. Orbitronics: Orbital currents in solids. EPL (Europhysics Lett. 135, 37001 (2021).
5. 
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Detection of long range orbital torques

Magnetic skyrmions are topological spin textures that can be stabilized by the Dzyaloshinskii-Moriya interaction [1,2]. There are particularly suitable for next generation spintronics devices, like the skyrmion-based racetrack memory [3]. Recent studies confirmed their current-driven skyrmion dynamics in ultrathin ferromagnets via spin-orbit torques [4], where the chirality and spin texture of the skyrmions are key. However, the topological Magnus effect leads to a transverse motion of ferromagnetic skyrmions due to their non-zero topological charge [5], which is disadvantageous for devices. Antiferromagnetically exchange-coupled skyrmions or compensated ferrimagnets could suppress this effect owing to an overall zero topological charge [6]. Here we explore a GdFeCo-based ferrimagnet system with perpendicular magnetic anisotropy. We demonstrate that in this system the chirality of worm domains as well as a magnetic skyrmionium can be observed using scanning electron microscopy with polarization analysis (SEMPA) (Figure) [7]. The high spatial resolution magnetic imaging technique reveals the domain wall (DW) spin structure as a function of temperature, with the observed structures found to be pure Néel-type spin textures, which is promising for devices. From the images we extract the DW width as a function of the temperature and we calculate the exchange stiffness for the system. Figure: Determination of the magnetization direction inside the DW of different ferrimagnetic chiral spin textures. a) The direction of the in-plane magnetization in the DW is displayed, as defined by the color wheel in the bottom left corner of the image. Scale bar in a): 1 µm. b) Distribution of the direction of the in-plane magnetization in the DW with respect to the local tangent at RT: angle Ψ (see inset where a clockwise angle is defined as negative following the usual convention). A Gaussian fit indicates a central value around −90°. c) Absolute in-plane magnetization intensity with the DW skeleton displayed in white and d) direction of the in-plane magnetization in the DW of a ferrimagnetic skyrmionium at 320 K. Scale bars in c,d): 500 nm. Extracted from [7].

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Direct imaging of chiral domain walls in ferrimagnetic alloys

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research [1]. It arises from the dependence of the resistance of a magnetically ordered solid on the relative angle between the applied electric field and the magnetization vector independent of their polarity. The origins of the AMR are usually ascribed to extrinsic effects (i.e. spin-dependent electron scattering), whereas intrinsic (i.e. scattering-independent) contributions are neglected [2]. Here, we perform a broadband measurement of the AMR of prototypical ferromagnets such as Co, Ni or NiFe alloys in a frequency range from DC to 28 THz [3]. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni and NiFe alloys is of predominantly extrinsic nature. However, the Co thin film exhibits a sizeable intrinsic, frequency-independent AMR contribution, which amounts to more than 2/3 of the DC AMR contrast of 1%. These features are attributed to the lower symmetry of the hexagonal structure of Co crystallites which allows for larger anisotropies in their bandstructure. The new insight into the nature of AMR is not only relevant for the fundamental understanding of the magnetotransport but might also advance the ultrafast probing techniques in spintronic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/b939fa383f615a86b5ac200134a15b4d.jpg)

Fig. 1. THz detection of the AMR. The incident THz pulse applies transient electric field in the direction of polarization, in which, as a consequence of the AMR, the electrical resistance and the transmission of the THz pulse depend on the angle of the magnetization. Variations in transmitted amplitude allow us to calculate the AMR contrast.


![](https://assets.underline.io/uploads/markdown_image/1/image/584dca932356943c776e64e9fc76bbd6.jpg)

Fig. 2. Spectral composition of the AMR in different metals. (a) The absolute value of the AMR contrast in permalloy versus frequency is well described within the Drude formalism (curve) assuming only magnetization-dependent electron scattering rates. (b) The same measurement for a cobalt sample shows a scalable frequency-independent offset which witnesses the intrinsic contribution to the AMR.

Broadband terahertz spectroscopy of anisotropic magnetoresistance reveals intrinsic contributions

Contemporary spintronics, utilizing the electronic spin for information processing and microelectronics, is mostly based on ferromagnetic device architectures. In view of long term perspectives to enable enhanced data processing speeds and downscaling for on-chip information processing, spintronics with antiferromagnets is a promising avenue [1]. Antiferromagnets exhibit the key advantage over ferromagnets that their resonance frequency is enhanced by the exchange coupling of the sublattices, and thus generally in the terahertz regime. In compensated antiferromagnets, the absence of a net moment however strongly impedes simple access to their ultrafast dynamics, especially in thin films, and the development of ultra-fast antiferromagnet-based devices [2]. Experimental access to the spin dynamics can be facilitated by spin-to charge conversion mechanisms such as the spin pumping effect [3,4] or by probing the light-matter interaction of the spin system via a coupling to cavity resonator photons [5]. The spin pumping effect generates alternating (AC) and continous (DC) spin currents in an adjacent conductor which can be electrically detected, for instance, by the inverse spin hall effect (ISHE) [6]. On the other hand, light-matter interaction is also at the heart of cavity spintronics which describes the generation and manipulation of cavity-magnon polaritons (CMP) for a general understanding of these interactions and applications for information processing and spintronics. The CMP is the associated quasiparticle to the hybridization of cavity resonator photons and collective spin excitations, i.e. magnon modes and the hallmark for a coherent information transfer in the strong coupling regime [5]. Such coherent information transfer is key for the implementation of microwave-to-optics transducers or quantum-based information processing schemes such as quantum memories [7]. Probing the spin dynamics electrically by spin-to-charge mechanisms or via the light-matter interaction of a strongly coupled photon-magnon system allows to study the physics of spin dynamics from different perspectives as it is at the crossroads of spintronics, magnonics and photonics. In this work, we access the spin dynamics of a collinear (easy axis) antiferromagnetic system by using a bulk chromium oxide (Cr2O3) sample as a model system. We both investigate the hybridization between the spin precession in the low frequency left-handed mode with cavity resonator photons and its temperature dependence via recording the corresponding ISHE voltages. We measure a CMP in Cr2O3 at 150 K and we observe several, small couplings to resonator modes below the spin flop field at 6 T. For instance, for a resonator mode ~39.6 GHz, we find a coupling strength of approximately 50 MHz. For the spin pumping experiment, we find a non-monotonous temperature dependence of the ISHE voltage signal with a peak at 30 K of the low frequency left-handed mode. The opposite signs of the ISHE voltages between the left-handed mode of Cr2O3 and the right-handed mode of ferrimagnetic YIG confirm the mode polarity. We compare our findings to the work of J.Li et al., who studied the temperature dependence of the high frequency right-handed mode of Cr2O3 [8]. We find qualitatively the same temperature dependence towards higher signals with the signal vanishing above 100 K as the net spin current polarization decreases to higher temperatures [8]. Hence, we study the spin dynamics of our model system Cr2O3 for GHz frequencies based on electrical detection and light matter interaction. Notably, establishing antiferromagnets in cavity spintronics could bridge frequency regimes from the GHZ to the optical range. Further, the demonstration of a coupling and, eventually, coherent information exchange is a promising way towards quantum information processing schemes with antiferromagnets [9].

**References:**

[1] V. Baltz, A. Manchon, M. Tsoi et al., Rev. Mod. Phys, 90, 015005 (2018). [2] T. Kampfrath, A. Sell, G. Klatt et al., Nat. Photonics 5, 31 (2011). [3] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Phys. Rev. B 66, 224403 (2002). [4] M. V. Costache, M. Sladkov, S. M. Watts et al., Phys. Rev. Lett. 97, 216603 (2006). [5] M. Harder and C. M. Hu, Solid State Physics, 69, 47 - 121 (2018). [6] E. Saitoh, M. Ueda, H. Miyajima et al., Appl. Phys. Lett. 88, 182509 (2006). [7] D. Lachance-Quirion, Y. Tabuchi, A. Gloppe et al., Appl. Phys. Expr. 12, 070101 (2019). [8] J. Li, C. B. Wilson, R. Cheng et al., Nature 578, 7793 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/64f1679716243edc0e3b382587c8dbac.jpg)
Dispersion relation of the CMP for a bulk chromium oxide sample (Cr2O3 ) at 150 K with several couplings for higher frequencies across the spin flop field at ~6 T. Inset displays the gradient calculated from the amplitude spectrum in the main. It shows expemplarily a zoom on one clearly separated anticrossing at 39.6 GHz with a coupling strength of ~50 MHz.

![](https://assets.underline.io/uploads/markdown_image/1/image/9eb4756535fc1148c73ad825bbc5330a.jpg)
Temperature dependence of the ISHE voltage from spin pumping with the same crystal up to 100 K.

 Cavity Magnon Polaritons and Inverse Spin Hall Effect in Easy-Axis Antiferromagnets

EOA: Antiferromagnetic Spintronics I

technical paper

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EOF: Spin-Orbitronics II

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SHORT BIO

Presentations

Néel spin orbit torque induced remnant switching of the Néel vector in antiferromagnetic Mn2Au

Skyrmion detection with U Net

Analyzing collective thermal Skyrmion Dynamics by coarse graining

Harnessing the spin and orbital dynamics of iron based 2D van der Waals ferromagnets

Brownian Reservoir Computing Realized Using Geometrically Confined Skyrmions

Detection of long range orbital torques

Direct imaging of chiral domain walls in ferrimagnetic alloys

Broadband terahertz spectroscopy of anisotropic magnetoresistance reveals intrinsic contributions

Cavity Magnon Polaritons and Inverse Spin Hall Effect in Easy-Axis Antiferromagnets

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