United States

Magnetic skyrmions [1] are promising candidates for next-generation spintronic devices. Their solitonic and chiral nature allows an efficient current-induced motion up to high velocities [2]. A fine tuning of their chirality would enable highly-manipulable magnetic skyrmions, which paves the way for new functionalities in spintronic memory, logic and neuromorphic devices [3,4]. This gives great importance to the control of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) [5,6] strength and sign, at the origin of magnetic skyrmions and their chirality. In this study, we directly demonstrate magnetic skyrmion’s chirality switching in a Ta/FeCoB/TaOx trilayer. In fact, by varying the thickness of FeCoB and the oxidation state at the FeCoB/TaOx interface, we are able to obtain different regions in which magnetic skyrmions are right-handed (iDMI&gt;0) or left-handed (iDMI&lt;0) (resp. regions #4 and #2 on Fig.1). This material-dependent chirality switching brings a new degree of freedom to the exciting physics of magnetic skyrmions. For the first time, we show that the iDMI sign crossover in the different regions, as measured by Brillouin-Light-Scattering (BLS), is accompanied by a change of skyrmion chirality, as confirmed by the opposite direction of current-induced motion of magnetic skyrmions (See Fig. 2). Notably, between the two regions of opposite iDMI sign we have found, as expected, a region where iDMI∼0. In this region, the control of the iDMI with an electric field [7] may lead to a voltage-induced chirality switching, which hasn’t been observed yet. This all-electrical control of magnetic skyrmions represents a cornerstone towards power efficient spintronic devices and multidirectional logic functionnalities.References: [1] A. N. Bogdanov and D. A. Yablonskii, J. Exp. Theor. Phys., Vol. 95, p101−103 (1989) [2] W. Jiang, P. Upadhyaya and W. Zhang et al., Science, Vol. 349 (2015) [3] X. Zhang, Y. Zhou and K. M. Song et al., J. Phys.: Condens. Matter, Vol. 32 (2020) [4] A. Fert, N. Reyren and V. Cros et al., Nat. Rev. Mater., Vol. 2 (2017) [5] I. E. Dzyaloshinskii, Sov. Phys. JETP, Vol. 5 (1957) [6] T. Moriya, Phys. Rev., Vol. 120 (1960) [7] T. Srivastava, M. Schott and R. Juge et al., Nano. Lett, Vol. 18 (2018)

MMM 2020

Chirality Switch of Magnetic Skyrmion in Ta/FeCoB/TaOx Trilayers

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Magnetic skyrmions [1] are promising candidates for next-generation spintronic devices. Their solitonic and chiral nature allows an efficient current-induced motion up to high velocities [2]. A fine tuning of their chirality would enable highly-manipulable magnetic skyrmions, which paves the way for new functionalities in spintronic memory, logic and neuromorphic devices [3,4]. This gives great importance to the control of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) [5,6] strength and sign, at the origin of magnetic skyrmions and their chirality. In this study, we directly demonstrate magnetic skyrmion’s chirality switching in a Ta/FeCoB/TaOx trilayer. In fact, by varying the thickness of FeCoB and the oxidation state at the FeCoB/TaOx interface, we are able to obtain different regions in which magnetic skyrmions are right-handed (iDMI>0) or left-handed (iDMI<0) (resp. regions #4 and #2 on Fig.1). This material-dependent chirality switching brings a new degree of freedom to the exciting physics of magnetic skyrmions. For the first time, we show that the iDMI sign crossover in the different regions, as measured by Brillouin-Light-Scattering (BLS), is accompanied by a change of skyrmion chirality, as confirmed by the opposite direction of current-induced motion of magnetic skyrmions (See Fig. 2). Notably, between the two regions of opposite iDMI sign we have found, as expected, a region where iDMI∼0. In this region, the control of the iDMI with an electric field [7] may lead to a voltage-induced chirality switching, which hasn’t been observed yet. This all-electrical control of magnetic skyrmions represents a cornerstone towards power efficient spintronic devices and multidirectional logic functionnalities.References: [1] A. N. Bogdanov and D. A. Yablonskii, J. Exp. Theor. Phys., Vol. 95, p101−103 (1989) [2] W. Jiang, P. Upadhyaya and W. Zhang et al., Science, Vol. 349 (2015) [3] X. Zhang, Y. Zhou and K. M. Song et al., J. Phys.: Condens. Matter, Vol. 32 (2020) [4] A. Fert, N. Reyren and V. Cros et al., Nat. Rev. Mater., Vol. 2 (2017) [5] I. E. Dzyaloshinskii, Sov. Phys. JETP, Vol. 5 (1957) [6] T. Moriya, Phys. Rev., Vol. 120 (1960) [7] T. Srivastava, M. Schott and R. Juge et al., Nano. Lett, Vol. 18 (2018)

technical paper

High-gyrotropy rare-earth iron garnets with low losses enable the realization of integrated optical isolators for laser-on-chip silicon photonics [1]. However, integrated isolators with magneto-optical materials have always required an external magnetic field. Novel waveguide isolator designs with Ce:TbIG can operate in the remnant magnetization state and overcome the magnetic field requirement [2,3]. Further magnetless design optimizations and device downscaling require a deeper understanding of the magnetic properties of the material in the bulk and at the interface. Here, sputter-deposited Ce:TbIG thin films with large Faraday rotation (∼-3500°/cm) are shown to have a near-ideal stoichiometry of Ce0.5Tb2.5Fe4.75O12 with a rare-earth to iron ratio (Ce+Tb/Fe) of 0.57. Magnetometry revealed a 44nm magnetic dead layer (MDL) at the garnet-silicon interface that can be detrimental to waveguide photonics relying on evanescent tail interaction. However, the large Faraday rotation (independent of film thickness) compensates for gyrotropy loss due to the MDL, Figure 1(a). The effective anisotropy of Ce:TbIG on Si is also important, but calculations using bulk thermal mismatch overestimate the effective anisotropy. X-ray diffraction measurements yield highly accurate measurements of strain that show anisotropy favors an in-plane magnetization in agreement with the positive magnetostriction of Ce:TbIG, Figure 1(b). Upon doping TbIG with Ce, a slight decrease in compensation temperature occurs which points to preferential rare-earth occupation in dodecahedral sites and an absence of cation redistribution between different lattice sites [4]. The high Faraday rotation, large remanent ratio, large coercivity, and preferential in-plane magnetization enable Ce:TbIG to be an in-plane latched garnet, immune to stray fields with magnetization collinear to the direction of light propagation.References: [1] K. Srinivasan and B. J. H. Stadler, Opt. Mater. Express, Vol. 8, p. 3307 (2018). [2] K. Srinivasan et al., ACS Photonics, Vol. 6, pp. 2455–2461 (2019). [3] C. Zhang et al., Sci. Rep., Vol. 7, pp. 1–8 (2017). [4] K. Srinivasan et al., Adv. Funct. Mater., Vol. 30, p. 2000409 (2020).

Interfacial and Bulk Magnetic Properties of Stoichiometric Cerium Doped Terbium Iron Garnet Polycrystalline Thin Films

We are presenting some results of our experiments image of formation using a device in which light is scattered by a thin film of ferrofluid subjected to an external magnetic field and comparing them with the effect of optical vortex [1-7]. When the light passes through this thin layer of ferrofluid, we can observe directly, with the naked eye, the formation of interesting luminous patterns. In addition to this, we can also observe the effects of light polarization in this system using a polariscope. Some interesting applications of this type of device include optical switches, or the simulation of atmospheric optics known as jumping laser dogs, just to cite few of them. Another potential application is characterizing and controlling the quality of compound magnets observing the patterns formed in this type of Hele-Shaw cell filled with ferrofluids, known as Ferrocell. Using polarized light, the pattern formation curiously resembles the nodal lines of the transverse electric and magnetic modes observed in optical resonant cavities, associated with the hypergeometric polynomials [6], such as Hermite-Gauss, Laguerre-Gauss, and Ince-Gauss polynomials. In this analogy, the magnetic field has an equivalent role of the Gaussian irradiance distribution, and the light source is a very flat lamp normal to z, which nearly corresponds to a plane wave with constant flux set up to the z-axis, analogous to a standing wave in the optical cavity. In this work, we are exploring the analogy between our experiment with the mode conversion in a laser cavity with our system, because mode convergion requires Hermite-Gaussian (HG) modes, which can easily be made inside the laser cavity and in our experiment using polarized light.References: 1) A. Tufaile, T. A. Vanderelli, A. P. B. Tufaile, Journal of Applied Mathematics and Physics 4, 1977-1988 (2016). 2) A. Tufaile, T. A. Vanderelli, A. P. B. Tufaile, Advances in Condensed Matter Physics 2017, Article ID 2583717 (2017). 3) Y. Shen, X. Wang , Z. Xie, C. Min, X. Fu, Q. Liu, M. Gong and X. Yuan, Light Science & Applications 8, 90 (2019). 4) A. Tufaile, A. P. B. Tufaile, T. A. Vanderelli, M. Snyder, Proceedings of Frontiers in Optics/Laser Science, JTu3A.17 (2019) 5) A.P.B Tufaile, T.A. Vanderelli, R. Amorim, A. Tufaile, Proceedings of the 12th Conference on Light and Color in Nature, 24–25 (2016). 6) A. Tufaile, T. A. Vanderelli, M. Snyder, A. P. B. Tufaile, Condens. Matter 2019, 4, 35. 7) A. Tufaile, T. A. Vanderelli, M. Snyder, A. P. B. Tufaile, Controlling Light Diffraction with Magnetic Nanostructures; TechConnect Briefs: Boston, MA, USA; ISBN 978-009988782-8-7, (2019).

Optical Vortex and Ferrocell: A Comparative Study

Magnetic field pulser generates magnetic field by driving current through an inductor. These pulsers have numerous applications based on the output magnetic field intensity and switching time. For communication systems, the pulser is central for an all-optical switching platform employing the theory of Faraday rotation [1]. In medical field, the pulser is used to perform transcranial magnetic stimulation to provide safer, noninvasive treatment for certain mental disorders [2]. Another application area is portable magnetic resonance systems where the pulser initiates magnetic resonance [3]. These are just some of the many fields where magnetic field pulsers have found applicability over the years. Recent developments in transistor technologies have enabled production of transistors with higher power and superfast switching. Among these is the introduction of gallium nitride (GaN) based transistors. Compared to silicon-based transistors, GaN transistors have higher electron mobility, a property which allows smaller size for a given on-resistance and breakdown voltage. GaN transistor also offers high drain current and extremely fast switching speed [4]. These are all desirable properties for a switch in the magnetic field pulser design. This investigation will explore the application of GaN transistor in a pulser design. Based on the advantages of GaN transistor, the investigation looks towards creating a pulser capable of producing magnetic field of 500 Gauss with a rise/fall time of less than 500 nanoseconds. This investigation will surpass findings from prior studies to build a pulser targeted for magneto-optic switching [5] applications. A preliminary result as seen in Figure 1 shows that at the same maximum current level, the GaN transistor displays steeper rise and fall when compared to a Si transistor. This result further highlights the potential of GaN transistor as the switching device where rapid switching is preferable.References: [1] J. W. Tioh, M. Mina, and R. J. Weber, “All-optical integrated switch utilizing Faraday rotation,” IEEE Trans. Magn., vol. 46, no. 6, pp. 2474–2477, Jun. 2010. [2] J. Selvaraj, P. Rastogi, N. P. Gaunkar, R. L. Hadimani, and M. Mina, “Transcranial magnetic stimulation: Design of a stimulator and a focused coil for the application of small animals,” IEEE Transactions on Magnetics, no. 99, pp. 1–5, 2018. [3] N. P. Gaunkar, J. Selvaraj, W.-S. Theh, R. Weber, and M. Mina, “Pulsed magnetic field generation suited for low-field unilateral nuclear magnetic resonance systems,” AIP Advances, vol. 8, no. 5, p. 056814, 2018. [4] L. F. Eastman and U. K. Mishra, "The toughest transistor yet [GaN transistors]," IEEE Spectrum, vol. 39, no. 5, pp. 28-33, May 2002, doi: 10.1109/6.999791. [5] J.W. Pritchard, M. Mina, and R.J. Weber, “Magnetic Field Generator Design for Magneto-Optic Switching Applications,” IEEE Transactions on Magnetics, Vol. 49, No. 7, pp. 4242-4244, 2013.

GaN Based High Strength Magnetic Field Pulser

Magnetic domain wall (DW) Skyrmions are 360° transitions of the internal magnetization in a Dzyaloshinskii DW. These structures are analogous to vertical Bloch lines (VBLs), which were once considered for magnetic memory storage. Although they have been predicted theoretically [1], DW skyrmions have only recently been observed experimentally [2]. Here we performed a systematic study of multiple iterations of [Pt/(Co/Ni)M/Ir]N multi-layers to formulate a phase diagram of magnetic excitations related to domain wall substructures, including DW skyrmions, via Lorentz transmission electron microscopy (LTEM). [Pt/(Co/Ni)M/Ir]N multi-layers were deposited by magnetron sputtering onto Si3N4 TEM membranes [3]. LTEM was performed using an aberration-corrected FEI Titan G2 80-300. In low DMI samples, VBLs are formed over all thicknesses, but are only seen to form pile-ups (i.e. n-π VBLs) in the case of ultrathin samples. We attribute this to the formation of hybrid DWs in thicker samples where the magnetization varies along the z-axis, thereby, creating low energy paths to annihiliation of 2π VBLs. Likewise, only pure Dzyaloshinskii DWs are observed in thicker samples with high DMI. It is only for lower thicknesses in this strong DMI regime that DW skyrmions are observed. Like their VBL counterparts, DW Skyrmions are found to have a pinning effect on the domain wall as seen through the in-situ application of a perpendicular magnetic field. The stability and dynamic response of DW skyrmions are examined further via micromagnetics. The energy barrier to annihilation was calculated using M3 [4] and a GNEB method and is observed to decrease with increased DMI strength. Most notably an energy barrier to annihilation of >60 kBT was found for D<1.0 mJ/m2 at room temperature giving confidence in the stability of DW Skyrmions for use in magnetic memory devices. Details regarding their dynamic response to electric current will be discussed. [5]References: [1] R. Cheng, M. Li, A. Sapkota, A. Rai, et al., Phys. Rev. B, Vol. 99, p. 184412 (2019). [2] M. Li, A. Sapkota, A. Rai, et al., ArXiV: 2004.07888 (2020). [3] M. Li, D. Lau, M. De Graef, and V. Sokalski, Phys. Rev. Mater., Vol. 3, p. 064409, (2019). [4] Micromagnetic Code M3: http://magneticslab.ua.edu/micromagnetics-code.html. [5] This research is supported by the Defense Advanced Research Project Agency (DARPA) program on Topological Excitations in Electronics (TEE) under grant number D18AP00011. The authors also acknowledge use of the Materials Characterization Facility at Carnegie Mellon University supported by grant MCF-677785.

Domain Wall Skyrmions: Examining Domain Wall Substructures in [Pt/(Co/Ni)M/Ir]N Multi-Layers

Magnetic skyrmions, which are chiral geometric spin textures with a non-trivial topology [1, 2], are shown to have the advantages of small size, low driven current density and robustness against external perturbations for potential applications in the next-generation spintronic devices. Generally, skyrmions are stabilized by the non-collinear Dzyaloshinskii–Moriya interaction (DMI), originates from the strong spin-orbit coupling (SOC) combined with the inversion symmetry breaking, the latter of which may be implemented with a heterostructure. We report the use of the heterostructure consisting of a high SOC topological insulator (TI) (BiSbTe) and the ferrimagnet (GdFeCo) to create the DMI and demonstrate the presence of skyrmions [3]. A sizable interfacial DMI coefficient (D = 35.5±0.25 μJ m-2) is obtained by the Brillouin-Light-Scattering spectrum from the topological surface states, as shown in Figure 1. Small sizes of skyrmions under 100 nm are observed by the scanning transmission X-ray microscopy (STXM) at room temperature, and the Néel-type spin spiral transition is verified by the line-scan spin profile of the single skyrmion. Furthermore, the ferrimagnet is shown to create the antiferromagnetic (AFM) skyrmions, for which the element-resolved STXM identifies the AFM-coupled Gd and FeCo skyrmion lattices, as shown in Figure 2, which promises the potential of vanishing skyrmion Hall effect and ultrafast skyrmion dynamics. This work demonstrates the Néel-type skyrmions at the topological interface, where small skyrmions combined the giant spin-orbit toque have potential applications for high-density and energy-efficient spintronic devices [4].References: [1] A. Fert, N. Reyren and V. Cros, Nature Reviews Materials 2 (7), 17031 (2017). [2] W. Jiang, G. Chen, K. Liu, J. Zang, S. G. E. te Velthuis and A. Hoffmann, Physics Reports 704, 1-49 (2017). [3] H. Wu, F. Groß, B. Dai, D. Lujan, S. A. Razavi, P. Zhang, Y. Liu, K. Sobotkiewich, J. Förster, M. Weigand, G. Schütz, X. Li, J. Gräfe, and K. L. Wang, Advanced Materials 2003380 (2020). [4] H. Wu, Y. Xu, P. Deng, Q. Pan, S. A. Razavi, K. Wong, L. Huang, B. Dai, Q. Shao, G. Yu, X. Han, J.-C. Rojas-Sánchez, S. Mangin and K. L. Wang, Advanced Materials 31 (35), 1901681 (2019).

Antiferromagnetically-coupled skyrmion lattices in topological insulator/ferrimagnet heterostructures

Magnetic skyrmions are topologically stable configuration with particle-like properties. Due to their efficient current-induced manipulation, they could act as carrier of information in futur devices. They where initially observed at low temperature and by applying an intense magnetic field in bulk materials exhibiting broken inversion symmetry[1-2] and in epitaxial ultrathin films with interfacial Dzyaloshinskii-Moriya interaction[3-4]. An intense research effort during the past few years has led to their stabilization at room temperature[5] but still by applying an external magnetic field. A promising strategy to overcome this last obstacle is to exploit the interlayer exchange coupling in magnetic heterostructure to produce an internal effective magnetic field Bint[6-7]. Here, we demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased Pt/Co/Ni80Fe20/Ir20Mn80 multilayerstack[8]. This is achieved through an advanced optimization of the multilayer-stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning nitrogen-vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. Simulation of the PL quenching image are also achieved and the results, in agreement with experimental data, allows us to extract informations such as skyrmion's size. These results establish exchange-biased multilayer stacks as a promising platform toward the effective realization of memory and logic devices based on magnetic skyrmions.References: [1] S. Mühlbauer et al., Science323, 915 (2009) [2] X. Z. Yu et al., Nature (London) 465, 901 (2010) [3] S. Heinze et al., Nature Phys.7, 713 (2011) [4] N. Romming et al., Science341, 636 (2013) [5] W. Jiang et al., Science349, 283 (2015) [6] G. Chen et al., Appl. Phys. Lett.106, 242404 (2015) [7] Y. Guang et al., Nat. Commun.11, 949 (2020) [8] K. Gaurav Rana et al., Phys. Rev. Applied 13, 044079 (2020)

Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

Skyrmions induced by the Dzyaloshinskii–Moriya (DM) interaction have the great potential for spintronic applications, owing to emergent electromagnetic phenomena, and ultra-low current driven motion. Although near room temperature skyrmions were reported for B20-type FeGe (ferromagnetic Curie temperature TC = 278 K)1, β-Mn type CoxZnyMnz (TC = 420 K)2 and tetragonal Heusler Mn1.4Pt1-yRhySn (TC = 390 K)3, their skyrmion size were found to be relatively large around 100 nm. It is of particular importance to develop new materials that exhibited skyrmions with small skyrmion sizes at and above room temperature. In this study, we focused on β-Mn type nitride A2Mo3N, because skyrmion phase was observed at 100 K by Lorenz transmission electron microscopy (LTEM) in FexCo1.5-xRh0.5Mo3N (x = 0.3 and 0.5)4. We prepared the epitaxial A2Mo3N films and investigated the effects of multi doping of A site on magnetic properties as well as the appearance of skyrmion phases. A series of A2Mo3N (A = Fe, Pd, Ni, Pt,) epitaxial thin films was fabricated on c-plane sapphire substrates with the (110) orientation by reactive magnetron sputtering. The skyrmion phase in a 50 nm thick film with Fe1.68Pd0.32Mo3N was determined by topological Hall effect (THE) and LTEM imaging. Topological Hall resistivity ρTH induced by skyrmions at 4 K and 300 K are shown in Fig. 1 as a function of applied magnetic field. Enormous ρTH at 4 K indicates the dense skyrmion phase, and reduced ρTH at 300 K indicates the isolated skyrmion phase with sign reversal, which is considered to be induced by the temperature-dependent electronic state. LTEM images of single skyrmion at 90 K and 300 K are shown in Fig. 2. Small size skyrmion (ca. 60 nm) was observed unaffected by the temperature. These results indicate that β-Mn-type A2Mo3N has a great potential and is a candidate as a host of new kind of skyrmion, which may help us to understand the fundamental mechanisms of DM interaction in the skyrmion.References: [1] P. Upadhyaya et al., Phys. Rev. B 92, 134411 (2015). [2] Y. Tokunaga et al., Nat. Commun. 6, 7638 (2015). [3] V. Kumar et al., Phys. Rev. B 101, 014424 (2020). [4] W. Li et al. Phys. Rev. B 93, 060409(R) (2016).

Magnetic skyrmions in epitaxial thin films of A2Mo3N with the filled β-Mn-type structure

Magnetic skyrmions are localized magnetic textures in magnetic films, behaving as particles and topologically different from the uniform ferromagnetic state, which have been identified to be extremely promising for applications, as well as of fundamental interest [1]. The magnetism community has provided a great effort in the last years to stabilize them at room temperature, most often by designing magnetic and heavy-metal multilayers combining perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI) [2]. The DMI promotes a unique chirality of the skyrmionic spin textures and, combined with spin-orbit torques generated in the heavy-metal layer(s), allows efficient current-induced motion of skyrmions [3]. However, recent observations have revealed that multilayered skyrmions are prone to present a hybrid chirality, due to the competition between dipolar interaction and DMI, which can negatively influence their current-induced motion [4] and also their size and stability [6]. We will discuss how those effects can be reduce thanks to synthetic antiferromagnets (SAF) based systems that allow the stabilisation of smaller antiferromagnetic skyrmions, due to the cancellation of the long range dipolar effects [7]. We will mainly focus on Pt/Co/Ru and Pt/CoFeB/Ru based systems and present the optimization process that has been done on the thicknesses of the different layer in order to maximize the antiferromagnetic RKKY coupling without cancelling the perpendicular magnetic anisotropy, needed to stabilize skyrmions. Those optimizations enabled for example the observation at room temperature of spin spiral, predicted by micromagnetic simulations, by reaching an effective anisotropy close to zero. We will then describe how, combined to a bias layer structure, multi-repeats SAF can host antiferromagnetic skyrmions and present their observation by MFM techniques without needs to go under vacuum. Those observations will also be completed by NV center microscopy images. The ANR grant TOPSky TOPSKY (ANR-17-CE24-0025), the SESAME Ile de France IMAGeSPIN (EX039175), the DARPA TEE program grant (MIPR#HR0011831554) and EU grant SKYTOP (H2020 FET Proactive 824123) are acknowledged for their financial support.References: [1] A. Fert, N. Reyren, V. Cros, Nat. Rev. Mat. 2 (2017), 17031. [2] C. Moreau-Luchaire et al, Nat. Nanotech, 11 (2016), 444 ; S. Woo et al, Nat. Mater. 15 (2016), 501 ; A. Soumyanarayanan et al, Nat. Mater. 16 (2017), 898 ; A. Hrabec et al, Nat. Comm. 8 (2017), 15765. [3] K. Litzius et al, Nat. Phys. 13 (2017), 170 ; S. Woo et al, Nat. Comm. 8 (2017), 15573. [4] W. Legrand et al, Sci. Adv. 4 (2018), eaat0415 ; Y. Dovzhenko et al, Nat. Comm. 9 (2018), 2712. [5] F. Büttner et al, Sci. Rep., 8 (2018), 4464 ; A. Bernand-Mantel et al, SciPost Phys. 4 (2018), 027. [6] W. Legrand et al, Phys. Rev. Applied 10 (2018), 064042. [7] W. Legrand et al, Nat. Mater. 19 (2019), 34–42.

Room-temperature magnetic imaging of antiferromagnetic skyrmions in multi-repeats synthetic antiferromagnets

Magnetic skyrmions are topologically nontrivial particles with a potential application as information elements in future spintronic device architectures [1, 2]. While they are commonly portrayed as two dimensional objects, in reality magnetic skyrmions are thought to exist as elongated, tube-like objects extending through the thickness of the host material, as seen in Fig. 1 [3]. These structures are theorised to exhibit Bloch points, responsible for the topological unwinding of skyrmions [4]. The study of this skyrmion tube state (SkT) is vital for furthering the understanding of skyrmion formation and dynamics for future applications. However, direct experimental imaging of skyrmion tubes has yet to be reported. Here, we demonstrate the real-space observation of skyrmion tubes in a lamella of FeGe using resonant magnetic x-ray imaging and comparative micromagnetic simulations, the horizontal stripes seen in Fig. 2, confirming their extended structure [5]. The formation of these structures at the edge of the sample highlights the importance of confinement and edge effects in the stabilisation of the SkT state, opening the door to further investigation into this unexplored dimension of the skyrmion spin texture.References: [1] Rößler, U. K., Bogdanov, A. N. & Pfleiderer, C. Spontaneous skyrmion ground states in magnetic metals. Nature 442, 797 (2006). [2] Nagaosa, N. & Tokura, Y. Topological properties and dynamics of magnetic skyrmions. Nat. Nanotechnol. 8, 899 (2013). [3] Seki, S. et al. Propagation dynamics of spin excitations along skyrmion strings. Nat. Commun. 11, 256 (2020). [4] Milde, P. et al. Unwinding of a skyrmion lattice by magnetic monopoles. Science 340, 1076 (2013). [5] Birch, M. et al. Real-space imaging of confined magnetic skyrmion tubes. Nat. Commun. 11, 1726 (2020).

Real-Space Imaging of Confined Magnetic Skyrmion Tubes INVITED

Imaging and manipulation of atomic-scale magnetism for future spintronic applications demands for a spin-sensitive technique with ultimate lateral resolution. Exploiting the tunneling effect between magnetic electrodes, as between a magnetic probe tip and a magnetic sample in a spin-polarized scanning tunneling microscopy (SP-STM) set up, has proven to be capable of resolving and switching magnetic structures down to the single atomic level. However, this approach disqualifies for many technical applications, since it requires tip-sample distances in the order of a few angstroms. For practical applications, technical feasible distances, like flying distances of read-write heads in current data storage devices, are in the range of a few nanometers. Electron tunneling over this distance is very unlikely and therefor not detectable. Spin-polarized vacuum states are unoccupied electronic states located nanometers apart from the underlying magnetic surface in vacuum [1]. In an SP-STM setup, these states can be addressed by spin-polarized electrons that tunnel resonantly from the magnetic tip via a vacuum state into the surface, resulting in a magnetic image [2]. As I will show in this talk, these states exhibit the same local spin quantization axis as the spin texture of the underlying sample surface, even when the spins are rotating on the atomic-scale [2]. Our SP-STM experiments at low temperatures on ultrathin films with non-collinear spin textures demonstrate that the spin-polarized resonant tunneling via the vacuum states allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm [3]. Acting as mediators for the spin contrast across the nm-spaced vacuum gap, the vacuum states provide thereby a loophole from the hitherto existing dilemma of losing spatial resolution when increasing the tip-sample distance in a scanning probe setup. Experimental results will be presented and discussed in terms of the vacuum states’ spin-splitting and the magnetic contrast as a function of bias and tip-sample distance, as well as in terms of the atomic-scale nature of the resonant tunneling condition between the probe tip and the sample. While spin-polarized resonant tunnel currents in the low nA regime are used for non-perturbative imaging, injecting high spin-polarized resonant tunnel currents (~100 nA) via vacuum states considerably affect the underlying magnetism. As I will demonstrate in this talk, atomic-scale ferromagnets can be efficiently switched via these states, with the underlying current-induced mechanism being based on thermally-assisted spin-transfer torque mediated by the vacuum states [4]. Consequently, spin-polarized vacuum states can be utilized for the readout and manipulation of atomic-scale magnetic objects from nanometer distances. Spin-polarized resonant tunneling qualifies for a spin-sensitive read-write technique with ultimate lateral resolution, potentially opening a pathway towards future technical applications.References: [1] M. Donath, C. Math, M. Pickel, A.B. Schmidt, and M. Weinelt, Surf. Sci. 601, 5701 (2007). [2] A. Schlenhoff, S. Kovarik, S. Krause, and R. Wiesendanger, Phys. Rev. Lett. 123, 087202 (2019). [3] A. Schlenhoff, S. Kovarik, S. Krause, and R. Wiesendanger, Appl. Phys. Lett. 116, 122406 (2020). [4] A. Schlenhoff, S. Krause, A. Sonntag, and R. Wiesendanger, Phys. Rev. Lett. 109, 097602 (2012).

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