On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.11.2021 - 15:00 German Time

Antiferromagnetic Skyrmionics: generating and controlling topological textures

Hariom Jani, National University of Singapore

Whirling magnetic textures such as skyrmions, bimerons and their anti-particles could emerge as topologically-protected information bits for next-generation memory and logic. However, their practical exploitation has been inhibited by susceptibility to stray magnetic fields, strong internal dipolar fields, slow speeds or sideway motion. To alleviate these issues, there has been a surge of interest in antiferromagnetic (AFM) analogues, predicted to be robust, scalable and ultra-fast1,2. Even so, experimental progress in this field has been curtailed by magnetic compensation in AFM systems, which makes it difficult to visualize and control AFM textures via standard magnetic techniques.
To this effect, I will firstly discuss a recently-developed AFM vector-mapping technique3,4 exploiting angle-dependent dichroism to image spatial variations of the AFM order. Then, I will present a general field-free approach, employing the Kibble–Zurek transition, that we used to reversibly create a wide multichiral family of topological AFM textures, including exotic merons or antimerons and bimerons5. In the earth-abundant oxide (α-Fe2O3) these nanoscale textures can be nucleated and stabilized at room temperature. Particularly, the presence of widely tunable anisotropy6 and exchange4,5,7 interactions in this system enable unprecedented reversible control over the dimensions and orientation of AFM textures. I will then present how we may realize the hitherto undiscovered AFM skyrmions in α-Fe2O3, by introducing new symmetry breaking interactions8. Lastly, I will outline the path ahead for AFM skyrmionics, discussing how our results may be translatable to a broad class of AFM systems – including orthoferrites, orthochromites and layered-ferrates6,9, and sharing how electrical pathways can be exploited to control members of the topological family10-12.

[1] J Barker et al., Physical Review Letters 116, 147203 (2016)
[2] V Baltz et al., Review of Modern Physics 90, 015005 (2018)
[3] NW Price et al., Physical Review Letters 117, 177601 (2016)
[4] FP Chmiel et al., Nature Materials 17, 581 (2018)
[5] H Jani et al., Nature 590, 74 (2021)
[6] H Jani et al., Nature Communications 12, 1668 (2021)
[7] PG Radaelli et al., Physical Review B 101, 144420 (2020)
[8] J Harrison et al., arXiv:2111.15520 (2021)
[9] ZS Lim et al., MRS Bulletin (In Press, 2021), arXiv:2111.10562
[10] L Baldrati et al., Physical Review Letters 123, 177201 (2019)
[11] P Zhang et al., Physical Review Letters 123, 247206 (2019)
[12] Y Cheng et al., Physical Review Letters 124, 027202 (2020)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 10.11.2021 - 15:00 (CET)

Altermagnetism: spin-momentum locked phase protected by non-relativistic symmetries

Tomas Jungwirth, Institute of Physics of the Science Academy of the Czech Republic

 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 17.11.2021 - 15:00 German Time

Magnetism and spin dynamics control by carrier doping in van der Waals magnet Cr2Ge2Te6

Hidekazu Kurebayashi, University College London

Two-dimensional (2D) van der Waals (vdW) materials have been intensively and extensively studied in the last two decades. A magnetic version of vdW systems has only gained attention since 2017 where a few mono-layers of exfoliated magnetic vdW ones were reported to sustain magnetism [1.2]. Since then, scientists started to seriously explore the physics and materials science of this new class of materials by applying their own research ideas and growth/measurement techniques. These material groups are ideal, for example, in studying magnetism and spin transport at the truly 2D limit, and in exploring how these materials can be responded by external stimuli such as current-induced torques and electric field. These experiments will also be enriched by an unlimited combination of their heterostructures that can be fabricated without significant lattice-matching constraints present in typical thin-film sample-growth techniques such as MBE and sputtering. Furthermore, inherent low symmetry nature of vdW materials will offer a wealth of spin-orbit Hamiltonians that are the backbone of current-induced magnetization switching research and future technologies [3].
We started to work on one of magnetic 2D vdW materials, Cr2Ge2Te6 (CGT), to study its spin dynamics and how to control the magnetism by any external stimuli. In this presentation, I will start with a brief introduction of magnetic 2D vdW materials and then move on to our latest work of controlling magnetism (Curie temperatures and magnetic anisotropies) in CGT by electric field [4] and chemical doping. Both doping techniques show the change of carrier density in CGT by orders of magnitude (from insulator to metallic). As a result, the exchange coupling strength has been greatly enhanced, leading to Curie temperature enhancement. The carrier doping also modifies the spin-orbit interaction within CGT which is measured by a significant change of the magnetic anisotropy parameters. These have been characterized by magneto-transport as well as spin dynamics techniques [5]. Furthermore, if time permits, I will also briefly show our photon-magnon coupling in CGT and on-chip resonator systems.

[1] Gong et al. Nature 546 265 (2017).
[2] Huang et al., Nature 546, 270 (2017).
[3] H. Kurebayashi et al., arXiv:2107.03763; Nat. Rev. Phys. (in-press).
[4] Verzhbitskiy et al., Nature Electron. 3, 460 (2020).
[5] For example, for undoped CGT, Khan et al., Phys. Rev. B 100, 134437 (2019);
arXiv:1903.00584.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.11.2021 - 15:00 German Time

Analytic and ab initio theory of magnetization dynamics

Peter Oppeneer, Uppsala University

The Landau-Lifshitz-Gilbert (LLG) equation forms a cornerstone of contemporary magnetism research, yet it was originally proposed on the basis of phenomenological considerations. To put the equation on a fundamental footing, we start from the relativistic Dirac-Kohn-Sham equation, consider the motion of spin angular momentum in an external electromagnetic field and show that it leads to the LLG equation with anisotropic damping, as well as to additional terms, such as the field-derivative torque, the optical spin-orbit torque, spin-transfer torque, and inertial term [1,2]. Besides providing a foundational basis for the LLG equation, our analytic theory predicts new effects that could be observed in experiments.
Electric field or current induced spin-orbit torques (SOTs) arising from the spin Hall effect or Rashba-Edelstein effect (REE) have recently emerged as promising tools to achieve efficient magnetization dynamics [3]. To explore the origin of SOTs on a materials’ specific level, we employ density functional and linear-response theory to calculated ab initio the electric field induced magnetic polarizations. For the noncentrosymmetric antiferromagnets CuMnAs and Mn2Au we compute the induced polarizations and find that there exists dominantly an orbital Rashba-Edelstein effect that is much larger than the spin REE and does not require spin-orbit coupling to exist [4]. The staggered, field-induced orbital polarization moreover exhibits Rashba-type symmetry in contrast to the induced spin polarization.
Considering typical bilayer systems consisting of Pt and 2 monolayers of a 3d element (Co, Ni, Cu) we compute in a layer-resolved manner the spin and orbital conductivities and spin and orbital moment accumulations. We identify the contributions that lead to the fieldlike SOT and the dampinglike SOT, which are mainly the spin REE and magnetic spin Hall effect, respectively. The current-induced orbital accumulation transverse to the electric field is always much larger than the corresponding spin accumulation and exist without spin-orbit interaction [5]. This exemplifies that the induced orbital polarization is the primary response to the electric field and suggests the possibility of utilizing large orbital effects in light-metal devices.

[1] R. Mondal, M. Berritta, A.K. Nandy, and P.M. Oppeneer, Phys. Rev. B 96, 024425 (2017).
[2] R. Mondal, M. Berritta, and P.M. Oppeneer, Phys. Rev. B 94, 144419 (2016); Phys. Rev. B 98, 214429 (2018).
[3] A. Manchon, J. Železný, I.M. Miron, T. Jungwirth, J. Sinova, A. Thiaville, K. Garello, and P. Gambardella, Rev. Mod. Phys. 91, 035004 (2019).
[4] L. Salemi, M. Berritta, A.K. Nandy and P.M. Oppeneer, Nature Commun. 10, 5381 (2019).
[5] L. Salemi, M. Berritta, and P.M. Oppeneer, Phys. Rev. Mater. 5, 074407 (2021).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.12.2021 - 15:00 German Time

Femto-magnetism meets spintronics: Towards integrated magneto-photonics

Bert Koopmans, Eindhoven University of Technology

Novel schemes for optically controlling ferromagnetic order at a femtosecond time scale [1] receive great scientific interest. In the strongly non-equilibrium regime, it has become possible not only to quench magnetic order, but even to deterministically switch the magnetic state by a single femtosecond laser pulses. Moreover, it has been shown that pulsed laser excitation can induce spin currents over several to tens of nanometers. This development triggered a merge of the fields of ‘femto-magnetism’ and spintronics – opening up a fascinating playground for novel physical phenomena. In this lecture I will discuss the underlying principles, but also envision their exploitation in THz magnonics and integrated spintronic-photonic memories.

After a brief review of the field, mechanisms for ultrafast loss of magnetic order upon fs laser heating [2] as well as all-optical switching will be explained. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer [3]. These fs spin currents are absorbed within a few nanometers, providing ideal conditions for exciting and exploring THz spin waves [4]. Finally, it will be argued that synthetic, layered ferrimagnets provide an ideal platform for combining fs optical control with advanced spintronic functionality. It will be shown how magnetic bits can be written ‘on-the-fly’ by fs laser pulses in a so-called magnetic racetrack, where they are immediately transported by a dc current [5]. Such schemes may lead to a novel class of integrated photonics, in which information is transferred back and forth between the photonic and magnetic domain without any intermediate electronic steps.

[1] E.E. Fullerton, H.A. Dürr, A.V. Kimel and B. Koopmans, Chapter VI “Interfacial effects in ultrafast magnetization dynamics”, in F. Hellman, et al., “Interface-induced phenomena in magnetism”, Rev. Mod. Phys. 89, 025006 (2017).
[2] B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Faehnle, T. Roth, M. Cinchetti, and M. Aeschlimann, “Explaining the paradoxical diversity of ultrafast laser-induced demagnetization”, Nature Materials 9, 259 (2010).
[3] A.J. Schellekens, K.C. Kuiper, R.R.J.C. de Wit, and B. Koopmans, “Ultrafast spin-transfer torque driven by femtosecond pulsed-laser excitation”, Nat. Commun. 5, 4333 (2014).
[4] M. L. M. Lalieu, R. Lavrijsen, R. A. Duine, and B. Koopmans, “Investigating optically excited terahertz standing spin waves using noncollinear magnetic bilayers”, Phys. Rev. B 99, 184439 (2019).
[5] M.L.M. Lalieu, R. Lavrijsen, and B. Koopmans, “Integrating all-optical switching with spintronics”, Nat. Commun. 10, 1038 (2019).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 20.10.2021 - 15:00 German Time

Archimedean screw and time quasi-crystals in driven chiral magnets

Achim Rosch, Institute for Theoretical Physics, University of Cologne

The Archimedean screw is one of the oldest machines of mankind. We show theoretically [1] how one can realize and drive such an Archimedean screw
in chiral magnets, where helical spin textures are realized. A small oscillating magnetic field at GHz frequencies induces a net rotation and screw-like motion of the magnetic texture. This effect arises from the coupling of the oscillating field to the Goldstone mode of the system. The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a large voltage in metallic systems. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing the oscillating field forming a `time quasicrystal' which oscillates in space and time for moderately strong drive.

[1] Nina del Ser, Lukas Heinen, Achim Rosch, SciPost Phys. 11, 009 (2021).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.10.2021 - 15:00 German Time

Magnetic skyrmion strings: how they bend, twist and vibrate

Markus Garst, KIT

Magnetic skyrmions are smooth topological textures of the magnetization that are localized within a two-dimensional plane. In bulk materials, they extend in the third direction forming an effective string. Such skyrmion strings either arise as excitations or they condense and form a crystal.
These strings can be dynamically excited resulting in various vibrational modes. We provide an overview of the dynamics of skrymion strings [1], that can be found in chiral magnets, and we compare theoretical predictions with magnetic resonance spectroscopy [2], spin-wave spectroscopy [3] and inelastic neutron scattering. At high energies, the spin-wave dynamics is governed by an emergent orbital magnetic field that is directly linked to the topological density of the skyrmions. As a result, magnon Landau levels emerge in skyrmion crystals. At low-energies the dynamics is determined by an effective elasticity theory of the strings. We focus, in particular, on the low-energy theory of a single string and demonstrate that it supports non-linear solitary waves [4] similar to vortex filaments in fluids. Finally, we discuss the influence of spin-transfer torques. Whereas it is well-known that a spin current flowing perpendicular to the string results in a skyrmion string motion, we demonstrate that a longitudinal current destabilizes the string.

[1] M. Garst, J. Waizner, and D. Grundler, J. Phys. D: Appl. Phys. 50, 293002 (2017).
[2] T. Schwarze et al., Nat. Mater. 14, 478 (2015).
[3] S. Seki et al., Nat. Commun. 11, 256 (2020).
[4] V. P. Kravchuk et al., Phys. Rev. B 102, 220408(R) (2020).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 27.10.2021 - 15:00 German Time

Nano-scale skyrmions and atomic-scale spin textures studied with STM

Kirsten von Bergmann, Universität Hamburg

Non-collinear magnetic order arises due to the competition of different magnetic interactions. Often the dominant interaction is the isotropic pair-wise exchange between neighboring atomic magnetic moments. An additional sizable contribution from anisotropic exchange (Dzyaloshinskii-Moriya-Interaction) typically leads to spin spiral ground states in the absence of magnetic fields. In applied magnetic fields such systems can transition into skyrmion lattices or isolated skyrmions with diameters down to a few nanometer.
In zero magnetic field single skyrmions can arise as metastable states, stabilzed by frustrated exchange interactions, which originate from competing non-negligible exchange interaction to more distant magnetic moments. Periodic two-dimensionally modulated magnetic states on the atomic scale can arise due to higher-order magnetic interactions. Such higher-order interactions can favor superpositions of spin spirals, so called multi-q states. Depending on the sample system atomic-scale non-collinear magnetic lattices of different symmetry and size can form. Higher-order interactions can also determine the type and width of domain walls in antiferromagnets.
I will present several examples of ultra-thin magnetic film systems studied with spin-polarized scanning tunneling microscopy, and discuss the origin of the different observed non-collinear magnetic states.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 29.09.2021 - 15:00 German Time

Inertial spin dynamics in ferromagnets

Stefano Bonetti, Stockholm University

The understanding of how spins move and can be manipulated at pico- and femtosecond timescales has implications for ultrafast and energy-efficient data-processing and storage applications. However, the possibility of realizing commercial technologies based on ultrafast spin dynamics has been hampered by our limited knowledge of the physics behind processes on this timescale. Recently, it has been suggested that inertial effects should be considered in the full description of the spin dynamics at these ultrafast timescales, but a clear observation of such effects in ferromagnets has been lacking for about a decade. In this presentation, I will first report on the first direct experimental detection of intrinsic inertial spin dynamics in ferromagnetic thin films, in the form of a forced nutation oscillation of the magnetization at THz frequency, that we observed at the TELBE facility in Dresden, Germany.
Then, I will show our most recent unpublished results on the detection of spin nutation using a table-top broadband THz source, with which we investigated epitaxial thin films of cobalt in its three crystalline phases. The terahertz magnetic field of the radiation generates a torque on the magnetization which causes it to precess for about 1 ps, with a sub-picosecond temporal lag from the driving force. Then, the magnetization undergoes natural damped THz oscillations at a frequency characteristic of the crystalline phase. We describe the experimental observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the results from the relativistic theory of magnetic inertia, we find that the angular momentum relaxation time is the only material parameter needed to describe all the experimental evidence. Our data suggest a proportionality between such time and the strength of the magneto-crystalline anisotropy, deepening our fundamental understanding of magnetic inertia.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.09.2021 - 15:00 German Time

Local magnetic measurements of quantum materials

Katja Nowack, Cornell University

Magnetic moments and moving charges produce magnetic fields. Probing these stray magnetic fields on a local scale can provide a unique window into a variety of phenomena in quantum materials. In this talk, I will discuss three examples of how we use a local magnetic probe to study different properties of quantum materials. First, we visualize a spatially modulated superconducting transition in microstructures fabricated from a heavy-fermion superconductor. Second, we visualize the current density in a quantum anomalous Hall insulator by imaging the magnetic field produced by the current. Third, we measure the superconducting diamagnetic response of an atomically thin van der Waals superconductor for the first time.

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