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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 22.09.2021 - 15:00 German Time

Reconfigurable Training, Vortex Writing and Noise-Tolerant Reservoir Computation via Spin-Wave Fingerprinting in an Artificial Spin-Vortex Ice

Jack C. Gartside, Imperial College London

Strongly-interacting artificial spin systems are moving beyond mimicking naturally-occurring materials to find roles as versatile functional platforms, from reconfigurable magnonics to designer magnetic metamaterials. Typically artificial spin systems comprise nanomagnets with a single magnetisation texture: collinear macrospins or chiral vortices.
By tuning array dimensions we achieve macrospin/vortex bistability and demonstrate a four-state metamaterial spin-system ‘Artificial Spin-Vortex Ice’ (ASVI). ASVI is capable of adopting Ising macrospins with strong ice-like vertex interactions, in addition to weakly-coupled vortices with low stray dipolar-field. The enhanced bi-texture microstate space gives rise to emergent physical memory phenomena, with ratchet-like vortex training and history-dependent nonlinear training dynamics.
We observe vortex-domain formation alongside MFM tip vortex-writing. Tip-written vortices dramatically alter local reversal and memory dynamics. Vortices and macrospins exhibit starkly-differing spin-wave spectra with analogue-style mode-amplitude control via vortex training and mode-frequency shifts of ∆f = 3.8 GHz. We employ spin-wave ‘spectral fingerprinting’ for rapid, scaleable readout of dynamic vortex and macrospin populations over complex training-protocols. The history-dependent spectral fingerprint is leveraged for a noise-tolerant reservoir computation scheme predicting and classifying time-series datasets.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 01.09.2021 - 15:00 German Time

Magnetic topological phases in dissipative systems

Benedetta Flebus, Boston College

While magnetic systems have been extensively studied both from a fundamental physics perspective and as building blocks for a variety of applications, their topological properties, however, remain relatively unexplored due to their inherently dissipative nature.
I will start this talk by showing how the recent introduction of non-Hermitian topological classifications has opened up opportunities for engineering topological phases in magnetic systems, and I will present our first proposal of a non-Hermitian topological magnonic system, i.e., a realization of a SSH non-Hermitian model via a one-dimensional spin-torque oscillator array.
In the second part of this talk, I will discuss the conditions under which magnetic insulating systems can host one of the most striking non-Hermitian phenomena with no Hermitian counterpart, i.e., the skin effect, which underlies the breakdown of the bulk-edge correspondence.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 14.07.2021 - 15:00 German Time

Topological protectorates of Fermi surfaces

Christian Pfleiderer, TU Munich

Following over a decade of intense research to enable major technological progress by means of materials in which the electronic structure exhibits non-trivial topological properties, three key challenges are still unresolved. First, the identification of topological band degeneracies that are generically rather than accidentally located at the Fermi level. Second, the ability to easily control such topological degeneracies. And third, to identify generic topological degeneracies in large, multi-sheeted Fermi surfaces.
Combining quantum oscillatory studies with density functional theory and comprehensive band-topology calculations, we report the identification of symmetry-enforced nodal planes in the B20 compounds CoSi and MnSi. The nodal planes enforce topological protectorates with substantial Berry curvatures at their intersection with the Fermi surface regardless of the complexity of the FS. In CoSi we show that the nodal planes provide the missing topological charges of an entire network of band-crossings comprising, in addition, multifold degeneracies and Weyl points, such that the fermion doubling theorem is satisfied. Moreover, in the ferromagnetic state of MnSi, the existence of the nodal planes may be controlled with the direction of the applied magnetic field.
The identification of symmetry-enforced topological protectorates of the Fermi surfaces of CoSi and MnSi suggests the existence of similar properties in a large number of materials. In particular, deriving the symmetry conditions underlying topological nodal planes, we show that the 1651 magnetic space groups comprise 7 grey groups and 26 black-and-white groups with topological nodal planes, including the space group of ferromagnetic MnSi.
[1] M. Wilde et al. Nature 594, 374 (2021)
[2] N. Huber et al., arXiv/2107.02820

 

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