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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 28.07.2021 - 15:00 German Time

Interacting and higher-order topological spin excitations

Alexander Mook, University of Basel

Quantum condensed matter admits topological collective excitations above a trivial ground state, an example being Chern insulators formed by Dirac bosons with a gap at finite energies. However, in contrast to electrons, there is no particle-number conservation law for collective excitations. This gives rise to particle number-nonconserving many-body interactions the influence of which on single-particle topology is an open issue of fundamental interest in the field of topological quantum materials.
Herein, I concentrate on magnons that are the elementary spin excitations of ferromagnets. A ferromagnet with Chern-insulating behavior of magnons exhibits a magnonic spectral gap hosting topologically protected chiral edge modes that unidirectionally revolve the sample. Since these chiral edge magnons may serve as directed information highways in next-generation technologies with ultralow energy consumption, a fundamental understanding of their formation and stability is at the very core of the topological magnonics paradigm.
I present topological magnons in three different setups: (i) skyrmion crystals [1], (ii) saturated chiral magnets [2], and (iii) stacks of honeycomb-lattice van der Waals magnets [3]. These setups respectively serve as platforms to study (i) quantum damping due to spontaneous quasiparticle decay, (ii) interaction-stabilized topological gaps in the magnon spectrum, and (iii) second-order topology in three-dimensional samples that admit chiral states along their hinges, where facets intersect.

[1] Alexander Mook, Jelena Klinovaja, and Daniel Loss, "Quantum damping of skyrmion crystal eigenmodes due to spontaneous quasiparticle decay," Phys. Rev. Research 2, 033491 (2020)
[2] Alexander Mook, Kirill Plekhanov, Jelena Klinovaja, and Daniel Loss, "Interaction-Stabilized Topological Magnon Insulator in Ferromagnets," Phys. Rev. X 11, 021061 (2021)
[3] Alexander Mook, Sebastián Díaz, Jelena Klinovaja, and Daniel Loss, "Chiral Hinge Magnons in Second-Order Topological Magnon Insulators," PRB (in press), arXiv:2010.04142 (2020)

 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 07.07.2021 - 15:00 German Time

Neuromorphic magnon-spintronic networks

Philipp Pirro, TU Kaiserslautern

Today’s computational technology based on CMOS has experienced enormous scaling of data processing capability as well as of price and energy consumption per logic element. However, new ways to process and analyze data like brain-inspired computing need novel hardware which implements the structure of the new logical concepts as directly as possible into a physical realization. In this context, spintronic systems are promising because of their intrinsic nonlinearity, low power consumption, scalability, ability to store information and to use multiplexing functionality.
I will discuss how the field of magnon-spintronics can contribute to this development using a novel hybrid system which combines nanoscaled ultralow damping magnonic systems [1,2] with spintronic auto-oscillators [3]. The proposed system uses guided coherent spin waves and their interference effects in magnetic insulators to interconnect metallic spintronic neurons. In this way, it takes advantage of the intrinsic nonlinearity of the spin system and the multiplexing functionality provided by the wave character. I will compare this type of network to the recently demonstrated optical neurosynaptic networks [4]. In addition, I will present the “inverse design” concept [5] which enables novel ways to efficiently design the building blocks needed for the proposed magnon-spintronic networks.

[1] Wang et al., Phys. Rev. Lett. 122, 247202 (2019) doi: 10.1103/PhysRevLett.122.247202
[2] Q. Wang, et al. Nature Electronics 3, 765–774, (2020) doi: 10.1038/s41928-020-00485-6
[3] M. Romera et al., Nature 563, 230 (2018) doi: 10.1038/s41586-018-0632-y
[4] Feldmann et al., Nature 569, 208 (2019) doi: 10.1038/s41586-019-1157-8
[5] Q. Wang, A. V. Chumak, and P. Pirro, Nat. Com. 12, 2636 (2021) doi: 10.1038/s41467-021-22897-4

 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 21.07.2021 - 15:00 German Time

Electrical manipulation of non-collinear antiferromagnet

Shunsuke Fukami, Tohoku University

Electrical manipulation of magnetic materials has been of paramount interest in the spintronics research for the last quarter of a century, and many interesting phenomena have been revealed, offering various opportunities of applications. Non-collinear antiferromagnet with chiral-spin structure is an attractive system showing intriguing properties that had been believed to be inherent to ferromagnets, such as the anomalous Hall effect [1]. A recent study demonstrated an electrical switching of chiral-spin structure in the same protocol with magnetization switching in ferromagnets [2].
In this seminar, I will show a new phenomenon unique to the non-collinear antiferromagnet, i.e., chiral-spin rotation [3]. We use Hall-bar devices with an epitaxial stack consisting of non-collinear antiferromagnetic Mn3Sn and heavy metals with large spin-orbit coupling [4,5]. An unconventional response of the Hall resistance under current applications is observed, which can be attributed to the continuous rotation of chiral-spin structure in Mn3Sn driven by the spin-orbit torque. We also find that the efficiency to manipulate the magnetic structure through this scheme is much higher than that in collinear ferromagnets and ferrimagnets.

[1] S. Nakatsuji et al., Nature 527, 212 (2015).
[2] H. Tsai et al., Nature 580, 608 (2020).
[3] Y. Takeuchi et al., Nature Materials, advanced online publication (2021). https://doi.org/10.1038/s41563-021-01005-3.
[4] J.-Y. Yoon et al., Appl. Phys. Express, 13, 013001 (2019); J.-Y. Yoon et al. AIP Adv. 11, 065318 (2021).

 

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