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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.06.2021 - 15:00 German Time

Electrical Generation of Spin Currents

Andrew Kent, New York University

Spin currents in magnetic random access memory (MRAM) devices being developed by the semiconductor industry are generated by passing an electrical current perpendicular to layers that form a magnetic tunnel junction [1]. However, it is now widely appreciated that current flow in the plane of a layer can generate significant spin currents through spin-orbit coupling, as first reported in heavy non-magnetic metal layers (e.g. Pt, Ta & W). In this case, however, the spin polarization is generally confined to the plane of the layers. An important research goal is to create a spin current with an arbitrary polarization, including one with a significant out-of-plane spin polarization to enable efficient switching and displacement of domain walls in perpendicularly magnetized layers. In this talk we discuss spin-orbit induced charge-to-spin conversion in various materials and nanostructures [2] and with magnetic materials. Specifically, we will report our observation of spin torques with a planar Hall effect symmetry from CoNi, with a spin polarization in the magnetization direction of the layer [3]. We found the strength of this effect to be comparable to that of the spin Hall effect in Pt, indicating that the planar Hall effect in ferromagnetic metals holds great promise as a spin current source with a controllable spin polarization direction.

[1] A. D. Kent and D. C. Worledge, Nature Nanotechnology 10, 187 (2015)
[2] J-W. Xu and A. D. Kent, Physical Review Applied 14, 014012 (2020)
[3] C. Safranski, J. Z. Sun, J-W., Xu and A. D. Kent, Physical Review Letters 124, 197204 (2020)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 30.06.2021 - 15:00 German Time

Building and investigating magnetic adatom chains on superconductors atom by atom

Katharina Franke, Freie Universität Berlin

Magnetic adatom chains on superconducting substrates are promising platforms for topological superconductivity and Majorana zero modes. Signatures of these have been found in densely-packed chains with direct exchange interaction among the adatoms. Here, we investigate adatom chains in the “dilute” limit. This means that the atoms are sufficiently far spaced that direct hybridization of their d orbitals is negligible, but close enough for sizeable substrate-mediated interactions. We build these chains from individual Fe atoms on a 2H-NbSe2 substrate. Using scanning tunneling microscopy and spectroscopy we first characterize the exchange coupling between the magnetic adatoms and the superconductor by detecting their Yu-Shiba-Rusinov states within the superconducting energy gap. We then use the tip of the STM to assemble dimers, trimers and chains of these Fe atoms. In each step, we track the evolution of the Yu-Shiba-Rusinov states and identify magnetic interactions, hybridization and band formation.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.06.2021 - 15:00 German Time

Spin And Charge Transport In Antiferromagnets

Vincent Baltz, SPINTEC, Grenoble

Antiferromagnets have attracted interest for use of their spin-dependent transport properties in electronic devices [1,2]. Towards this end, determining the characteristic lengths promoting spin dependent transport as well as understanding how antiferromagnetic spin structures [3,4] and spin textures [5,6] influence transport are some of the basic points that deserve to be investigated.
In this talk, I will first discuss experiments of spin injection and propagation in antiferromagnets. I will show how we demonstrated experimentally [7] the theoretical prediction [8] of the interplay between linear spin fluctuations and the spin mixing conductance, therefore opening perspectives for studies of critical phenomena in thin films of antiferromagnets. In search of non-linear spin fluctuations [9], I will then detail how we found experimental evidence of an overlooked effect: self-induced spin-charge conversion in the ferromagnetic spin-injector, corroborating the results of first-principle calculations [10]. Beyond extrinsic scattering, recent experimental findings relating to the interplay between the antiferromagnetic order and crystal symmetries [4] will be briefly announced.
In a second part, I will introduce a stimulating example of how antiferromagnets and superconductors [11] may envision a common future by showing how we inferred essential information using Cooper pair transport through antiferromagnets [12].
Finally, in search of the nucleation of skyrmions in antiferromagnets to study the associated spintronic effects, I will show how we took advantage of the exchange bias interaction between an antiferromagnet and an adjacent ferromagnet to stabilize several types of spin textures at the interface of the antiferromagnet [6,13,14].

[1] T. Jungwirth et al, Nat. Nanotechnol. 11, 231 (2016)
[2] V. Baltz et al, Rev. Mod. Phys. 90, 015005 (2018)
[3] L. Šmejkal et al, Nat. Phys. 14, 242 (2018)
[4] H. Reichlová et al, arXiv:2012.15651 (2020)
[5] S.-W. Cheong, et al, npj Quantum Materials 5, 3 (2020)
[6] K. G. Rana et al, arXiv:2009.14796 (2020)
[7] L. Frangou et al, Phys. Rev. Lett. 116, 077203 (2016); Phys. Rev. B 98, 094422 (2018)
[8] Y. Ohnuma et al, Phys. Rev. B 89, 174417 (2014)
[9] D. H. Wei et al, Nat. Commun. 3, 1058 (2012)
[10] O. Gladii et al, Phys. Rev. B 100, 1174409 (2019)
[11] A. I. Buzdin, Rev. Mod. Phys. 77, 935 (2005)
[12] R. L. Seeger et al, arXiv:2102.03425 (2021)
[13] S. Brück et al, Adv. Mater. 17, 2978 (2005)
[14] G. Salazar-Alvarez et al, Appl. Phys. Lett. 95, 012510 (2009)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.06.2021 - 15:00 German Time

Spintronic microwave and THz detectors: state-of-the art and future!

Giovanni Finocchio, University of Messina

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to their charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. Those devices realized with magnetic tunnel junctions exhibit high-detection sensitivity >200kV/W at room temperature, without any external bias fields, and for low-input power (micro-Watts or lower). In the first part of the talk, I will discuss our recent results in the field of microwave detectors based on spin diodes and possible implementations of THz detectors based on antiferromagnets.
Another application of spintronic diodes, when they have a broadband frequency response, is as electromagnetic energy harvesting, which offers an attractive energy source for applications in self-powered portable electronics in the “internet of things” era. Here I will show the development of a bias-field-free spin-torque diodes based on a magnetic tunnel junction having a canted magnetization in the free layer, and demonstrate that those devices could be an efficient harvester of broadband ambient RF radiation, capable to efficiently harvest microwave powers of microWatt and below and to power a black phosphorous nanodevice. The frequency response of spin-torque diodes and their current tunability can be also used as building blocks of the hardware realization of neurons and synapses in neuromorphic applications. Finally, I will show how to implement hardware multiplication with spintronic diodes by using the concept of degree of rectification.

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