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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.05.2021 - 15:00 German Time

Antiferromagnetic Switching Driven by the Collective Dynamics of Correlated Spin Textures

James Analytis, University of California

The theory behind the electrical switching of antiferromagnets is premised on the existence of a well-defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. Here, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet \Fex{1/3+\delta}, rooted in the electrically-stimulated collective winding of the spin glass. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium orientation of the antiferromagnetic state. The use of a spin glass' collective dynamics to electrically manipulate antiferromagnetic spin textures has never been applied before, opening the field of antiferromagnetic spintronics to many more material platforms with complex magnetic textures.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.06.2021 - 15:00 German Time

Ultrafast coupled charge, spin and nuclear dynamics: ab-initio description

Sangeeta Sharma, MBI-Berlin

Laser induced ultrafast dynamics is a burgeoning field of condensed matter physics promising the ultimate short time control of light over matter. From the outset of research into femtomagnetism, the field in which spins are manipulated by light on femtosecond or faster time scales, several questions have arisen and remain highly debated: How does the light interact with spin moments? How is the angular momentum conserved between the nuclei, spin, and angular momentum degrees of freedom during this interaction? What causes the ultrafast optical switching of magnetic structures from anti-ferromagnetic to ferromagnetic and back again? What is the ultimate time limit on the speed of spin manipulation? What is the impact of nuclear dynamics on the light-spin interaction?
In my talk I will advocate a parameter free ab-initio approach to treating ultrafast light-matter interactions, and discuss how this approach has led both to new answers to these old questions but also to the uncovering of novel and hitherto unsuspected early time spin dynamics phenomena. In particular I will demonstrate OISTR (optical inter-site spin transfer)[1,2] to be one of the fastest means of spin manipulation via light [4,7,8,9], with changes in magnetic structure occurring on attosecond time scales [8]. I will also discuss the impact of nuclear dynamics on laser induced spin dynamics and demonstrate how selective phonon modes can be used to enhance the OISTR effect.
The ability to measure and calculate the same physical quantity forms the cornerstone of the vital collaboration between theory and experiment, and I will discuss recent work where we have ab-initio calculated the real time response functions of L-edge and M-edge semi-core states during spin dynamics, demonstrating both good quantitative agreement with experiment [5,6] but also showing how theory can actually predict new phenomena and guide new experiments.
[1] Dewhurst et al. Nano Lett. 18, 1842, (2018)
[2] Elliott et al. Scientific Reports 6, 38911 (2016)
[3] Shokeen et al. Phys. Rev. Lett. 119, 107203 (2017)
[4] Chen et al. Phys. Rev. Lett. 122, 067202 (2019)
[5] Willems et al. Nat. Comm. 11, 1 (2020)
[6] Dewhurst et al. Phys. Rev. Lett. 124, 077203 (2020)
[7] Hofherr et al. Sci. Advs. 6, eaay8717 (2020)
[8] Siegrist et al. Nature 571, 240 (2019)
[9] Golias et al. Phys. Rev. Lett. 126, 107202 (2021)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.05.2021 - 15:00 German Time

Magnetic skyrmions for unconventional computing and revealing latent information

Karin Everschor-Sitte, University of Duisburg-Essen

Novel computational paradigms in combination with proper hardware solutions are required to overcome the limitations of our state-of-the-art computer technology. [1-3] In this talk, I will focus on the potential of topologically stabilized magnetic whirls – so-called skyrmions for reservoir computing. Reservoir computing is a computational scheme that allows to drastically simplify spatial-temporal recognition tasks. We have shown that random skyrmion fabrics provide a suitable physical implementation of the reservoir. [4,5] They allow to classify patterns via their complex resistance responses either by tracing a signal over time or by a single spatially resolved measurement. [6] In a second part of the talk, I will introduce two recently developed data analysis tools. [7, 8] While often a significant effort is made in enhancing the resolution of an experimental technique to obtain further insight into the sample and its physical properties, an advantageous data analysis has the potential to provide deep insights into given data set.

[1] J. Grollier, D. Querlioz, K.Y. Camsari, KES, S. Fukami, M.D. Stiles, Nat. Elect. 3, 360 (2020)
[2] E. Vedmedenko, R. Kawakami, D. Sheka, ..., KES, et al., J. of Phys. D 53, 453001 (2020)
[3] G. Finocchio, M. Di Ventra, K.Y. Camsari, KES, P. K. Amiri, Z. Zeng, JMMM 521, 167506 (2021)
[4] D. Prychynenko, M. Sitte, et al, KES, Phys. Rev. Appl. 9, 014034 (2018)
[5] G. Bourianoff, D. Pinna, M. Sitte and KES, AIP Adv. 8, 055602 (2018)
[6] D. Pinna, G. Bourianoff and KES, Phys. Rev. Appl. 14, 054020 (2020)
[7] I. Horenko, D. Rodrigues, T. O’Kane and KES, arXiv:1907.04601
[8] D. Rodrigues, KES, S. Gerber, I. Horenko, iScience 24, 102171 (2021)

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