On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.03.2024 - 15:00 CET

Spin-Orbit torque driven antiferromagnetic oscillator

Joerg Wunderlich, University of Regensburg

P. K. Rout1, J. Godinho1, F. Vilsmeier2, R. Salikhov3, Z. Soban4, R. M. Otxoa5, C. Back2, O. Hellwig3,6, J. Wunderlich1,4

Antiferromagnetic materials have unique properties due to their alternating exchange-coupled magnetic moment arrangements, leading to exchange-field enhanced fast and complex spin dynamics. Most intriguingly, excitation in antiferromagnets with locally broken inversion symmetry can be realized by current-induced spin-orbit torque (SOT), and complex self-oscillation modes near the critical spin-flop transition have been predicted when excited by antidamping SOT.
In this work, we realize an antiferromagnetic oscillator within a nanoconstriction patterned from a synthetic antiferromagnetic (SAF) multilayer. By exploiting the magnetic rectification effect (MRE), we first identify spin-orbit torque-driven excitations of optical and acoustic antiferromagnetic modes. Then, by adding a DC current to our radiofrequency excitation, we identify damping and anti-damping like SOT contributions by both MRE and Brillouin light scattering (BLS). Using spatially and temporally resolved magneto-optical Kerr effect (tr-MOKE) measurements, we observe pi-phase shifted current-induced oscillations of the Néel order in individual reversed antiferromagnetic domains at zero applied magnetic field. Finally we find first indications of self-oscillations near the critical spin-flop transition by MRE measurements, which appear only for DC currents above a critical current density.

1 University of Regensburg (Germany)
2 Technical University of Munich (Germany)
3 Helmholtz-Zentrum Dresden-Rossendorf (Germany)
4 Institute of Physics, Czech Academy of Sciences, Prague, (Czech Republic)
5 Hitachi Cambridge Labyoratory, Cambridge (United Kingdom)
6 Chemnitz University of Technology (Germany)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 31.01.2024 - 15:00 CET

Terahertz spinorbitronics - driving and probing spin and orbital currents at highest frequencies

Tom Seifert, FU Berlin

Launching terahertz (THz) angular-momentum currents from a magnet into a nearby material can be accomplished by laser-induced ultrafast magnetization quenching. Two channels for those ultrafast currents can be distinguished: spin and orbital angular momentum, i.e, S and L, respectively. In my talk, I will focus on the generation, propagation and detection of such laser-induced THz S and L currents in prototypical thin-film heterostructures. In detail, I will show how THz emission spectroscopy led to the development of efficient spintronic terahertz emitters relying on the spin degree of freedom [1,2]. Recently, an inversion of this emitter principle allowed for a broadband spintronic terahertz detection [3]. Finally, I will show how this experimental technique helped revealing THz L currents with a giant decay length in tungsten [4], and enabled us to measure THz spin conductances of antiferromagnetic insulators.

[1] Seifert, Tom, et al. "Efficient metallic spintronic emitters of ultrabroadband terahertz radiation." Nature photonics 10 (2016).
[2] Seifert, Tom S., et al. "Spintronic sources of ultrashort terahertz electromagnetic pulses." Applied Physics Letters 120 (2022).
[3] Chekhov, A. L., et al. "Broadband spintronic detection of the absolute field strength of terahertz electromagnetic pulses." Physical Review Applied 20 (2023).
[4] Seifert, Tom S., et al. "Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten." Nature Nanotechnology 18 (2023).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.12.2023 - 15:00 CET

First-principles calculations of spin transport and spin-orbit torque in metallic heterostructures

Kirill Belashchenko, University of Nebraska–Lincoln

I will discuss first-principles calculations of spin transport and spin-orbit torques in disordered films and multilayers within the nonequilibrium Green's function technique with supercell disorder averaging. I will present the results for ferromagnet/nonmagnet bilayers, for a single platinum film, and for ferromagnet/nonmagnet/ferromagnet trilayers, highlighting the features that can't be easily explained within the conventional spin-diffusion model. I will also discuss the possibility of using ferromagnets with anisotropic transport spin polarization as sources of exchange-driven transverse spin current, supported by a computational screening of suitable materials.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 22.11.2023 - 15:00 CET

Spin transport in graphene-based van der Waals heterostructures

Talieh Ghiasi, Harvard

Decades of research in graphene nanodevices have shown that graphene is an excellent material for charge and spin transport thanks to its high charge carrier mobility and long spin lifetime. However, practical applications of graphene-based spintronic devices require efficient electrical control of the spin information. This sought-after goal is now achievable through the proximity of graphene to other two-dimensional materials in van der Waals heterostructures. In this talk, I will show how we enrich the properties of graphene by the proximity effect and induce coupling between charges and spins via spin-orbit [1, 2] and exchange [3, 4] interactions.

These interactions result in the emergence of various unprecedented phenomena in graphene that showcase its active role in generating spin currents, both electrically and thermally [3, 4]. We further explore quantum Hall transport in proximitized graphene aiming to achieve quantum coherent spin propagation in these heterostructures. These experimental advancements in spin-related functionalities of graphene-based nanodevices can have potential applications in future ultra-compact memory and computing systems.

[1] Ghiasi, TS, et al. Nano Letters 17, 7528 (2017)
[2] Ghiasi, TS, et al. Nano Letters 19, 5959 (2019)
[3] Ghiasi, TS, et al. Nature Nanotechnology 16, 788 (2021)
[4] Kaverzin, AA, et al. 2D Materials 9, 045003 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 29.11.2023 - 15:00 CEST

Thermal and Electrical Probes of Spin Effects in Antiferromagnets: a Revisitation and a New Idea?

Barry Zink, University of Denver

The thermal injection of spin currents across an interface from insulators with a wide range of magnetic ordered states into metals that convert this spin to measurable charge, is known as the longitudinal spin Seebeck effect (LSSE). This effect has proven to be a powerful means to probe the fundamental spin properties of magnetically ordered materials. In recent years, the LSSE and its related electrical effect, spin Hall magnetoresistance (SHMR), have proven especially interesting in a range of antiferromagnetic materials, which can be difficult to probe using more traditional magnetic characterization techniques. In this talk, the first focus is on spin-charge conversion in polycrystalline chromium, an antiferromagnetic metal. Our recent studies using standard [1] and local-heating LSSE techniques [2] show that antiferromagnetism may play a role in the spin-conversion, which was not apparent from earlier work on this material. SHMR measurements made as part of the locally-heated LSSE show unexpected symmetries that may also relate to a role for AFM order. I will then discuss ongoing work on related probes of antiferromagnetic spin effects in field-controllable coupled AFM/FM perovskite oxide systems. Here, dramatic field dependence of the electrical Hall signals are seen when Pt is in contact with the material, while absence of Pt or presence of metals that disturb the AFM order change both the signal sizes and field symmetry. This suggests new routes to low-field control of spin transport in such coupled FM/AFM oxide systems. This work is supported by the US National Science Foundation (DMR-2004646 and EECS-2116991)

[1]   S. M. Bleser et al, Journal of Applied Physics. 131, 113904 (2022).
[2]   S. M. Bleser, et al, in preparation

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.11.2023 - 15:00 CET

Probabilistic Computing with p-bits: Optimization, Machine Learning and Quantum Simulation

Kerem Çamsarı, University of California

The slowing down of Moore's era has coincided with escalating computational demands from Machine Learning and Artificial Intelligence. An emerging trend in computing involves building physics-inspired computers that leverage the intrinsic properties of physical systems for specific domains of applications. Probabilistic computing with p-bits, or probabilistic bits, has emerged as a promising candidate in this area, offering an energy-efficient approach to probabilistic algorithms and applications.

Several implementations of p-bits, ranging from standard CMOS technology to nanodevices, have been demonstrated. Among these, the most promising p-bits appear to be based on stochastic magnetic tunnel junctions (sMTJ). sMTJs harness the natural randomness observed in low barrier nanomagnets to create energy-efficient and fast fluctuations, up to GHz frequencies. In this talk, I will discuss how magnetic p-bits can be combined with conventional CMOS to create hybrid probabilistic-classical computers for various applications. I will provide recent examples of how p-bits are naturally applicable to combinatorial optimization, such as solving the Boolean satisfiability problem, energy-based generative machine learning models like deep Boltzmann machines, and quantum simulation for investigating many-body quantum systems.

Through experimentally-informed projections for scaled p-computers using sMTJs, I will demonstrate how physics-inspired probabilistic computing can lead to GPU-like success stories for a sustainable future in computing.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.01.2024 - 15:00 CET

Orbital Angular Momentum for Spintronics

Mathias Kläui, JGU Mainz

Novel spintronic devices can play a role in the quest for GreenIT if they are stable and can transport and manipulate spin with low power. Devices have been proposed, where switching by energy-efficient approaches is used to manipulate topological spin structures [1,2].
We combine ultimate stability of topological states due to chiral interactions [3,4] with ultra-efficient manipulation using novel spin torques [3-5]. In particular orbital torques [6] increase the switching efficiency by more than a factor 10. We probe the materials dependence of the orbital current generation and conversion and show that the switching current density is strongly reduced compared to state-of-the-art spin-orbitronic devices.
We use skyrmion dynamics for non-conventional stochastic computing applications, where we developed skyrmion reshuffler devices [7] based on skyrmion diffusion, which also reveals the origin of skyrmion pinning [7]. We go beyond simple ferromagnets and study multilayers with antiferromagnetic coupling termed synthetic antiferromagnets. We find that the diffusion dynamics is drastically enhanced due to the topology and efficient dynamics can be induced by spin torques [8]. Finally, we find novel topological spin structures, such as bi-merons that are stabilized in synthetic antiferromagnets [9].

References
[1] G. Finocchio et al., J. Phys. D: Appl. Phys., vol. 49, no. 42, 423001, 2016.
[2] K. Everschor-Sitte et al., J. Appl. Phys., vol. 124, no. 24, 240901, 2018.
[3] S. Woo et al., Nature Mater., vol. 15, no. 5, pp. 501–506, 2016.
[4] K. Litzius et al., Nature Phys., vol. 13, no. 2, pp. 170–175, 2017.
[5] K. Litzius et al., Nature Electron., vol. 3, no. 1, pp. 30–36, 2020.
[6] S. Ding et al. Phys. Rev. Lett. 125, 177201, 2020; Phys. Rev. Lett. 128, 067201, 2022.
[7] J. Zázvorka et al., Nature Nanotechnol., vol. 14, no. 7, pp. 658–661, 2019;
R. Gruber et al., Nature Commun. vol. 13, pp. 3144, 2022.
[9] T. Dohi et al., Nature Commun. vol. 14, pp. 5424, 2023.
[9] M. Bhukta et al., arxiv:2303.14853 (Nature Commun. in press 2024)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 25.10.2023 - 15:00 CEST

Spintronics with low-symmetry materials

Felix Casanova, CIC nanoGUNE

Two-dimensional materials are an exciting new material family in which the proximity effect is especially important and opens ways to transfer useful spintronic properties from one 2D material into another. For instance, transition metal dichalcogenides (TMD) can be used to enhance the spin-orbit coupling of graphene. The spin-orbit proximity in such graphene/TMD van der Waals heterostructures leads to spin-to-charge conversion (SCC) of out-of-plane spins due to spin Hall effect (SHE), first observed by our group using MoS2 as the TMD [1]. The combination of long-distance spin transport and SHE in the same material gives rise to an unprecedented figure of merit (product of spin Hall angle and spin diffusion length) of 40 nm in graphene proximitized with WSe2, which is also gate tunable [2].
The low symmetry present in many of these low-dimensional materials allows the creation of spin polarizations in unconventional directions and enables new fundamental effects and configurations for devices. In this regard, chiral systems are the ultimate expression of broken symmetry, lacking inversion and mirror symmetry. One way to achieve this is by twisting a graphene/TMD heterostructure. We use twisted graphene/WSe2 to observe SCC arising from Rashba-Edelstein effect (REE) from spins not only perpendicular to the current (conventional configuration), but also parallel to the current (unconventional configuration) [3]. Furthermore, we can tune the twist angle between graphene and WSe2 to control the helicity of the Rashba spin texture, which even changes sign, in excellent agreement with theoretical predictions [4].
Another way to exploit chirality is by directly using materials with a chiral crystal structure, such as elemental tellurium (Te), a 1D van der Waals material. We have recently demonstrated a gate-tunable chirality-dependent charge-to-spin conversion in Te, [5], detected by recording a large unidirectional magnetoresistance (up to 7%). The orientation of the electrically generated spin polarization is determined by the crystal handedness, while its magnitude can be manipulated by an electrostatic gate.
Our results pave the way for the development of chirality-based spintronic devices.

References
[1] C. K. Safeer, FC et al., Nano Lett. 19, 1074 (2019).
[2] F. Herling, FC et al. APL Mater. 8, 071103 (2020).
[3] H. Yang, FC et al. submitted.
[4] S. Lee, FC et al. Phys. Rev. B 106, 165420 (2022).
[5] F. Calavalle, FC et al., Nat. Mater. 21, 526 (2022).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 18.10.2023 - 15:00 CEST

Spin and Charge Pumping in the Presence of Spin-Orbit Coupling in THz Spintronics with Antiferromagnets

Branislav K. Nikolić, University of Delaware

The interaction of fs light pulses with magnetic materials has been intensely studied for more than two decades to understand ultrafast demagnetization in single magnetic layers or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. Despite long history, microscopic understanding of ultrafast-light-driven magnets is incomplete due to numerous competing effects and with virtually no study reporting calculation of output THz radiation. This talk presents a recently developed [1] multiscale quantum-classical formalism where conduction electrons are described by quantum master equation of the Lindblad type; classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and incoming light is described by classical vector potential while outgoing electromagnetic radiation is computed using the Jefimenko equations for retarded electric and magnetic fields. We illustrate it by application to a bilayer of Weyl antiferromagnet Mn3Sn with noncollinear local magnetization in contact with SO-coupled nonmagnetic material, revealing new mechanisms of THz radiation due to direct charge pumping by local magnetization dynamics of Mn3Sn in the presence of its strong intrinsic SO coupling. I also discuss how to modify this approach when LLG equation becomes inapplicable and local magnetization must be treated by quantum many-body techniques including dissipation [2], as is the case of strongly correlated antiferromagnet NiO. Finally, the simplest example of dynamics of magnetization leading to spin and charge pumping is that of microwave-driven uniformly precessing local magnetization within ferro- or antiferromagnets, where we show how to handle the presence of SO coupling using the Floquet-Keldysh formalism [3] with possible first-principles Hamiltonian as an input [4]. This yields a new prediction of high harmonics [3] in pumped spin and charge currents due to peculiar motion of flowing electron spins generated by the SO coupling.

[1] A. Suresh and B. K. Nikolić, Phys. Rev. B 107, 174421 (2023)
[2] F. Garcia-Gaitan and B. K. Nikolić, https://doi.org/10.48550/arXiv.2303.17596 (2023)
[3] J. Varela-Manjarres and B. K. Nikolić, J. Phys. Mater. 6, 045001 (2023)
[4] K. Dolui, A. Suresh, and B. K. Nikolić, J. Phys. Mater. 5, 034002 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 21.06.2023 - 15:00 CEST

All electrical magnon transport experiments in magnetically ordered insulators

Matthias Althammer, Walther-Meißner-Institut

Pure spin currents, i.e. the flow of angular momentum without an accompanying charge current represents a new paradigm in the field of spintronics. Most importantly, pure spin currents can be transported by fermions, i.e. by electrons, in electrical conductors as well as by bosons, i.e. by magnons, the quantized spin excitations in magnetically ordered systems. Interestingly, heterostructures consisting of spin-orbit coupled metals with magnetically ordered insulators allow to investigate pure spin current transport in both regimes and their interconversion at the interface. This approach enables an all electrical injection and detection scheme to study magnon transport phenomena in magnetically ordered insulators [1]. I will present recent results focusing on ferrimagnetic and antiferromagnetic insulators. First, I will present spin conductance measurements in yttrium iron garnet thin films. Here, I will highlight how charge currents can control the magnon spin conductance realizing a compensation of magnon damping via spin-orbit torques [2,3]. In the second part, I will focus on magnon spin transport in antiferromagnetic insulators. The quantized spin excitations of an ordered antiferromagnet with opposite chirality represent pairs of spin-up and -down magnons and this two-level nature can be characterized by a magnonic pseudospin. In the last years, first descriptions and observations of the associated dynamics of antiferromagnetic pseudospin have been reported [1,2,3,4]. I will introduce these magnon pseudospin dynamics and describe how they lead to the manifestation of the magnon Hanle effect in hematite thin films.

[1] M. Althammer, Phys. Stat. Sol. RRL 15, 2100130 (2021)
[2] T. Wimmer et al., Phys. Rev. Lett. 123, 257201 (2019)
[3] J. Gückelhorn et al., Phys. Rev. B 104, L140404 (2021)
[4] T. Wimmer et al., Phys. Rev. Lett. 125, 247204 (2020)
[5] A. Kamra et al., Phys. Rev. B 102, 174445 (2020)
[6] J. Gückelhorn et al., Phys. Rev. B 105, 094440 (2022)
[7] J. Gückelhorn et al., arXiv:2209.09040 (accepted in PRL 2023)

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