On-line SPICE-SPIN+X Seminars

On-line Seminar: 28.02.2024 - 15:00 CET

Towards magnonic memory: reversal of nanomagnets on yttrium iron garnet by propagating spin waves

Dirk Grundler, EPFL

D. GRUNDLER1,2

1Institute of Materials (IMX), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
2Institute of Electrical and Micro Engineering (IEM), EPFL, 1015 Lausanne, Switzerland

Magnonics based computing has regained increasing interest when micromagnetic simulations showed that ferromagnetic nanoelements control the interference of spin waves (magnons) in low-damping yttrium iron garnet (YIG) and give rise to a neural network [1]. Magnonics-based in-memory computation would be even more promising if nonvolatile magnetic bits could store directly magnon signals. I will report on our experiments which show that magnons with wavelengths down to 99 nm in YIG induce the reversal of bistable nanomagnets assisted by a small bias field [2]. We combine broadband spin-wave spectroscopy, micro-focus Brillouin light scattering and magnetic force microscopy and study the magnon-induced reversal depending on the YIG thickness, interface properties, nanomagnet shape, the magnon amplitude and their propagation length over 100 m. The magnon-induced reversal is found to be a robust effect [2] and contributes to the progress of on-chip devices which combine the concept of a neural network with an embedded magnonic memory. The work was supported by SNSF via grant 197360.

References:
[1] Papp A., Porod W., Csaba G. (2021). Nat. Commun. 12, 6422.
[2] Baumgaertl K., Grundler D. (2023). Nat. Commun. 14, 1490; Joglekar S. et al. (2023). https://arxiv.org/abs/2312.09177; Mucchietto A. et al. (2023). https://arxiv.org/abs/2312.15107 .

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 07.02.2024 - 15:00 CET

Magnon-exciton coupling in a magnetic semiconductor

Youn Jue Bae, Cornell

While magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. The ability to overcome energy mismatch between magnon and exciton combined with optical excitation and detection renders 2D magnetic semiconductors attractive candidates in quantum transducers. In this presentation, I will discuss strong magneto-electronic and magnetoelastic coupling and the implications of these couplings in the 2D van der Waals antiferromagnetic semiconductor, CrSBr. Because of both magnetic and semiconducting properties in CrSBr, excitons are highly sensitive to spin environments. Optical excitation of coherent spin waves can dynamically modulate the dielectric environment and we can probe excitons to obtain spin information. I will also discuss strong magnetoelastic coupling in CrSBr that induces transient strain fields to selectively launch a narrow range of wavevector and frequency of both coherent magnons and acoustic phonons.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 14.02.2024 - 15:00 CET

Energy, geometry, and topology of collective magnetic dynamics

Yaroslav Tserkovnyak, UCLA

I will review our recent work on transport phenomena engendered by large-angle magnetic dynamics, combining insights from differential geometry, topology, and overarching thermodynamic considerations. As a specific illustrative case, we study the transport of vorticity on curved dynamical two-dimensional magnetic membranes. We find that topological transport can be controlled by geometrically reducing symmetries, which enables processes that are not present in flat magnetic systems. To this end, we construct a vorticity 3-current obeying a continuity equation, which is immune to arbitrary local disturbances of the magnetic texture as well as spatiotemporal fluctuations of the membrane. We show how electric current can manipulate vortex transport in geometrically nontrivial magnetic systems. As an example, we propose a minimal setup that realizes an experimentally feasible energy storage device and discuss its thermodynamic efficiency in terms of a vorticity-transport counterpart of the thermoelectric "ZT" figure of merit.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.
PDF file of the talk available here

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)

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

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).

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

PDF file of the talk available here

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.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

PDF file of the talk available here

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)

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

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

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

PDF file of the talk available here

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.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

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)

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

PDF file of the talk available here