On-line SPICE-SPIN+X Seminars

On-line Seminar: 10.04.2024 - 15:00 CEST

Mapping and controlling topological textures
in 3D magnetic systems

Claire Donnelly, MPI CPfS

Three dimensional magnetic systems promise significant opportunities for applications, for example providing higher density devices and new functionalities associated with complex topology and greater degrees of freedom [1,2]. Extending to three dimensions allows for the formation of new topologies of spin textures, for example containing defects in 3D such as Bloch point singularities, or truly three-dimensional topological structures such as magnetic torons or hopfions.
In this talk, I will address two main questions: first, can we observe and understand such three-dimensional topological magnetic textures, and second, can we control them?
For the observation and understanding of these three-dimensional textures, we have developed magnetic X-ray tomographic techniques, that open the possibility to map both the three-dimensional magnetic structure [3], and its dynamical response to external excitations [4,5]. In this way, we have observed 3D magnetic solitons which we identify as nanoscale magnetic vortex rings, as well as torons that contain Bloch point singularities [6,7].
However, while X-ray magnetic tomography is now a relatively well-established technique, high resolution imaging of extended magnetic systems has so far been limited to rare-earth containing materials. To this end, I will present recent results of soft X-ray dichroic ptychography where the phase dichroism offers a route to imaging magnetic systems that until now have not been accessible [8].
As well as naturally existing within the bulk, 3D spin textures can be introduced and controlled via the patterning of 3D curvilinear geometries [9]. I will discuss how, in this way, not only can new states be realized [10], but the energy landscape of topological defects can be designed through the local patterning of curvature induced chirality [11].
This new understanding and control of topological textures in 3D magnetic systems paves the way not only for enhanced understanding of these systems, but also towards the next generation of technological devices.

[1] Fernández-Pacheco et al., Nature Communications 8, 15756 (2017).
[2] C. Donnelly and V. Scagnoli, J. Phys. D: Cond. Matt. 32, 213001 (2020).
[3] C. Donnelly et al., Nature 547, 328 (2017).
[4] C. Donnelly et al., Nature Nanotechnology 15, 356 (2020).
[5] S. Finizio et al., Nano Letters (2022)
[6] C. Donnelly et al., Nat. Phys. 17, 316 (2021)
[7] N. Cooper, PRL. 82, 1554 (1999).
[8] Neethirajan et al., arXiv:2309.14969 [cond-mat.mes-hall]

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.04.2024 - 15:00 CEST

Magnetic hopfion rings

Nikolai Kiselev, FZ Juelich

Magnetic solitons are localized magnetization field configurations in crystals that exhibit properties similar to ordinary particles. For example, under different external stimuli such as magnetic fields, temperature gradients, or electric currents, magnetic solitons can move and interact with one another. These characteristics make magnetic solitons promising candidates for applications in information transfer and data storage. The most well-known example of magnetic solitons is skyrmions in chiral magnets. Skyrmions in 2D materials and skyrmion strings in bulk samples represent quasi-two-dimensional configurations. The three-dimensional magnetic solitons, also known as hopfions, were theoretically predicted in several magnetic systems. Hopfions can be conceptualized as closed twisted skyrmion strings. In the simplest scenario, hopfions form toroidal or ring-like structures localized within a small volume of the magnetic sample. We present the first experimental observation of 3D topological magnetic solitons in magnetic crystals, particularly hopfions linked to skyrmion strings in B20-type FeGe, through high-resolution transmission electron microscopy. I will discuss various aspects of hopfion rings, including a highly reproducible protocol for hopfion ring nucleation, the diversity of configurations of hopfion rings linked with one or a few skyrmion strings, hopfion ring zero modes, etc.

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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 .

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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.

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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.

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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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