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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 22.03.2023 - 15:00 German Time

Strong coupling of microwaves and magnons in YIG microstructures

Georg Schmidt, Martin Luther Univ. Halle

Strong coupling between microwaves and magnons has already been demonstrated. So, magnon modes in an yttrium iron garnet (YIG) sphere of several hundred micrometer diameter were successfully coupled to the microwave modes of a large microwave cavity[1]. Also coupling of magnons in YIG to phonons[2] or to optical photons[3] has already been shown. All these experiments have in common that rather macroscopic pieces of YIG were used. This is unfavorable if the effect is to be integrated for device purposes, both in terms of size and technology.
On the other hand coupling between magnetic microstructures and superconducting resonators has been reported making use of ferromagnetic metals that can easily be patterned[4,5]. Nevertheless, the lifetime of spin waves in ferromagnetic metals is rather small and although strong coupling could be demonstrated, it would be desirable to use microscopic YIG resonators instead.
We have realized coupling between microwave photons in superconducting lumped element resonators and magnons in Permalloy and YIG nanostructures, respectively. With the metallic ferromagnet we realize an unusual coupling behavior because of the strong shape anisotropy in an elongated structure. With YIG, we are able to reach the strong coupling regime. This is possible because of an optimized lumped element resonator that concentrates the magnetic field in the magnetic microstructure.

[1] e.g. Y. Tabuchi et al., Phys. Rev. Lett. 113, 083603 (2014)
[2] X. Zhang et al., Science Advances 2, e1501286
[3] A. Osada et al., Phys. Rev. Lett. 116, 223601 (2016)
[4] Y. Li et al., Phys. Rev. Lett. 123, 107701 (2019)
[5] J.T. Hou et al., Phys. Rev. Lett. 123, 107702 (2019)
 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.05.2023 - 15:00 CEST

Spin-orbit coupling: an endless source of exotic phenomena in 2D magnets

Silvia Picozzi, D'Annunzio University

During the last decades, the spin-orbit coupling (SOC) has played an increasingly crucial role in condensed matter physics, thanks to its relevance as a rich microscopic mechanism from the fundamental point of view and as a driving force for innovative spintronic applications on the technological side. Combined with the global thrust towards miniaturization and with the ubiquitous research in two-dimensional (2D) materials, the talk will focus on the modelling of 2D magnets with emphasis on SOC-induced effects. In particular, I will focus on the magnetic and ferroelectric properties of transition-metal monolayers (mostly halides) and discuss the role of SOC in the magnetoelectric coupling. The reports of multiferroicity in NiI2 layers [1], obtained via a joint theory-experiments approach down to the single-layer limit, show the potentiality of cross-coupling phenomena in van der Waals magnets. If time permits, other recent examples – such as SOC-induced effects in CrSBr monolayers - will be discussed.

[1] Song, Q., Occhialini, C.A., Ergecen, E., Ilyas, B., Amoroso, D., Barone, P., Kapeghian, J., Watanabe, K., Taniguchi, T., Botana, A. S., Picozzi, S., Gedik, N., Comin, R., Evidence for a single-layer van der Waals multiferroic, Nature 602, 601 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.02.2023 - 15:00 German Time

Machine learning as a tool to accelerate magnetic materials discovery

Stefano Sanvito, Trinity College Dublin

The process of finding new materials, optimal for a given application, is lengthy, often unpredictable, and has a low throughput. Here I will describe a collection of numerical methods, merging advanced electronic structure theory and machine learning, for the discovery of novel compounds, which demonstrates an unprecedented throughput and discovery speed. This is applied here to magnetism, but it can be used for any materials class and potential application.
Firstly, I will discuss a machine-learning scheme for predicting the Curie temperature of ferromagnets, which uses solely the chemical composition of a compound as feature and experimental data as target[1]. In particular, I will discuss how to develop meaningful feature attributes for magnetism and how these can be informed by experimental and theoretical results.
Then, I will describe how an accurate description of the structure of materials, which is amenable to be used with machine learning, can offer a quantum-chemistry-accurate description of local properties at virtually no computational costs. The method is not just suitable for building energy models[2], namely force fields to used across a broad spectrum of conditions[3], but also for any other local electronic quantity. These models may then be employed to design new materials, as demonstrated here for magnetic molecules with enhanced uniaxial anisotropy[4].
Finally, I will present a novel rotationally invariant representation for generic vector fields. This can be used to generate linear and non-linear machine-learning models, where the total energy depends both on the atomic position and the vector field direction[5]. The scheme will be put to the test against a hierarchy of simple spin models, demonstrating an impressive ability to extrapolate away from the training region of the data. Application to complex potential energy surfaces, as those extracted from DFT are then envisioned.

[1] J. Nelson and S. Sanvito, Predicting the Curie temperature of ferromagnets using machine learning, Phys. Rev. Mat. 3, 104405 (2019)
[2] Alessandro Lunghi and Stefano Sanvito, A unified picture of the covalent bond within quantum-accurate force fields: from simple organic molecules to metallic complexes reactivity, Science Advances 5, eaaw2210 (2019).
[3] Yanhui Zhang, Alessandro Lunghi and Stefano Sanvito, Pushing the limits of atomistic simulations towards ultra-high temperature: a machine-learning force field for ZrB2, Acta Materialia 186, 467 (2020).
[4] Alessandro Lunghi and Stefano Sanvito, Surfing multiple conformation-property landscapes via machine learning: Designing magnetic anisotropy, J. Phys. Chem. C 124, 5802 (2019).
[5] Michelangelo Domina, Matteo Cobelli and Stefano Sanvito, Spectral neighbor representation for vector fields: Machine learning potentials including spin, Phys. Rev. B 105, 214439 (2022).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.03.2023 - 15:00 German Time

Hidden magnetoelectric order

Nicola Spaldin, ETH Zurich

Most magnetic materials, phenomena and devices are well described in terms of magnetic dipoles of either spin or orbital origin. There is mounting evidence, however, that the existence and ordering of higher-order magnetic multipoles can lead to intriguing magnetic behaviors, which are often attributed to "hidden order" since they are difficult to characterize with conventional probes. In this talk I will discuss the relevance of the so-called magnetoelectric multipoles, which form the next-order term, after the magnetic dipole, in the multipolar expansion of the energy of a magnetization energy in a magnetic field. First I will describe how magnetoelectric multipoles underlie multiferroic behavior and in particular how they determine the magnetic response to applied electric fields. Then I will discuss signatures of hidden magnetoelectric multipolar order, how it can be unearthed using density functional calculations and possibilities for its direct measurement. Finally, I will show that the bulk magnetoelectric multipolization manifests at surfaces as a magnetization, and explore an analogy with the bulk electric polarization and its associated surface charge.
 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 22.02.2023 - 15:00 German Time

Exploring spintronics at unconventional hybrid interfaces

Angela Wittmann, JGU

Controlled manipulation of a system allows for systematic investigation of the underlying interactions and phenomena. Simultaneously, tunability also enables the development of novel materials systems and devices customized for specific applications. Here, we will focus on materials systems that conventionally have not been used as active components in spintronic devices. We will explore the impact of strain on the antiferromagnetic domain structure via magneto-elastic coupling [1]. Furthermore, we will delve into hybrid molecule-magnetic interfaces. Molecules offer a unique way of controlling and varying the structure at the interface making it possible to precisely tune the spin injection and diffusion by molecular design [2]. In particular, chirality has gained recent interest in the context of the chiral-induced spin selectivity effect [3]. Here, we will explore signatures of spin filtering at a non-magnetic chiral molecule-metal interface paving the path toward novel hybrid spintronics.

[1] Wittmann, A. et al. Role of substrate clamping on anisotropy and domain structure in the canted antiferromagnet a-Fe2O3. Phys. Rev. B 106, 224419 (2022).
[2] Wittmann, A. et al. Tuning Spin Current Injection at Ferromagnet-Nonmagnet Interfaces by Molecular Design. Phys. Rev. Lett. 124, 027204 (2020).
[3] Naaman, R., Paltiel, Y. & Waldeck, D. H. Chiral molecules and the electron spin. Nat. Rev. Chem. 3, 250–260 (2019).

 

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