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 German Time


Silvia Picozzi, D'Annunzio University

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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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 29.03.2023 - 15:00 German Time

Probabilistic spintronics – Computing and Device Physics

Shunsuke Fukami, Tohoku University

Probabilistic behavior of physical system is mostly regarded as a nuisance in conventional electronics. Contrary to this perception, here I show that properly designed probabilistic systems is even useful for unconventional computers that address complex problems more efficiently than conventional computing hardware, and spintronic systems can be a prime candidate on that front, opening a new paradigm, probabilistic spintronics.
In this seminar, I will show some proof-of-concepts of the spintronic probabilistic computers and describe how the computers can be constructed from probabilistic spintronic devices and how it solves computationally hard problems [1-4]. I will also discuss the physics governing the probabilistic behavior of spintronics devices and strategy to develop the devices for high-performance probabilistic computers [5-9].

[1] K. Camsari et al., Phys. Rev. X 7, 031014 (2017).
[2] W. A. Borders et al., Nature 573, 390 (2019).
[3] J. Kaiser et al., Phys. Rev. Appl. 17, 014016 (2022).
[4] A. Grimardi et al., IEEE IEDM 2022, 22.4 (2022).
[5] S. Kanai et al., Phys. Rev. B 103, 094423 (2021).
[6] K. Hayakawa et al., Phys. Rev. Lett. 126, 117202 (2021).
[7] K. Kobayashi et al., Appl. Phys. Lett. 119, 132406 (2021).
[8] T. Funatsu et al., Nat. Comm. 13, 4079 (2022).
[9] K. Kobayashi et al., Phys. Rev. Appl. 18, 054085 (2022).


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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.02.2023 - 15:00 German Time

A stride down the quantum materials roadmap

Alberta Bonanni, Johannes Kepler University

Lately, condensed matter physics has witnessed the emergence of material systems in which quantum effects persist over a wide range of energy and length scales [1]. Such quantum materials include, among others, topological insulators, topological crystalline insulators, magnetically doped topological quantum materials, superconductors, 2-dimensional (2D). van der Waals, Kitaev and spin-orbit systems [2]. Here, the striking properties of quantum materials will be highlighted through a collection of relevant examples overarching the Rashba spin-orbit coupling in wurtzite n-GaN:Si [3,4] and the intriguing electronic properties of the magnetically doped topological crystalline insulator SnTe [5] and of the intrinsic ferromagnetic topological insulator MnSb2Te4. Striding further down the quantum materials road, the emergence of quantum chiral anomaly in 2D Weyl semimetal Td-WTe2with a record temperature of 100 K will be addressed [6]. Moreover, a bosonic island percolation model for Fe-doped superconducting NbN thin films will be presented [7]. Finally, an outlook of emergent phenomena in hybrid quantum structures with particular attention to topology, symmetry, spin-orbit coupling and superconductivity will be provided.

[1] B. Keimar et al. Nat. Phys. 13, 1045 (2017)
[2] F. Giustino et al. J. Phys. Mater. 3, 042006 (2020)
[3] W. Stefanowicz et al. Phys. Rev. B 89, 205201 (2014)
[4] R. Adhikari et al. Phys. Rev. B 94, 085205 (2016)
[5] R. Adhikari et al. Phys. Rev. B 100, 134422 (2019)
[6] R. Adhikari et al. Nanomaterials 11, 2755 (2021)
[7] R. Adhikari et al. Nanomaterials 12, 3105 (2022)


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On-line SPICE-SPIN+X Seminars

On-line Seminar: 25.01.2023 - 15:00 German Time

Developments in ultrafast electron microscopy

Claus Ropers, MPI for Biophysical Chemistry and University of Göttingen

Providing the most detailed views of atomic-scale structure and composition, Transmission Electron Microscopy (TEM) serves as an indispensable tool for structural biology and materials science. The combination of electron microscopy with pulsed electrical or optical stimuli allows for the study of transient phenomena, involving magnetization dynamics, strain evolution and structural phase transformations. Ultrafast transmission electron microscopy (UTEM) is a pump-probe technique, in which non-equilibrium processes can be tracked with simultaneous femtosecond temporal and nanometer to atomic-scale spatial resolutions.
This talk will cover recent methodical developments and applications in UTEM based on laser-triggered field emitters, including real-space imaging [1] and ultrafast nanobeam diffraction [2] of a structural phase transition. Moreover, the mechanisms involved in free-electron beams interacting with optical fields at photonic structures will be discussed, emphasizing quantum effects. In particular, recent progress in the coupling of electron beams to whispering gallery modes [3] and integrated photonic resonators [4] will be presented. Finally, using event-based electron spectroscopy, electron-energy loss measured in coincidence with cathodoluminescence is used to demonstrate the preparation and characterization of electron-photon pair states [5].

[1] "Ultrafast nanoimaging of the order parameter in a structural phase transition”, T. Danz, T. Domröse, C. Ropers, Science 371, 6527 (2022)
[2] "Light-induced hexatic state in a layered quantum material", T. Domröse et al., arXiv:2207.05571(2022)
[3] "Controlling free electrons with optical whispering-gallery modes", O. Kfir et al., Nature 582, 46 (2020)
[4] "Integrated photonics enables continuous-beam electron phase modulation", J.-W. Henke et al., Nature 600, 653 (2021)
[5] A. Feist et al., “Cavity-mediated electron-photon pairs”, Science 377, 777 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 07.12.2022 - 15:00 German Time

Light-driven phonomagnetism

Dmytro Afanasiev, Radboud University

Light in the form of ultrashort pulses made it possible to control magnetism at the ultimate time and length scales where traditional excitation with magnetic fields fails. In particular, it has opened a novel pathway to highly efficient control antiferromagnets – materials that do not possess any net magnetization and thus are notoriously known to be insensitive to magnetic fields. One of the latest breakthroughs in the optical control of magnetism is the resonant driving of elementary lattice excitations - phonons. While phonons are intuitively associated with heating, which only destroys magnetism, recent experiments have shown that when driven by the light the phonons can be used to control and even induce magnetic order.
Here I will show you how light-driven phonons can lead to a net distortion of the crystalline lattice, able to switch between various symmetry antiferromagnetic phases and even induce a net magnetization in initially not magnetized media, all on the ultrafast timescale.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.11.2022 - 15:00 German Time

Towards a "complete" picture of ultrafast dynamics in the 2D ferromagnet FGT

Yoav William Windsor, FH Institute of the MPS and TU Berlin

Two dimensional materials have been the focus of intense study in recent years, with extensive effort invested in transition metal dichalcogenides. A recent addition to this is the study of 2D magnetic materials. Here we focus on one such ferromagnetic material: Fe3GeTe2. We present an overview of its ultrafast response to photoexcitation using three time-resolved probes: angle resolved photoemission (ARPES), X-ray magnetic circular dichroism (XMCD), and ultrafast electron diffraction (UED). These reflect the response of different material degrees of freedom, namely the carriers, the spins and the phonons. I will then focus primarily on the study of lattice dynamics using UED.

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