On-line SPICE-SPIN+X Seminars

On-line Seminar: 01.03.2023 - 15:00 German Time

Coherent manipulation of spins in diamond via spin-wave mixing

Toeno van der Sar, TU Delft

Coherent manipulation of spins in diamond via spin-wave mixing Magnetic imaging based on nitrogen-vacancy (NV) spins in diamond enables probing condensed matter systems with nanoscale resolution[1]. In this talk I will introduce NV magnetometry as a tool for imaging spin waves – the wave-like spin excitations of magnetic materials. Using the NV sensitivity to microwave magnetic fields, we can map coherent spin waves[2] and incoherent magnon gases[3] and provide insight into their interaction and damping[4]. By using a single NV in a scanning diamond tip we gain access to spin-wave scattering at the nanoscale[5]. I will highlight how we can use spin-wave mixing to generate frequency combs that enable high-fidelity, coherent control of the NV spins even when the applied microwave drive fields are far detuned from the NV spin resonance frequency 6 (Fig. 1). These results form a basis for developing NV magnetometry into a tool for characterizing spin-wave devices and expand the control and sensing capabilities of NV spins.

[1] Casola, F., Van Der Sar, T. & Yacoby, A. Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond. Nat. Rev. Mater. 3, 17088 (2018).
[2] Bertelli, I. et al. Magnetic resonance imaging of spin-wave transport and interference in a magnetic insulator. Sci. Adv. 6, eabd3556 (2020).
[3] Simon, B. G. et al. Directional Excitation of a High-Density Magnon Gas Using Coherently Driven Spin Waves. Nano Lett. 21, 8213–8219 (2021).
[4] Bertelli, I. et al. Imaging Spin‐Wave Damping Underneath Metals Using Electron Spins in Diamond. Adv. Quantum Technol. 4, 2100094 (2021).
[5] Simon, B. G. et al. Filtering and imaging of frequency-degenerate spin waves using nanopositioning of a single-spin sensor. Nano Lett. 22, 9198 (2022).
[6] Carmiggelt, J. J. et al. Broadband microwave detection using electron spins in a hybrid diamondmagnet sensor chip. Nat. Commun. 14, 490 (2022)

 

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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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 30.11.2022 - 15:00 German Time

Multilayer spintronic neural networks with radio-frequency connections

Alice Mizrahi, CNRS-Thales

The combination of the two key effects of spintronics, magnetization dynamics and magneto-resistive effects, allows the realization of nano-neurons and nano-synapses with high computational capabilities. However, multilayer spintronic neural networks have never been realized with these nanodevices because there is no way to connect them that can work on a large scale. I will show that it is possible to exploit the two key effects of spintronics to connect together magnetic tunnel junctions that implement both neurons and synapses of a multilayer network. The magnetic tunnel junctions perform neural operations on DC signals and output the result as RF, operate synaptic operations on RF signals and output the result as DC thus giving rise to multilayer networks based on successive, clean, and fast conversions from RF to DC and from DC to RF. I will give a proof of concept with a two-layer neural network composed of nine interconnected magnetic tunnel junctions functioning as both synapses and neurons and experimentally demonstrate its ability to solve non-linear tasks with high performance. I will show that with junctions downscaled to 20 nm, such a network would consume 10fJ per synaptic operation and 100fJ per neuronal operation, several orders of magnitude less than current software neural network implementations. Finally, I will show through physical simulations that these networks, which can process DC data, can also natively classify RF inputs, achieving state-of-the-art drone identification from their RF transmissions. This study lays the foundation for deep spintronic neural networks at the nanoscale.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 14.12.2022 - 15:00 German Time

Skyrmions in chiral magnetic multilayers

Christopher Marrows, University of Leeds

Magnetic skyrmions are topologically-nontrivial spin textures with particle-like properties [1]. Their size, topological stability, and mobility suggest their use in future generations of spintronic devices, the prototype of which is the skyrmion racetrack [2]. To realise a racetrack requires three basic operations: the nucleation (writing), propagation (manipulation), and detection (reading) of a skyrmion, all by electrical means. Here we show that all three are experimental feasible at room temperature in Pt/Co/Ir or Pt/CoB/Ir multilayers in which the different heavy metals above and below the magnetic layer break inversion symmetry and induce chirality by means of the Dzyaloshinskii-Moriya interaction, defining the structure of Néel skyrmion spin textures [3]. We show deterministic nucleation on nanosecond timescales using an electrical point contact on top of the multilayer [4] (Figure 1), current-driven propagation along a wire in which the skyrmions are channelled by defects in the multilayer [5], and their detection by means of the Hall effect (Figure 2) that reveals an unexpectedly large contribution to the Hall signal that correlates with the topological winding number [6].

[1] N. Nagaosa & Y. Tokura, Nat. Nanotech. 8, 899 (2013)
[2] A. Fert et al. Nature Nanotech. 8, 152 (2013)
[3] K. Zeissler et al. Sci. Rep. 7, 15125 (2017)
[4] S. Finizio et al., Nano Lett. 19, 7246 (2019)
[5] K. Zeissler et al., Nature Comm. 11, 428 (2020)
[6] K. Zeissler et al. Nature Nanotech. 13, 1161 (2018)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.10.2022 - 15:00 German Time

Ultrafast optoelectronic probes of quantum materials

James McIver, MPSD and Columbia University

Ultrafast optoelectronic circuits offer new opportunities for investigating the electrical response of microstructured quantum materials and heterostructures on femtosecond timescales and at terahertz frequencies. Based on waveguides and laser-triggered photoconductive switches, these circuits can be used to directly probe the ultrafast flow of electrical currents in materials [1], or perform near-field spectroscopy on length scales orders of magnitude smaller than the diffraction limit [2]. In this talk, I will show how using this circuitry we observed an anomalous Hall effect in graphene driven by a femtosecond pulse of circularly polarized light [3]. The dependence of the anomalous Hall effect on a gate potential used to tune the Fermi level revealed multiple features that reflect the formation of photon-dressed (‘Floquet-engineered’) topological band structure [4], similar to the band structure originally proposed by Haldane [5]. This included an approximately 60 meV wide conductance plateau centered at the Dirac point, where a gap of equal magnitude was predicted to open. We found that when the Fermi level was tuned within this plateau, the anomalous Hall conductance saturated around 1.8 ± 0.4 e^2/h.

In the second part of the talk, I will share our progress on using these ultrafast circuits to perform near-field time-domain terahertz spectroscopy on graphene heterostructures, where we observe a coherent plasmonic response that can be tuned with electrostatic gating. This near-field technique, which we are extending to mK temperatures and strong magnetic fields, could be used to investigate a wide range of topological and strongly correlated phenomena in microstructured quantum materials and heterostructures that often fall on the gigahertz-terahertz energy scale.

[1] D.H. Auston, Appl. Phys. Lett. 26, 101–103 (1975)
[2] Z. Zhong et al., Nature Nano. 3, 201-205 (2008)
[3] J.W. McIver et al., Nature Physics 16, 38 (2020)
[4] T. Oka & H. Aoki. Phys. Rev. B 79, 081406 (2009)
[5] F.D.M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.11.2022 - 15:00 German Time

Stripe domain phases in chiral magnetic systems with perpendicular anisotropy

Eric Fullerton, UC San Diego

Stripe domain phases arise in magnetic films with perpendicular magnetic anisotropy due to the competition between long-range dipolar fields and short-range exchange and have been studied dating back to the early work of Kittel [1] in the 1940s. Introducing interfacial Dzyaloshinskii-Moriya interactions (DMI) to the system alters the energy balance by lowering the domain wall energy and further fixes the chirality of the domain walls. These both effect the magnetic response of the stripe phase to fields, currents, and temperature. I will first discuss the implications of DMI on the stripe domain phase in a Pt/Co/Ni-based multilayer structures and their response to magnetic field and electrical current pulses [2, 3]. In particular, I will discuss the novel growth direction symmetries of the stripe phase in response to a combined in-plane and out-of-plane magnetic fields [3]. I will then discuss of the emergence of the stripe phase in ultrathin ferromagnetic layers in Pt/Co and related structures [4]. This results from low ferromagnetic exchange stiffness that lowers the domain wall energies and triggers a domain morphological transition either by changing temperature or magnetic layer thickness. This transition is accompanied by two distinct types of temperature-dependent fluctuations – faster-scale fluctuations in the domain wall positions and slower-scale fluctuations in the configuration of the overall stripe domain morphology. While the domain wall fluctuations demonstrate properties of Brownian motion modified by a sporadic pinning potential, the slower-scale morphological fluctuations exhibit characteristics typically ascribed to more solid-like, collective dynamics. At higher temperatures, these results suggest a novel fluctuating stripe phase emerges.

[1]. C. Kittel, Phys. Rev. 70, 965; (1946) and Rev. Mod. Phys. 21, 541 (1949)
[2]. J. A. Brock et al., Phys. Rev. Mater. 4, 104409 (2020)
[3]. J. A. Brock et al., Adv. Mater. 33, 2101524 (2021)
[4]. J. A. Brock and E. E. Fullerton, Adv. Mater. Inter. 9, 2101708 (2022)

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