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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.10.2022 - 15:00 German Time

Generation of electric field induced unconventional spin-current

Arnab Bose, JGU Mainz

Symmetry plays a central role in determining the form of electrically-generated spin torques in magnetic devices. Here, we show that an unconventional out-of-plane damping-like torque can be generated in ruthenium oxide (RuO2)/permalloy devices when the Néel vector of the collinear antiferromagnet RuO2 is canted relative to the sample plane [1]. By measuring characteristic changes in all three components of the electric-field-induced torque vector as a function of the angle of the electric field relative to the crystal axes, we find that the RuO2 generates a spin current with a well-defined tilted spin orientation that is approximately parallel to the Néel vector. This dependence is the signature of an antiferromagnetic spin-Hall effect predicted to arise from momentum-dependent spin splitting within the bandstructure of RuO2, rather than from spin-orbit coupling [2]. The unconventional components are absent in the isostructural but non-magnetic rutile oxide IrO2. The out-of-plane antidamping component of the spin torque from RuO2 is among the strongest measured in any material even with the antiferromagnetic domain structure uncontrolled, suggesting that high efficiencies useful for switching magnetic devices with perpendicular magnetic anisotropy might be achieved by controlling the domain structure.

[1] A. Bose, et al., Nature Electronics (2022). doi.org/10.1038/s41928-022-00744-8
[2] R. González-Hernández, et al., Phys. Rev. Lett. 126, 127701 (2021)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 29.06.2022 - 15:00 German Time

How to engineer non-equilibrium crystal and magnetic structures with light

Ankit Disa, MPSD / Cornell University

The crystal structure is a key ingredient determining the macroscopic properties of any solid. In the context of magnetism, the local moments, anisotropy, and exchange parameters are all strongly dependent on the bonding environment and the symmetry of the lattice, which can be tuned, for example, by chemical composition or external pressure. Such quasi-static approaches are limited in their speed and efficacy, however. In this talk, I describe how we can instead manipulate the crystal structure of materials dynamically using light, which enables one to induce, enhance, and control magnetic states in ways not possible in equilibrium. The approach is based on selectively exciting optical phonons with resonant THz pulses and exploiting nonlinearities of the crystal lattice [1]. We used this approach to realize a light-induced transition to a ferrimagnetic phase in the antiferromagnet CoF2 [2]. The resultant non-equilibrium magnetization is optically switchable and has a magnitude 100-fold larger than achievable via strain. More recently, we demonstrated the ability to strengthen magnetic order in the strongly correlated ferromagnet YTiO3 [3]. By driving specific lattice distortions, the low-temperature moment was enhanced and a non-equilibrium ferromagnetic state was stabilized even at temperatures well in excess of the equilibrium Tc. These experiments show that optically engineering the crystal structure provides a versatile and powerful tool for emerging magnetic and spintronic technologies.

[1] A.S. Disa, T.F. Nova, A. Cavalleri, Nature Phys. 17, 1087 (2021)
[2] A.S. Disa, et al., Nature Phys. 16, 937 (2020)
[3] A.S. Disa, et al., arXiV: 2111.13622

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.06.2022 - 15:00 German Time

Nonreciprocal transport and topological band structure through interactions of magnonic multilayers

Luqiao Liu, MIT

Dipolar interaction, one of the most classical interaction mechanisms among magnetic moments, acts as a ‘spin-orbit-coupling’ like mechanism on magnons, by coupling their precession handedness with the propagation direction. This has given rise to the well-known surface magnetostatic wave (Damon-Eshbach mode) in thin film ferromagnet with chiral surface states. Recently, dipolar interaction induced magnon couplings between different magnetic layers have drawn interests, for realizing non-reciprocal transmission of microwave signals and non-Hermitian Hamiltonian systems. In a recent work, via the coupling of magnons between a ferrimagnetic insulator Y3Fe5O12 (YIG) and a magnetic alloy NiFe, we showed that tunable, nonreciprocal propagation can be realized in spin Hall effect-excited incoherent magnons, whose frequencies cover the spectrum from a few gigahertz up to terahertz [1]. In the diffusion transport, the nonreciprocity is reflected as asymmetric magnon diffusion lengths, which are unequal along opposite transmission directions. The diffusive nonreciprocity is closely related to the change of the magnon damping through the chiral dipolar coupling. Extending this bi-layer structure into an antiparallelly aligned magnetic multilayer, we show theoretically that the interlayer dipolar interaction generates bulk bands with non-zero Chern integers and magnonic surface states carrying chiral spin currents [2]. The surface states are highly localized and can be easily toggled between non-trivial and trivial phases through an external magnetic field. Our experimental and theoretical findings enrich knowledge on diffusive transport of magnons and provide an easy-to-implement system for topological magnonic states, which can be used for the design of passive signal isolation devices.

[1] J. Han, Y. Fan, B. C. McGoldrick, J. Finley, J. T. Hou, P. Zhang, and L. Liu, Nano Letters, 21, 7037 (2021)
[2] Z. Hu, L. Fu, L. Liu, “Tunable Magnonic Chern Bands and Chiral Spin Currents in Magnetic Multilayers,” arXiv:2201.00312 (2022)

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