On-line SPICE-SPIN+X Seminars
On-line Seminar: 02.10.2024 - 15:00 CEST
TBA
Hans-Gregor Huebl, Walther-Meißner-Institut
TBA
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Spin-X-Abstracts
On-line SPICE-SPIN+X Seminars
On-line Seminar: 12.06.2024 - 15:00 CEST
TBA
José J.Baldoví, University of Valencia
TBA
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On-line SPICE-SPIN+X Seminars
On-line Seminar: 29.05.2024 - 15:00 CEST
Fluctuation-driven phenomena in the kagome-net magnets RMn_6Sn_6
Igor Mazin, George Mason University
I will discuss four different nontrivial manifestations of spin fluctuations in 166 materials:
1) Mn spin fluctuation in Y166 and Er166 generate (at finite temperature) topological Hall effect
2) Tb spin fluctuations in Tb166 trigger a spontaneous spin-reorientation transitions
3) Er spin fluctuations trigger a ferrimagnet-spiral transition
4) RE and Mn spin fluctuations modify the conventional scaling of the anomalous Hall effect.
This shows that spin fluctuations can have qualitatively diverse and nontrivial effects already on the mean field level, i.e. outside of the standard framework of suppressing the long-range order and triggering order from disorder transitions.
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On-line SPICE-SPIN+X Seminars
On-line Seminar: 27.03.2024 - 15:00 CET
Chiral spin textures on the racetrack
Stuart Parkin, MPI Halle
The simplest chiral spin texture is a one-dimensional Néel magnetic domain wall that separates two magnetic regions that are magnetized in opposite directions. Under the influence of spin orbit torques, that are derived from spin currents that carry angular momentum, these walls can be driven at high speeds exceeding 1 km/sec along magnetic nano-wires that, thereby, form “magnetic racetracks”. This is the basic principle of the magnetic racetrack memory that stores digital data in the form of the presence or absence of such chiral domain walls. We discuss recent developments including the scaling of racetrack to sub-100 nm widths and the first 3D racetrack memory devices. Chiral domain walls are, however, just one member of an ever-expanding family of chiral spin textures that are of great interest from both a fundamental as well as a technological perspective. Recently a zoology of complex 2D and 3D spin textures stabilized by volume or interface Dzyaloshinskii-Moriya vector exchange interactions have been discovered including, in our work, anti-skyrmions, elliptical Bloch skyrmion, two-dimensional Néel skyrmions and fractional antiskyrmions. Such nano-objects are potential candidates as magnetic storage bits on the racetrack. Another class of chiral spin textures are Kagome antiferromagnets: we have recently shown how their complex spin textures can be manipulated by a previously unobserved seeded spin orbit torque (SSOT) mechanism. These chiral spin textures become superconducting when placed in proximity to a conventional superconductor and support long range triplet supercurrents. Triplet supercurrents are highly interesting in that they can carry spin angular momentum, unlike conventional superconductors, and, therefore, could be used, in principle, to manipulate chiral spin textures at ultra-low temperatures. This is the basic principle of the SUPERTRACK memory device that I have recently proposed [1].
[1] European Research Council Advanced Grant– SUPERMINT awarded to Stuart S.P. Parkin April, 2022.
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On-line SPICE-SPIN+X Seminars
On-line Seminar: 26.06.2024 - 15:00 CEST
Exploring d-d Transition Dynamics in FePS3: A Journey through Magneto-Optical and Photoelectron Spectroscopy Investigations
Mirko Cinchetti, TU Dortmund University
Excitations between localized 3d states of transition metal ions within crystalline solids, commonly known as d-d transitions, play a pivotal role in diverse phenomena across solid-state physics, materials science, and chemistry. These transitions contribute significantly to the optical properties of transition metal oxides, catalytic activity on oxide surfaces, high-temperature superconductivity, and magnetic behaviors, facilitating spin-crossover transitions and linking optical excitation to quantized phenomena such as phonons and magnons. The discovery of unique effects in two-dimensional (2D) antiferromagnets, such as electron-phonon bound states, sub-terahertz (sub-THz) frequency magnon modes, and hybridized phonon-magnon modes, highlights the complex phenomenology driven by d-d transitions.
In this presentation, I will discuss our recent investigations into FePS3, selected for its promise as a scalable van der Waals antiferromagnetic semiconductor that retains magnetic order even at the 2D limit. We employed two complementary experimental approaches. Initially, pump-probe magneto-optical measurements were conducted to observe laser-driven lattice and spin dynamics. Pumping in resonance with a d-d transition within the Fe2+ multiplet induced a coherent phonon mode oscillating at 3.2 THz. Remarkably, this mode is excitable in a low optical absorption regime, safeguarding even single antiferromagnetic layers from damage. The mode's amplitude diminishes with increasing temperature, disappearing at the Néel temperature as the system transitions to a paramagnetic phase, thereby illustrating its connection to long-range magnetic order. Furthermore, in an external magnetic field, this 3.2 THz phonon mode hybridizes with a magnon mode, enabling optical excitation of the resultant phonon-magnon hybrid mode [1].
Additionally, we utilized angle-resolved photoelectron spectroscopy (ARPES) to probe the electronic structure in its ground state [2] and employed time-resolved ARPES to capture the ultrafast dynamics of selected spin-allowed and spin-forbidden d-d transitions in FePS3 [3]. The insights from magneto-optical experiments, juxtaposed with ARPES findings, shed light on the intricate quasiparticle dynamics underpinning d-d transitions in FePS3, offering a deeper understanding of their role in quantum material behaviors.
[2] J.E. Nitschke, et al. Materials Today Electronics (2023). https://doi.org/10.1016/j.mtelec.2023.100061
[3] J.E. Nitschke, et al. arXiv (2024). arXiv:2402.03018
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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.
[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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PDF file of the talk available here
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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PDF file of the talk available here