On-line SPICE-SPIN+X Seminars

On-line Seminar: 05.03.2025 - 15:00 CET

On the Origin of Electron-Electron Interactions in Bi2Se3 Topological Thin Films

Bryan J Hickey, University of Leeds

We are using Bi2Se3 as a platform in a number of applications in two large collaborative projects: NAME (Nanoscale Advanced Materials Engineering, https://name-pg.uk) and CAMIE ( Combining Advanced Materials with Interface Engineering https://camie.leeds.ac.uk). In our four-chamber deposition system we grow Bi2Se3 by MBE and transfer the samples under UHV to other chambers where we deposit additional layers such as ferromagnets, antiferromagnets, skyrmion bearing multilayers as well as organic layers such as C60. The groundwork for these projects required a growth campaign to obtain material of world class standard and we have characterised our Bi2Se3 using a wide range of techniques. We have achieved excellent epitaxy using a seed layer of (Bi,In)2Se3 and most of the results there are on layers of 20nm Bi2Se3.
Research into the transport properties of Bi2Se3 has been ongoing for many years but there are still questions to be answered about the nature of the conduction in this interesting material. For example, the spin-orbit lifetime is often assumed to be very short but results can be difficult to interpret when the number of conduction channels is reported to be other than 1 or 2, and frequently, it is less than 1. Equally, the spin-orbit scattering should be independent of temperature but it often cannot be seen to be so in many results. Although several papers have suggested that electron-electron interaction effects are observed in, especially the zero-field low-temperature upturn in the resistivity, the nature and origin of these interactions remains unreported.
We have extracted the lifetime of the spin-orbit interaction (!"), by fitting the full expression of the Hikami, Larkin and Nagaoka (HLN) theory for the MR, which is indeed short in the best materials ~ 10-14 s but can be longer in others. We show that fits to the MR can be achieved with a temperature independent value of !". In the strong spin-orbit limit, the approximate HLN function applies and then the fits return only a single conduction channel. The full analysis allows us to extract the electron-electron interaction time (## )
as a function of temperature and hence determine its origin in terms of Fermi liquid theory
and the effects of a finite mean free path, i.e. ($ ℓ).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.01.2025 - 15:00 CET

Probing 2D Magnetic Materials with magnetotransport

Alberto Morpurgo, University of Geneva

The ability to exfoliate van der Waals crystals of magnetic compounds is giving access to a vast, unexplored family of two-dimensional magnetic materials, with a variety of different magnetic ground states. Most of these compounds are semiconductors that offer –besides the possibility to explore magnetism in highly controlled 2D crystals— a new playground to combine magnetic and semiconducting functionalities. In this talk I will discuss how magnetotransport experiments allow the investigation the magnetic phase diagram of 2D magnetic material down to the ultimate limit of individual monolayers, to reveal phenomena that are difficult –or cannot—be accessed with other existing experimental techniques. After a short introduction, in my talk I will discuss vey recent experiments on field effect transistors realized on exfoliated crystals of CrPS4 –ranging from relatively thick multilayers, to double-gated bilayers, and to individual monolayers– and discuss results that illustrate the wealth of physical phenomena that become accessible with these systems.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 30.10.2024 - 15:00 CET

Quantum Functionalities of Magnetic Skyrmions

Christina Psaroudaki, ENS Paris

In this talk, I will discuss the development of magnetic nano-skyrmions as promising candidates for quantum logic elements, focusing on their potential applications in quantum computing. Nano-skyrmions possess quantized helicity excitations, and quantum tunneling between skyrmions with distinct helicities highlights their quantum nature. By harnessing these unique properties, we propose skyrmion qubits where information is stored in the quantum degree of helicity. Electric and magnetic fields can adjust the logical states of these qubits, offering a versatile operation regime with high anharmonicity.

I will explore the role of electrical control over helicity, opening new pathways for functionalizing collective spin states. Additionally, I will discuss the microwave pulses necessary to generate single-qubit gates and multiqubit schemes that promise scalable architectures with tailored couplings. Scalability, controllability by microwave fields, and nonvolatile readout techniques converge to make skyrmion qubits highly attractive for quantum processors. This talk will highlight the exciting developments, challenges, and potential breakthroughs in quantum magnetism and quantum information using skyrmions.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 27.11.2024 - 15:00 CET

Fractional Charges in 2D magnets & Aharonov-Bohm scattering

Nina del Ser, Caltech

Magnetic skyrmions are characterised by an integer topological charge, Q=1, while merons have half-integer winding numbers, Q=1/2. In this talk, I will describe the physics of magnetic textures with fractional topological charge, which is neither integer nor half-integer. Examples of generic magnetic systems which can host such fractional charges include the meeting points between domains in ferromagnetic films with cubic anisotropy or exploding skyrmions. Only fractionally charged defects give rise to an Aharonov-Bohm effect for incident magnons. We investigate this in a numerical scattering experiment by tracking the magnon-induced forces.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.10.2024 - 15:00 CEST

Exploring 3D Spin Structures and Dynamics in Chiral Magnets with Advanced Synchrotron X-ray Techniques

Thorsten Hesjedal, University of Oxford

Recent advances in the study of magnetic skyrmions, topologically protected spin textures, have unlocked new possibilities for innovative low-power, high-speed spintronic devices. This talk presents our recent advancements in using resonant elastic x-ray scattering (REXS) to explore 3D spin structures, such as skyrmions, chiral bobbers, emergent monopoles, and other non-collinear magnetic textures, along with their dynamic behaviors across different timescales.

We have developed cutting-edge 3D REXS techniques [1-4] that enable not only the detailed study of ordered 2D skyrmion lattices but also offer deep insights into microscopic properties like helicity angles and topological winding number [5]. Utilizing circular dichroism REXS (CD-REXS) and the depth sensitivity of soft x-rays, we uncovered surprising long-range surface effects, such as the transformation of Bloch-type skyrmions into Néel-type at the surface of the prototypical chiral magnet Cu2OSeO3 [3,4]. These findings led us to investigate exotic magnetic textures at interfaces in engineered heterostructures, including chiral bobber lattices [6], hybrid skyrmions [7], and the dynamic folding and unfolding of skyrmion strings [8].

The distinctive topology of skyrmions endows them with unique dynamical properties that hold promise for next-generation spintronic devices. In circular magnetic field gradients, skyrmion lattices exhibit controlled rotational dynamics [9]. Most importantly, the role of topological defects is crucial in understanding the slow relaxation dynamics of moving skyrmion lattices, influencing their behavior and stability under external perturbations [10]. On the other hand, understanding the fast, intrinsic magnetization dynamics of skyrmions is crucial for their controlled engineering in high-speed applications. We have pioneered techniques combining ferromagnetic resonance (FMR) with resonant magnetic x-ray reflectivity and diffraction with ferromagnetic resonance (RFMR [11] and DFMR [12]), offering novel pathways for probing real-space spin dynamics and unlocking new opportunities for spintronic device development [13].

[1] S.-L. Zhang et al., Phys. Rev. B 93, 214420 (2016).
[2] S.L. Zhang et al. Phys. Rev. B 96, 094401 (2017).
[3] S.L. Zhang et al., Phys. Rev. Lett, 120, 227202 (2018).
[4] S.L. Zhang et al., Proc. Natl. Acad. Sci. U.S.A. 115, 6386 (2018).
[5] S.L. Zhang et al., Nature Commun. 8, 14619 (2017).
[6] K. Ran et al., Phys. Rev. Lett. 126, 017204 (2020).
[7] K. Ran et al., Nano Lett. 22, 3737 (2022).
[8] H. Jin et al., Nano Lett. 23, 5164 (2023).
[9] S.L. Zhang et al., Nature Commun. 9, 2115 (2017).
[10] H. Jin et al., Nano Lett., in press (2024).
[11] D.M. Burn et al., Phys. Rev. Lett. 125, 137201 (2020).
[12] D.M. Burn et al., Nano Lett. 20, 345 (2020).
[13] G. van der Laan and T. Hesjedal, Nucl. Instrum Methods Phys. Res. B 540, 85 (2023).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.10.2024 - 15:00 CEST

Quantum hybrids: connecting spin excitations to resonators

Hans-Gregor Huebl, Walther-Meißner-Institut

Magnons - the quantized excitations of a magnetic system - are not only pivotal for spintronics applications, but also probe the fundamental properties of magnetic systems. For example, the field of magnetization dynamics deduces from the dynamic response of the spin system the type of magnetic order and relevant magnetic anisotropy parameters. Moreover, this allows to extract the interaction of the magnetization dynamics with other degrees of freedom such as solid-state excitations. Typically, the electromagnetic waves used for this purpose are thought as weakly perturbing stimuli and probes. However, if the interaction between magnetic excitations and the drive becomes strong, collective effects can arise and have the ability to modify or completely alter the character of the magnetization dynamics. One such example is a hybrid system formed by a spin excitation and a resonator.

In my presentation, I will discuss the impact on magnetic excitations due to their coupling to an auxiliary system using the example of spin-excitations in a microwave cavity. In this context, I will introduce characteristic fingerprints, a categorization scheme of the coupling regime, and frequently used figures of merit like the cooperativity or coupling rate.

In addition, the magnetization excitations are naturally linked to a sense of precession via the gyromagnetic ratio. This offers the opportunity to access the angular momentum carried by other excitations, even for an engineered hybrid system. In the second part of my presentation, I will discuss the coupling of magnons to the extrinsic phonons of a bulk acoustic resonator via the magnetoelastic interaction. Here, I will focus on the presently demonstrated coupling regimes, and the ability to use this system as a probe for the angular momentum properties of the phonons.

I will conclude the presentation with an outlook on future opportunities for the field of quantum science and, in particular, for sensing and transduction applications.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 12.06.2024 - 15:00 CEST

Playing with magnetism in 2D van der Waals materials via first principles

José J. Baldoví, University of Valencia

The recent isolation of two-dimensional (2D) magnets offers tantalizing opportunities for spintronics, magnonics and quantum technologies at the limit of miniaturization. [1] Among the key advantages of atomically-thin materials are their flexibility, which provides an exciting avenue to control their properties by strain engineering, and the more efficient tuning of their properties with respect to their bulk counterparts.

In this presentation, I will provide an overview of our recent results on this fascinating topic. First, we will take advantage of the outstanding deformation capacity of 2D materials to answer the question: Can we use strain engineering to control spin waves propagation? [2] For that, we will focus on the magnetic properties, magnon dispersion and spin dynamics of the air-stable 2D magnetic semiconductor CrSBr, investigating their evolution under mechanical strain and Coulomb screening using first-principles. Then, we will introduce the modulation of the magnetic properties, magnon dispersion and spin dynamics of this 2D magnet after the deposition of sublimable organic molecules in a journey towards molecular controlled magnonics. [3] On the other hand, we will look for topological magnons in chromium trihalides (CrX3), [4] investigate magnetostriction effects in 2D van der Waals antiferromagnets such as FePS3 and CoPS3, [5] create new Janus 2D magnetic materials based in MPS3 in order to answer: what are the effects of mirror broken symmetry on the magnetic properties? [6], and finally, we will delve into the origin of above-room-temperature magnetism in Fe3GaTe2 [7].

[1] B. Huang et al., Nature, 546, 270–273 (2017).
[2] D. L. Esteras et al., Nano Lett. 22, 8771–8778 (2022).
[3] A. M. Ruiz et al., Nanoscale Adv. DOI: 10.1039/d4na00230j (2024).
[4] D. L. Esteras et al., Materials Today Electronics, 6, 100072 (2023).
[5] M. Houmes et al., Nature Commun. 14, 8503 (2023).
[6] A. M. Ruiz et al., Dalton Trans. 51, 16816-16823 (2022).
[7] A. M. Ruiz et al., Nano Lett. DOI: 10.1021/acs.nanolett.4c01019 (2024).

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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.

[1] F. Mertens, et al. Advanced Materials (2023).https://doi.org/10.1002/adma.202208355
[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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