On-line SPICE-SPIN+X Seminars

On-line Seminar: 25.06.2025 - 15:00 CEST

Magnetism in Moiré Materials

Allan MacDonald , University of Texas

Two-dimensional van der Waals crystals that are overlaid with a difference in lattice constant or a relative twist form a moiré pattern. In semiconductors and semimetals, the low-energy electronic properties of these systems are accurately described by Hamiltonians that have the periodicity of the moiré pattern creating artificial crystals with lattice constants on the 10 nm scale. Recent progress in fabricating two-dimensional material devices has made it possible to use moiré patterns to design quantum metamaterials in which electrons exhibit strongly-correlated and topologically non-trivial properties that are rare in naturally occuring crystals. Since the miniband widths in both graphene and TMD moiré materials can be made small compared to interaction energy scales (by mechanisms [1,2] that differ), these materials can be used both for quantum simulation and for quantum design. An important property of moiré materials is that their band filling factors can be tuned over large ranges without introducing chemical dopants, simply by using electrical gates.
In this talk I will focus on magnetism in moiré materials, which is sometimes similar to that found in atomic scale crystals and sometimes unusual. In many cases the magnetic order is purely orbital – opening the door to electrical manipulation of magnetic states. Orbital magnetic order combined with non-trivial topology in single-particle bands [3] helps to make quantum anomalous Hall effects common and gives rise to the fractional quantum anomalous Hall effect. The role of band topology is natural in graphene moirés, where it derives from the interesting band topology of graphene monolayers, but has been an unexpected bonus [3] in the case of TMD moires where it derives from the layer degree of freedom.
[1] R. Bistritzer, and A.H.MacDonald, Proceedings of the National Academy of Sciences 26, 12233 ( 2011).
[2] F. Wu, T. Lovorn, E. Tutuc, and A.H.MacDonald, Phys. Rev. Lett. 121, 026402 (2018).
[3] F. Wu, T. Lovorn, E. Tutuc, I. Martin, and A.H.MacDonald, Phys. Rev. Lett. 122, 086402 (2019).

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.
PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.07.2025 - 15:00 CEST

Superconductivity at interfaces of the quantum paraelectric KTaO3

Anand Bhattacharya, Argonne National Laboratory

In this talk I will discuss the recently discovered two-dimensional superconductivity found at interfaces of the incipient ferroelectric KTaO3 (KTO). In its pristine insulating state, KTO is believed to be a ‘quantum paraelectric’, where the onset of ferroelectricity at low temperatures is thwarted by quantum fluctuations. A metallic electron gas can be obtained at interfaces of KTO by depositing a variety of insulating metal-oxide overlayers. Electron microscopy studies reveal the presence of both oxygen vacancies near the interface of KTO and diffusion of cations into KTO from the oxide overlayers, which dope the interfacial region of KTO with electrons. These interfacial electron gases were found to be superconducting up to temperatures as high has 2.2 K. Remarkably, the superconducting state is orientation selective, where electron gases formed at the (111) and (110) crystalline interfaces of KTO are robust two-dimensional superconductors, with Tc as high as 2.2 K and 1 K respectively, while electron gases formed at the (001) interface of KTO and oxide overlayers remain normal down to 25 mK. In this light, I will present a proposed mechanism for superconductivity at KTO interfaces where pairing involves an inter-orbital coupling mechanism mediated by the same soft phonon that is responsible for the incipient ferroelectricity in KTO. This mechanism favors superconductivity in states with maximal orbital degeneracy, and the lifting of this degeneracy due to quantum confinement effects explains the orientation selective nature of superconductivity at KTO interfaces. The broken inversion symmetry and strong spin-orbit coupling in KTO interfacial electron gases also lead to a spin-textured Fermi surface. I will outline how orbital degeneracy gives rise to a uniaxial ‘in-plane Ising’ spin texture for electron gases formed at KTO (110) interfaces, evidenced by their interaction with an insulating magnetic overlayer in both their superconducting and normal states.

References:
1. C. Liu et al., Science 371, 716 (2021).
2. C. Liu et al., Nature Communications 14, 951 (2023).
3. J. Yang et al., arXiv 2502.19599.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 04.03.2026 - 15:00 CET

The Chiral Induced Spin Selectivity- Why it is so special?

Ron Naaman, Department of Chemical and Biological Physics, Weizmann Institute, Israel

The Chiral Induced Spin Selectivity (CISS) effect was first observed more than 25 years ago. Despite numerous experimental verifications across chemistry, physics, and biology, many feel that the underlying mechanism remains poorly understood. This talk will address the question: what makes CISS fundamentally different from conventional electron transport? and will provide new insights into its mechanism. It will be demonstrated how chirality necessitates a fresh perspective on electron transfer and present experimental evidence for the key parameters that enable the CISS effect, challenging our conventional understanding of electronic transport phenomena.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.04.2025 - 15:00 CEST

van der Waals Magnets and Antiferromagnets Interacting with Electron Spins

Daniel Ralph, Cornell University

This talk will discuss two projects. The first concerns topological insulator/magnet samples made by mechanical stacking of exfoliated van der Waals flakes. The proximity interaction of the magnet with the topological surface state allows realization of the “parity anomaly” half-quantized Hall effect state at temperatures as high as 10 K -- the first materials system with a quantized anomalous Hall signal well above liquid-helium temperature. Direct capacitive measurements also indicate a large value of the exchange gap of approximately 10 meV. We speculate the reason for the higher temperature scale compared to previous topological insulator/magnet samples deposited by molecular beam epitaxy is that even a small amount of disorder due to interfacial mixing in deposited samples can push the topological surface state away from the interface and thereby weaken the exchange coupling to the magnet. The pristine interfaces formed by mechanical stacking of van der Waals layers eliminate this intermixing.

The second project concerns 3-terminal tunnel junctions (PtTe2/bilayer CrSBr/graphite) in which the A-type van der Waals antiferromagnet CrSBr acts as the tunnel barrier. Spin-filter tunneling through the CrSBr bilayer allows direct electrical measurements of antiferromagnetic resonance in the frequency domain. Furthermore, spin-orbit torque from the PtTe2 electrode provides electrically-tunable control over the magnetic damping and resonance linewidth. We find the interesting result that the spin-orbit torque is highly local, with the spin current from the PtTe2 electrode acting only on the individual spin sublattice layer adjacent to that electrode, with a negligible amount propagating to the next van der Waals layer.

 

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.
PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 21.05.2025 - 15:00 CEST

Quantum Sensing of Broadband Spin Dynamics and Magnon Transport in Antiferromagnets

P. Chris Hammel, Ohio State University

Optical detection of magnetic resonance using quantum spin sensors (QSS) provides a spatially local and sensitive technique to probe spin dynamics in magnets. However, its utility as a probe of antiferromagnetic resonance (AFMR) remains an open question. We report the first experimental demonstration of optically detected AFMR in layered van der Waals antiferromagnets (AF) up to frequencies of 24 GHz. We leverage QSS spin relaxation due to low-frequency magnetic field fluctuations arising from collective dynamics of magnons excited by the uniform AFMR mode. First, through AFMR spectroscopy we characterize the intrinsic exchange fields and magnetic anisotropies of the AF. Second, using the localized sensitivity of the QSS we demonstrate magnon transport over tens of micrometers. Finally, we find that optical detection efficiency increases with increasing frequency. This showcases the dual capabilities of QSS as detectors of high frequency magnetization dynamics and magnon transport, paving the way for understanding and controlling the magnetism of antiferromagnets.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.02.2025 - 15:00 CET

Altermagnetism imaged and controlled down to the nanoscale

Peter Wadley, University of Nottingham

Altermagnetism is a newly identified class of magnetism which combines properties from both ferromagnets and antiferromagnets, making them highly promising candidates for spintronic applications[1,2]. We recently demonstrated the spin split nature of the altermagnetic electronic band structure in MnTe[3]. In this talk, I will discuss how the unique resultant properties of altermagnets can be used to image them in unprecedented details, and also to control them in unique ways.
Utilising a combination of linearly and circularly polarised x-rays we can generate a full Neel vector map of the magnetic domain in MnTe, showing all 6 domain types and revealing vortices and their vorticity. In addition, a combination of patterning and field cooling can control the domain formation to nucleate single domains of our choosing from the micron to nanoscale. This incluides generation and control of the position and vorticity of single vortices. These experiments showcase the unique properties of altermagnets and also provide a platform for the next stages of research and application[4].

 

 

 

 

 

 

 

 

 

Figure1 – Altermagnetic domain structure in open space (a) showing a vortex antivortex pair and (b) in micro-fabricated field-cooled triangles showing single vortices with opposite vorticity. Adapted from [4].

 

 

References
1. Smejkal, L., Sinova, J. & Jungwirth, T. Beyond Conventional Ferromagnetism and Antiferromagnetism: A Phase with Nonrelativistic Spin and Crystal Rotation Symmetry. Physical Review X 12, 031042 (2022).
2. Smejkal, L., Sinova, J. & Jungwirth, T. Emerging Research Landscape of Altermagnetism. Physical Review X 12, 040501 (2022).
3. Altermagnetic lifting of Kramers spin degeneracy
J. Krempaský, L. Šmejkal, S. W. D’Souza, M. Hajlaoui, G. Springholz, K. Uhlířová, F. Alarab, P. C. Constantinou, V. Strocov, D. Usanov, W. R. Pudelko, R. González-Hernández, A. Birk Hellenes, Z. Jansa, H. Reichlová, Z. Šobáň, R. D. Gonzalez Betancourt, P. Wadley, J. Sinova, D. Kriegner, J. Minár, J. H. Dil & T. Jungwirth
Nature 626, 517–522 (2024)
https://doi.org/10.1038/s41586-023-06907-7
4. Altermagnetism imaged and controlled down to the nanoscale
O. J. Amin, A. Dal Din, E. Golias, Y. Niu, A. Zakharov, S. C. Fromage, C. J. B. Fields, S. L. Heywood, R. B. Cousins, J. Krempasky, J. H. Dil, D. Kriegner, B. Kiraly, R. P. Campion, A. W. Rushforth, K. W. Edmonds, S. S. Dhesi, L. Šmejkal, T. Jungwirth, P. Wadley
Nature 636, pages348–353 (2024)

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.02.2025 - 15:00 CET

New materials and interface effects in charge and spin transport in magnetic heterostructures

Günter Reiss, University of Bielefeld

Magnetic heterostructures are key devices for spinelectronics. Their preparation requires a combination of thin film deposition with sub-Å control, field-annealing and nanopatterning. If fully functional, they can help fundamental research on new materials and effects as well open applications in sensors, memories, logic and oscillators. An introduction will present examples for basic effects and their applications.
We then will discuss several novel materials and interface induced effects occurring in magnetic heterostructures:
- The growth of altermagnetic thin films and their integration in magnetic tunnel junctions using the example of RuO2. Such altermagnets are at present intensively investigated due to their potentially spin split band structure and related spin currents. The X-ray analysis reveals a high crystalline quality of the films with or without twinning depending on the choice of the substrate. When integrated with an MgO tunnel barrier and a ferromagnetic counter electrode, signatures of a tunneling magnetoresistance can be found that strongly depend on the bias voltage and are not yet fully understood. When integrated with ferromagnets (Ni80Fe20) or heavy metals (Pt), an analysis based on the 2ω method shows the presence of torques in accordance with a spin current at the interface.
- When replacing the alter- by a ferromagnet, the heavy metal can show a proximity induced ferro-magnetism at the interface that substantially influences the results of well-known phenomena such as the spin Seebeck, anomalous Nernst or anomalous Hall effect. Examples will be discussed using metallic as well as insulating ferro- or ferrimagnets and recipes for disentangling the zoo of effects will be given.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

PDF file of the talk available here.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 12.03.2025 - 15:00 CET

Static and Dynamic Properties of Insulating Antiferromagnetic Cr2O3

Jing Shi, University of California

This talk explores two distinct aspects of insulating antiferromagnetic Cr2O3. First, we demonstrate the electrical detection of the Néel vector in single-domain Cr2O3. While previous Hall effect measurements have shown promise, signal cancellation due to multi domains can reduce the signal and therefore limit unambiguous Néel vector determination. By fabricating small Pt detectors, we isolate individual magnetic domains and observe a complete reversal of the anomalous Hall signal upon switching the Néel vector. Second, I will present our study of the spin Seebeck effect in bulk Cr2O3 coupled to a homoepitaxial Cr2O3 film. We observe a significant suppression of bulk magnon transport as the film thickness increases to 9 nm, indicating that point defects within the film impede antiferromagnetic magnon diffusion.

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.04.2025 - 15:00 CEST

Chiral phononics

Dominik Juraschek, Eindhoven University of Technology

Chiral phononics is an emerging field that utilizes the angular momentum of circularly polarized lattice vibrations to manipulate the properties of quantum materials. When phonons are driven resonantly with an ultrashort circularly polarized terahertz pulse, light makes the ions in the material behave like electromagnetic coils, producing circular motions of the atoms around their equilibrium positions in the crystal. This motion induces real and effective magnetic fields that have been calculated and measured in a range up to the tesla scale, providing a new tool for the control of magnetic order. Here, I provide an introduction to the field and present recent theoretical predictions of novel phenomena arising from chiral phonon driving. These include the light-induced magnetization in antiferromagnets [1] and cavity-engineered phonon chirality [2]. Further, I will show that nonlinear phonon excitation can be utilized to make achiral materials chiral on demand [3].

[1] Kahana, Bustamante Lopez, Juraschek, Science Advances 10, eado0722 (2024)
[2] Yaniv, Juraschek, in preparation
[3] Romao, Juraschek, ACS Nano 18, 29550 (2024)

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.
PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 05.02.2025 - 15:00 CET

Spintronics with van der Waals heterostructures

Sergio Valenzuela, ICREA and ICN2

Van der Waals (vdW) heterostructures provide a versatile platform for investigating spintronic phenomena, particularly through their atomically sharp interfaces and tunable properties [1,2]. Such heterostructures allow for the design of proximity effects via short-range interactions, enabling the exploration of spin-orbit coupling and spin-dependent transport in ways not easily achieved with conventional materials [1].
In this talk, I will begin by addressing the importance of boundary states and the quality of the topological insulator (TI)/ferromagnet (FM) interface in maximising spin-orbit torques (SOT). For example, vdW TIs such as (Bi,Sb)2Te3 can influence spin transport and charge-to-spin conversion processes due to spin-momentum locking. I will show how introducing a non-magnetic metallic [3] or, in particular, graphene [4] interlayer between the TI and FM, when the FM is a transition metal, significantly modifies the nature and enhances the efficiency of SOTs [3,4]. Similar enhancements observed with sharp interfaces between TIs and vdW FMs [5] further illustrate the potential of interfacial engineering in shaping spintronic functionalities.
Building on these examples, I will then discuss how proximity effects in graphene can be identified through spin transport dynamics, focusing on our findings on spin relaxation anisotropy [6] and charge-to-spin interconversion [7,8]. I will highlight the role of crystal symmetry, showing how systems with reduced symmetry give rise to diverse spin-orbit fields and unconventional charge-to-spin conversion components, alongside methods for determining their underlying mechanisms. Furthermore, I will demonstrate that electrostatic gating can tune spin relaxation anisotropy, as well as spin Hall and spin galvanic effects, with these phenomena remaining robust up to room temperature [6-8].

[1] J. F. Sierra et al., Nature Nano. 16, 856–868 (2021)
[2] H. Yang, S. O. Valenzuela et al., Nature 606, 663 (2022)
[3] F. Bonell et al., Nano Lett. 20, 5893 (2020)
[4] R. Galceran et al., Adv. Mater. Interfaces 9, 2201997 (2022): T. Guillet, V. Zatko et al., unpublished (2025)
[5] T. Guillet et al., Nano Lett. 24, 822 (2024)
[6] B. Raes et al. Nature Commun. 7, 11444 (2016); L. A. Benítez et al., Nature Phys. 14 (2018); APL Materials 7, 120701 (2019); J. F. Sierra, J. Světlík et al., Nature Mater. (in press 02/2025)
[7] L. A. Benítez et al., Nature Mater. 19, 170 (2020)
[8] L. Camosi et al., 2D Mater. 9, 035014 (2022)

Please sign up here in order to get the Zoom link and regular announcements of the upcoming talks.