On-line SPICE-SPIN+X Seminars
On-line Seminar: 21.05.2025 - 15:00 CEST
TBA
P. Chris Hammel, Ohio State University
TBA
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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)
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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.
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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.
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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].
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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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PDF file of the talk available here
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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PDF file of the talk available here