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.

 

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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: 13.12.2023 - 15:00 CET

First-principles calculations of spin transport and spin-orbit torque in metallic heterostructures

Kirill Belashchenko, University of Nebraska–Lincoln

I will discuss first-principles calculations of spin transport and spin-orbit torques in disordered films and multilayers within the nonequilibrium Green's function technique with supercell disorder averaging. I will present the results for ferromagnet/nonmagnet bilayers, for a single platinum film, and for ferromagnet/nonmagnet/ferromagnet trilayers, highlighting the features that can't be easily explained within the conventional spin-diffusion model. I will also discuss the possibility of using ferromagnets with anisotropic transport spin polarization as sources of exchange-driven transverse spin current, supported by a computational screening of suitable materials.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 01.06.2022 - 15:00 German Time

Domain walls and skyrmions: From ferromagnets to ferrimagnets

Geoffrey Stephen Beach, MIT

Tremendous progress has been made in engineering highly mobile domain walls and skyrmions in room temperature materials for racetrack-based applications. Most recent efforts have focused on heavy-metal/ferromagnet heterostructures with Dzyaloshinskii-Moriya interactions and spin-orbit torques, in which chiral domain walls and skyrmions can be stabilized at room temperature and readily manipulated [1,2]. However, ferromagnets possess fundamental limitations on spin texture speed and size owing to stray fields and precessional dynamics [3]. Antiferromagnets, on the other hand, possess no stray fields, and are angular-momentum-compensated, yielding extremely fast dynamics. Ferrimagnets exhibit similar behaviors at compensation, but are more readily probed since the individually sublattices are detectible and addressable owing to the fact that the electronic and optical properties of the elements on these sites are typically different. Here, I describe ferrimagnetic spin textures and dynamics in metallic and insulating ferrimagnets. Using Pt/GdCo/TaOx films with sizable Dzyaloshinskii-Moriya interaction, we realize current-driven domain wall motion with a speed of 1.3 km/s near the angular momentum compensation temperature (TA) and room-temperature stable skyrmions with minimum diameters close to 10 nm near magnetic compensation (TM) [4]. By using temperature as a knob, the roles of compensation on the dynamics can be clearly extracted. I then describe recent work on insulating magnetic garnets with perpendicular anisotropy, in which we have discovered an interfacial Dzyaloshinskii-Moriya interaction [5,6] which, combined with low damping and pure spin current injection mediated by a Pt overlayer [7], leads to exceptionally fast motion at extremely low current densities [8]. Finally, I will discuss all-optical manipulation of skyrmions using ultrafast laser excitations, including picosecond generation of topological charge [9] tracked in real time via single-shot soft x-ray scattering, and all-optical writing, deleting, and two-dimensional steering of skyrmions by light alone [10]. Recent progress and future directions in these areas will be discussed.

[1] S. Emori, et al., Nature Mater. 12, 611 (2013)
[2] S. Woo, et al., Nature Mater. 15, 501 (2016)
[3] F. Büttner, et al., Sci. Rep. 8, 4464 (2018)
[4] L. Caretta, et al., Nature Nano. 13, 1154 (2018)
[5] C. O. Avci, et al., Nat. Mater. 14, 561 (2019)
[6] L. Caretta, et al., Nat. Comm. 11, 1090 (2020)
[7] C.O. Avci, et al., Nature Mater. 16, 309 (2017)
[8] L. Caretta, et al., Science 18, 1438 (2020)
[9] F. Buttner, et al., Nat. Mater. 20, 30 (2021)
[10] L. Caretta, et al., to be submitted (2020)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.01.2021 - 15:00 German Time

Chirality Induced Spin Selectivity: Open questions and challenges

Bart van Wees, University of Groningen

The phenomenon of chirality induced spin selectivity (CISS) has been known for over two decades [1]. A strong coupling between spin and charge transport has been observed, which depends on the chirality (or helicity) of the molecules. These systems are non-magnetic, so the origin of CISS must lay in manifestations of the spin-orbit interaction between moving electrons and the chirality induced electric fields present in the chiral systems.
In this talk I will give an overview of the manifestations of chirality in nature, as well as the experimental status of CISS in electronic and spintronic transport experiments. In particular I focus on experiments where CISS is observed as a spin-valve magneto resistance (MR) in two-terminal devices, where a ferromagnet is used to polarize or analyze the spins transmitted through the chiral systems. I will discuss our recent theory work, where we emphasize the important symmetry differences between the spin-charge coupling in ferromagnets, which breaks time reversal symmetry, and in (non-magnetic) chiral systems, which preserves time reversal symmetry. This prohibits the observation of CISS induced MR in two-terminal systems (but still allows it in multiterminal devices)[2]. Experiments on CISS also explore the non-linear transport regime, where voltage biases are employed which exceed the thermal energy. In this regime a CISS induced MR is possible. However, its sign depends not only on the chirality, but also on the nature of the transport through the ferromagnet and chiral system[3].
Finally I will discuss a possible connection between the absence/presence of CISS induced MR in electronic transport measurements with experiments which demonstrated magnetization/magnetic field self-assembly of chiral molecules. [4].
[1] K. Ray, S.P. Ananthavel, D.H. Waldeck, R. Naaman, Asymmetric scattering of Polarized electrons by organized organic films made of chiral molecules, Science, 283, 814 (1999).
[2] X. Yang, C.H. van der Wal, and B.J. van Wees, Spin-dependent electron transmission model for chiral molecules in mesoscopic devices, Phys. Rev. B 99, 024418 (2019)
[3] X. Yang, C. H. van der Wal, and B. J. van Wees, Detecting Chirality in Two-Terminal Electronic Nanodevices, Nano Lett. 20, 8, 6148–6154 (2020)
[4] K. Banerjee Kosh et al., Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates, Science 10 May (2018)

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 11.11.2020 - 15:00 (CET)

Old 2degs with new tricks: Antiferromagnetic order and magnetoelectricity of 2D charge carriers

Ulrich Zuelicke, Victoria University of Wellington

In magnetoelectric media, an electric field can induce a magnetization and a magnetic field can induce an electric polarization, while the system remains in thermal equilibrium.  This effect requires that both space-inversion and time-reversal symmetry are broken.  I will present a comprehensive theory for magnetoelectricity in magnetically ordered quasi-2D systems.  Considering ferromagnetic (FM) zincblende and antiferromagnetic (AFM) diamond structures, quantitative expressions for the magnetoelectric responses due to electric and magnetic fields are obtained that reveal explicitly the inherent duality of these responses required by thermodynamics.  The magnitude of magnetoelectric effects in quasi-2D systems is tunable, and typical values are sizable in quasi-2D hole systems where moderate electric fields can induce a magnetic moment of one Bohr magneton per charge carrier.  For the microscopic understanding of magnetoelectric responses in these systems,  AFM order plays a central role.  We define a Néel operator t that describes AFM order, in the same way a magnetization mreflects FM order.  While m is even under space inversion and odd under time reversal, t describes a toroidal moment that is odd under both symmetries. Thus m and t quantify complementary aspects of magnetic order in solids.  In quasi-2D systems, FM order can be attributed to dipolar equilibrium currents that give rise to a magnetization.  In the same way, AFM order arises from quadrupolar currents that generate the toroidal moment.  The electric-field-induced magnetization can then be attributed to the electric manipulation of the quadrupolar currents.  Our theory provides a broad framework for the manipulation of magnetic order by means of external fields.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 14.10.2020 - 15:00 (CET)

Current fluctuations driven by ferromagnetic and antiferromagnetic resonance

Arne Brataas, NTNU Trondheim

When spins in magnetic materials precess, they emit currents into the surrounding conductors. We will explain how dynamical magnets also induce current noise. The shot noise characterizes and detects magnetic resonance and new aspects of electron transport in magnetic nanostructures.

We generalize the description of current fluctuations driven by spin dynamics in three ways using scattering theory. First, our approach describes a general junction with any given electron scattering properties. Second, we consider antiferromagnets as well as ferromagnets. Third, we treat multiterminal devices.

We give results for various junctions, such as ballistic and disordered contacts. Finally, we discuss the experimental consequences.

PDF file of the talk available here