On-line SPICE-SPIN+X Seminars

On-line Seminar: 17 June 2020 - 15:00 (CET)

Coherent Sub-Terahertz Spin Pumping from an Insulating Antiferromagnet

Enrique del Barco, University of Central Florida

Emerging phenomena, such as the spin-Hall effect (SHE), spin pumping, and spin-transfer torque (STT), allow for interconversion between charge and spin currents and the generation of magnetization dynamics that could potentially lead to faster, denser, and more energy efficient, non-volatile memory and logic devices. Present STT-based devices rely on ferromagnetic (FM) materials as their active constituents. However, the flexibility offered by the intrinsic net magnetization and anisotropy for detecting and manipulating the magnetic state of ferromagnets also translates into limitations in terms of density (neighboring elements can couple through stray fields), speed (frequencies are limited to the GHz range), and frequency tunability (external magnetic fields needed). A new direction in the field of spintronics is to employ antiferromagnetic (AF) materials. In contrast to ferromagnets, where magnetic anisotropy dominates spin dynamics, in antiferromagnets spin dynamics are governed by the interatomic exchange interaction energies, which are orders of magnitude larger than the magnetic anisotropy energy, leading to the potential for ultrafast information processing and communication in the THz frequency range, with broadband frequency tunability without the need of external magnetic fields.

PDF file of the talk available here

I will present evidence of sub-terahertz coherent spin pumping at the interface of a uniaxial insulating antiferromagnet MnF2 and platinum thin films, measured by the ISHE voltage signal arising from spin-charge conversion in the platinum layer. The ISHE signal depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized sub-THz irradiation (see figure). Contrary to the case of ferromagnets, antiferromagnetic spin pumping exhibits a sign dependence on the chirality of dynamical modes, allowing for the unambiguous distinction between coherent spin pumping and the thermally-driven, chirality-independent spin Seebeck effect. Our results open the door to the controlled generation of coherent pure spin currents with antiferromagnets at unprecedented high frequencies.

This work has been primarily supported by the Air Force Office of Scientific Research under Grant FA9550-19-1-0307.

References

• Priyanka Vaidya, Sophie A. Morley, Johan van Tol, Yan Liu, Ran Cheng, Arne Brataas, David Lederman, and Enrique del Barco, Subterahertz spin pumping from an insulating antiferromagnet, Science 368, 160-165 (2020)

On-line SPICE-SPIN+X Seminars

On-line Seminar: 10 June 2020 - 15:00 (CET)

Magnetic tunnel junctions and magnetic logic circuits driven by spin-orbit torques

Pietro Gambardella,

Department of Materials, ETH Zurich, Switzerland

Current-induced spin-orbit torques (SOTs) enable the switching of magnetic tunnel junctions (MTJs) in nonvolatile magnetic random access memories as well as the all-electrical operation of magnetic logic circuits based on domain wall manipulation. In this talk, I will present time-resolved measurements of magnetization reversal driven by SOTs in 3-terminal MTJ devices and discuss how the combination of SOT, spin transfer torque, and voltage control of magnetic anisotropy leads to reproducible sub-ns magnetization reversal with a very narrow spread of the switching time distributions [1]. Further, I will show how SOTs and the chiral coupling between neighbouring magnetic domains induced by the interfacial Dzyaloshinskii–Moriya interaction [2] allow for realizing an electrically-driven domain-wall inverter. Starting from this basic building block, it is possible to fabricate reconfigurable NAND and NOR logic gates, and therefore a complete family of logic gates, which perform operations with current-induced domain-wall motion [3]. Opportunities for scalable all-electric magnetic memories and memory-in-logic applications will be discussed.

PDF file of the talk available here

References

• [1] Single-shot dynamics of spin-orbit torque and spin transfer torque switching in 3-terminal magnetic tunnel junctions, E. Grimaldi, V. Krizakova, G. Sala, F. Yasin, S. Couet, G. S. Kar, K. Garello and P Gambardella, Nat. Nanotech. 15, 111 (2020).
• [2] Chirally Coupled Nanomagnets, Z. Luo, T. Phuong Dao, A. Hrabec, J. Vijayakumar, A. Kleibert, M. Baumgartner, E. Kirk, J. Cui, T. Savchenko, G. Krishnaswamy, L. J. Heyderman, and P. Gambardella, Science 363, 1435 (2019).
• [3] Current-driven magnetic domain-wall logic, Z. Luo, A. Hrabec, T. P. Dao, G. Sala, S. Finizio, J. Feng, S. Mayr, J. Raabe, P. Gambardella, L. J. Heyderman, Nature 579, 214 (2020).

Figure 1. a, Scanning electron microscope image of a 3-terminal magnetic tunnel junction device with injection electrodes for spin-orbit torque (SOT) and spin transfer torque (STT) switching. b, Detail of the magnetic tunnel junction pillar and W current line. c, Electrical setup for the time-resolved measurements of the tunneling magnetoresistance during SOT and/or STT switching. d, Examples of ten different single-shot switching events induced by spin-orbit torques. Adapted from [1].

On-line SPICE-SPIN+X Seminars

On-line Seminar: 3 June 2020 - 15:00 (CET)

Probing ultrafast spin transport with terahertz electromagnetic pulses

Tobias Kampfrath, Freie Unversität Berlin and Fritz Haber Institute of the Max Planck Society, Berlin

Transport of spins is often driven by heat gradients and electric fields. To probe the initial elementary steps which lead to the formation of spin currents, we need to launch and measure transport on femtosecond time scales. This goal is achieved by employing both ultrashort optical and terahertz electromagnetic pulses. We illustrate our experimental approach by several examples including the spin Seebeck effect (see figure) and anisotropic magnetoresistance.

References

• [1] Seifert et al., Nature Comm. 9, Article number: 2899 (2018)
• [2] Seifert et al., J. Phys. D: Appl. Phys. 51, 364003 (2018)

PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 27.05.2020 - 15:00 (CET)

Half-integer charge transfer by Majorana edge modes

Carlo Beenakker, Leiden University

A Josephson junction in a chiral p-wave superconductor can inject a charge e/2 into a normal-metal contact, carried by chiral Majorana edge modes. We address the question whether this half-integer charge is a sharp observable, without quantum fluctuations. Because the Majorana modes are gapless, they support charge fluctuations in equilibrium at zero temperature. But we find that the excess noise introduced out of equilibrium by the e/2 charge transfer vanishes. We discuss a strategy to reduce the equilibrium fluctuations, by means of a heavy-tailed time-dependent detection efficiency, to achieve a fully noiseless half-integer charge transfer.

PDF file of the talk available here

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Program - Elastic Tuning and Response of Electronic Order