On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.08.2020 - 15:00 (CET)

Magnetic Matchmaking: Hybrid Magnon Modes

Axel Hoffmann, University of Illinois

Hybrid dynamic excitations have gained increased interest due to their potential impact on coherent information processing. Towards this end, magnons, the fundamental excitation quant of magnetically ordered systems, are of particular interest, since they can be easily tuned by external magnetic fields and interact with a wide range of other excitations, such as microwave and optical photons, phonons, and other magnons.1 We have explored recently the integration of permalloy (Ni80Fe20) thin film structures into hybrid magnon systems. By combining permalloy structures with high-quality superconducting microwave resonators, we demonstrated strong magnon-photon coupling in co-planar, on-chip geometry, which is readily scalable to more complex devices.2 Furthermore, we demonstrated strong coupling of permalloy magnons to standing magnon modes in yttrium iron garnet films, which revealed the importance of dampin-like torques originating from spin pumping.3 Lastly, we demonstrated how the coupling between magnons in Ni and surface acoustic waves in LiNbO3 can be used to modulate phonon propagation.4
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.

PDF file of the talk available here

References:
1. Y. Li, et al., arXiv:2006.16158.
2. Y. Li, et al., Phys. Rev. Lett. 123, 107701 (2019).
3. Y. Li, et al., Phys. Rev. Lett. 124, 117202 (2020).
4. C. Zhao, et al., Phys. Rev. Appl. 13, 054032 (2020).

On-line SPICE-SPIN+X Seminars

On-line Seminar: 12.08.2020 - 15:00 (CET)

Using magnetic tunnel junctions to compute like the brain

Mark Stiles, NIST

Computers, originally designed to do precise numerical processing, are now widely used to do more cognitive tasks. These include categorical challenges like image and voice recognition, as well as robotic tasks like driving a car and making real-time decisions based on sensory input. While the human brain does not do precise numerical processing well, it excels at these other tasks, leading researchers to look to the brain for inspiration on efficient ways to engineer cognitive computers. Of particular interest are energy and space optimization. Computers can now perform many of these cognitive tasks as well as humans, and often faster, but at the cost of much higher total energy consumption and much greater space. Some improvements are being found at the top of the computational stack from algorithms that are more brainlike, and some at the bottom from novel electronic devices that emulate features of the brain. However, the greatest progress can be found by working simultaneously across the computational stack.

Magnetic tunnel junctions have several features that make them attractive potential devices for these applications. One feature is that they are already integrated into fabrication plants for complementary-metal-oxide-semiconductor (CMOS) integrated circuits. They can be readily integrated with existing CMOS technology to take advantage of its many capabilities. Another feature is that they are multifunctional. With only slight changes in fabrication details, they can be modified to provide non-volatile memory, truly random thermal fluctuations, or gigahertz oscillations. Magnetic tunnel junctions can be used as a memory to store synaptic weights, but when the weights change too frequently the energy cost of repeatedly writing them becomes inefficient. Reducing the retention time of the memory reduces the cost of writing them, leading to a trade-off between energy efficiency and reliability. The seemingly random patterns of neural spike trains have inspired a number of computational approaches based on the random thermal fluctuations of superparamagnetic tunnel junctions. I discuss some of these approaches and the design choices we have made in implementing a neural network based on superparamagnetic tunnel junctions.

PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 29 July 2020 - 15:00 (CET)

Macroscopic magnonic quantum states

Burkard Hillebrands, Technische Universität Kaiserslautern

Finding new ways for fast and efficient processing and transfer of data is one the most challenging tasks nowadays. Elementary spin excitations ‒ magnons (spin wave quanta) ‒ open up very promising directions of high-speed and low-power information processing.
Magnons are bosons, and thus they can spontaneously form a spatially extended coherent ground state ‒ a Bose-Einstein condensate (BEC) – which can be established independently of the magnon excitation mechanism even at room temperature. An extraordinary challenge is the use of macroscopic quantum phenomena such as the magnon BEC for the information transfer and processing.
Very promising is the use of magnon supercurrents driven by a phase gradient in the magnon BEC. Imposing such a phase gradient onto the BEC’s wave function we have found using Brillouin light scattering spectroscopy that local heating by a probing laser beam leads to an excessive decay of the freely evolving magnon BEC. This is a fingerprint of the supercurrent efflux of condensed magnons. Moreover, we revealed that the condensed magnons being pushed out from the heated area form compact density humps, which propagate over long distances through the thermally homogeneous magnetic medium. We refer to them as a superposition of Bogoliubov waves with oscillations of both the amplitude and the phase of the magnon BEC’s wave function. In the long-wavelength limit, these waves have a linear dispersion law and can be considered as a magnon second sound potentially featuring viscosity-free propagation.
A further consequence of a magnon BEC is the prediction of the existence of a magnon ac Josephson effect. Recently, we discovered this effect in a room-temperature magnon BEC. The magnon condensate was prepared in a parametrically populated magnon gas around a potential trench created by a dc electric current. The appearance of the magnonic Josephson effect is manifested by oscillations of the magnon BEC density in the trench, caused by a coherent phase shift between magnon condensates from the left and right zones of the trench.

 

PDF file of the talk available here

 

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 22 July 2020 - 15:00 (CET)

Transversal transport coefficients and topological properties

Ingrid Mertig, Martin Luther University Halle-Wittenberg, Germany

Spintronics is an emerging field in which both charge and spin degrees of freedom of electrons are utilized for transport. Most of the spintronic effects—like giant and tunnel magnetoresistance—are based on spin- polarized currents which show up in magnetic materials; these are already widely used in information technology and in data storage devices.
The next generation of spintronic effects is based on spin currents which occur in metals as well as in insulators, in particular in topologically nontrivial materials. Spin currents are a response to an external stimulus—for example electric field or temperature gradient—and they are always related to the spin-orbit interaction. They offer the possibility for future low energy consumption electronics.
The talk will present a unified picture, based on topological properties, of a whole zoo of transversal transport coefficients: the trio of Hall, Nernst, and quantum Hall effects, all in their conventional, anomalous, and spin flavour. The formation of transversal charge and spin currents and their interconversion as response to longitudinal gradients is discussed.

PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 15 July 2020 - 15:00 (CET)

Crystal time-reversal symmetry breaking and spin splitting in collinear antiferromagnets

Libor Šmejkal, JGU, Mainz

Relativistic bandstructure of solids generates functionalities of modern quantum, topological and spintronics materials. Common collinear antiferromagnets exhibit Kramers spin degenerate bands and for many decades were believed to be excluded from spin splitting physics and spontaneous Hall effects. Our recent prediction of crystal time-reversal symmetry breaking by anisotropic magnetization densities (see picture) due to the collinear antiferromagnetism combined with nonmagnetic atoms changes this perspective. Unlike the conventional relativistic spin-orbit interaction induced spin splitting, our crystal antiferromagnetic spin splitting is of exchange origin, can reach giant eV values, and can preserve spin quantum number.
In this talk, we will discuss the basic properties of this new type of antiferromagnetic spin splitting, its local magnetic symmetry origin and symmetry criteria for its emergence and we will catalogue broad class of material candidates. Furthermore, we will show that this antiferromagnetic spin splitting can generate a crystal Hall effect controllable via rearrangement of nonmagnetic atoms. Finally, we will present an experimental discovery of crystal Hall effect in ruthenium dioxide antiferromagnet.

 

 

Šmejkal, L., Mokrousov, Y., Yan, B. & MacDonald, A. H. Topological antiferromagnetic spintronics. Nat. Phys. 14, 242 (2018).
Šmejkal, L., Železný, J., Sinova, J. & Jungwirth, T. Electric Control of Dirac Quasiparticles by Spin-Orbit Torque in an Antiferromagnet. Phys. Rev. Lett. 118, 106402 (2017), arXiv (2016)
Šmejkal, L., González-Hernández, R., Jungwirth, T. & Sinova, J. Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets. Sci. Adv. 6, eaaz8809 (2020), arXiv (2019)
Feng, Z.*, Zhou, X.*, Šmejkal, L.*, González-Hernández, R., Sinova, J., Jungwirth, T., Liu, Z. et al. Observation of the Crystal Hall Effect in a Collinear Antiferromagnet. arXiv (2020).

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 8 July 2020 - 15:00 (CET)

Towards deep neural networks with nanoscale spintronic oscillators as neurons

Julie Grollier, CNRS-Thales

Spintronic oscillators are nanoscale devices realized with magnetic tunnel junctions which have the potential to be integrated by hundreds of millions in electronic chips. Their non-linear dynamical properties are rich and tunable, and can be leveraged to imitate different features of biological neurons. High performance pattern recognition was achieved through the coupled dynamics of the oscillators in small circuits. The transient dynamics of a single spintronic nano-oscillator has been used to implement reservoir computing, achieving state-of-the-art results on a simple spoken digit recognition task [1], [2]. Four spintronic nano-oscillators have been trained to classify spoken vowels by phase locking their oscillations to the strong input signals produced by external microwave sources [3]. Three spintronic nano-oscillators did bind temporal data through their mutual synchronization [4].

These demonstrations now need to be scaled to deep networks to establish their potential definitely. The neocortex, the seat of higher cognitive functions in the brain, has a hierarchical structure of six layers of neurons. Adopting such a layered structure in artificial neural networks was the key to their fantastic progress in the last ten years. Neuromorphic systems need to be scalable to deep networks to truly establish their promises.

PDF file of the talk available here

A key asset of spintronic nano-oscillators towards this goal is their ability to emit radio-frequency (RF) signals. These oscillators indeed produce microwave voltages with varying amplitude and frequency in response to direct current inputs. They could therefore potentially communicate through radio-frequencies signals, allowing fully parallel operation with minimized wiring, at a speed seven orders of magnitude faster than the brain. But for this, it is necessary to devise radio-frequency synapses that can interconnect the oscillators.

In this talk, I will rapidly review recent results on neuromorphic computing with spintronic nano-oscillators. I will then describe how they can be interconnected layer-wise through RF spintronic nano-synapses, and present our recent simulation results of classification with these novel RF synapses.

[1]  J. Torrejon et al., « Neuromorphic computing with nanoscale spintronic oscillators », Nature, 547,  428‑431 (2017).
[2]   S. Tsunegi et al., « Physical reservoir computing based on spin torque oscillator with forced synchronization », Appl. Phys. Lett., 114,  164101 (2019).
[3]   M. Romera et al., « Vowel recognition with four coupled spin-torque nano-oscillators », Nature, 563, 230,(2018).
[4]   M. Romera et al., « Binding events through the mutual synchronization of spintronic nano-neurons », arXiv:2001.08044 (2020).

On-line SPICE-SPIN+X Seminars

On-line Seminar: 1 July 2020 - 15:00 (CET)

Magnons as Probes of Strongly Correlated Electron Physics

Amir Yacoby, Harvard University

Scattering experiments have revolutionized our understanding of nature. Examples include the discovery of the nucleus, crystallography, and the discovery of the double helix structure of DNA. Scattering techniques differ by the type of the particles used, the interaction these particles have with target materials and the range of wavelengths used. Here, we demonstrate a new 2-dimensional table-top scattering platform for exploring magnetic properties of materials on mesoscopic length scales. Long lived, coherent magnonic excitations are generated in a thin film of YIG and scattered off a magnetic target deposited on its surface. The scattered waves are then recorded using a scanning NV center magnetometer that allows sub-wavelength imaging and operation under conditions ranging from cryogenic to ambient environment. While most scattering platforms measure only the intensity of the scattered waves, our imaging method allows for spatial determination of both amplitude and phase of the scattered waves thereby allowing for a systematic reconstruction of the target scattering potential. Our experimental results are consistent with theoretical predictions for such a geometry and reveal several unusual features of the magnetic response of the target, including suppression near the target edges and gradient in the direction perpendicular to the direction of surface wave propagation. Our results establish magnon scattering experiments as a new platform for studying correlated many-body systems.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 24 June 2020 - 15:00 (CET)

Chiral spintronics: non collinear spin textures with application to Racetrack Memory

Stuart Parkin, Max Planck Institute of Microstructure Physics and
Martin Luther University Halle-Wittenberg, Halle

Magnetic non-collinear spin textures that have chiral structures are of great current interest.  Coupled chiral domain walls in synthetic antiferromagnetic (SAF) racetracks and compensated ferrimagnets can be moved with current at very high speeds via a novel giant exchange torque^1,2. When the antiferromagnetic coupling between two domain walls in a SAF racetrack is weakened unusual magnetization dynamics takes place due to chiral domain wall drag³. The same type of Dzyaloshinskii-Moriya (DMI) vector exchange interactions that stabilize chiral Néel domain walls in magnetic multilayers results in the formation of topological spin textures in bulk compounds. We recently discovered magnetic antiskyrmions in a tetragonal inverse Heusler compound Mn_1.4Pt_0.9Pd_0.1Sn using Lorentz transmission electron microscopy (LTEM) ⁴. The size of the anti-skyrmion can be tuned by varying the thickness of the host material allowing for sizes varying from nanometer to microns in the same material⁵.  This is due to long range magneto-dipole interactions that are important in this compound that has a complex DMI interaction with a symmetry that follows the D_2d symmetry of its crystalline structure. The same symmetry ensures that anti-skyrmions are robust to temperature and magnetic field⁶. The magnetic dipole-dipole interactions also allow for the formation of metastable “elliptical skyrmions” in this same material⁷.  These have Bloch-like boundaries that we can directly observe using LTEM. Finally, we discuss our recent discovery of novel Néel like skyrmions in a metallic compound that exist almost to room temperature and that are also tunable in size⁸.  Chiral spin textures in ferro-, ferri- and anti-ferrimagnetic materials and thin film heterostructures are of fundamental interest with potential for spintronic applications.

PDF file of the talk available here

 

References

1  Yang, S.-H., Ryu, K.-S. & Parkin, S. S. P. Domain-wall velocities of up to 750 ms⁻¹ driven by exchange-coupling torque in synthetic antiferromagnets. Nat. Nano. 10, 221-226, (2015).
2  Bläsing, R. et al. Exchange coupling torque in ferrimagnetic Co/Gd bilayer maximized near angular momentum compensation temperature. Nat. Commun. 9, 4984, (2018).
3  Yang, S.-H., Garg, C. & Parkin, S. Chiral Exchange Drag and Chirality Oscillations in Synthetic Antiferromagnets Nat. Phys. 15, 543–548, (2019).
4  Nayak, A. K. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561-566, (2017).
5  Ma, T. et al. Tunable Magnetic Antiskyrmion Size and Helical Period from Nanometers to Micrometers in a D_2d Heusler Compound. Adv. Mater., 2002043, (2020).
6  Saha, R. et al. Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn_1.4Pt_0.9Pd_0.1Sn Nat. Commun. 10, 5305, (2019).
7  Jena, J. et al. Elliptical Bloch skyrmion chiral twins in an antiskyrmion system. Nat. Commun. 11, 1115, (2020).
8  Srivastava, A. K. et al. Observation of Robust Néel Skyrmions in Metallic PtMnGa. Adv. Mater. 32, 1904327, (2020).

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].