On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.01.2022 - 15:00 German Time

Theory of magnetic interactions in real materials

Mikhail Katsnelson, Radboud University

Magnetic ordering and related phenomena are of essentially quantum and essentially many-body origin and require strong enough electron-electron interactions. Also, they are very sensitive to the details of electronic structure of specific materials. This makes a truly microscopic description of exchange interactions a challenging task. Long ago we suggested a general scheme of calculations of exchange interactions responsible for magnetism based on the “magnetic force theorem”. It was formulated originally as a method to map the spin-density functional to effective classical Heisenberg model, the exchange parameters turned out to be, in general, essentially dependent on initial magnetic configuration and not universal. However, they are directly related to the spin-wave spectrum and, thus, can be verified experimentally. This approach also lies in the base of “ab initio spin dynamics” within the density functional approach.
It is well known now that this scheme is, in general, insufficient for strongly correlated systems and should be combined with the mapping to the multiband Hubbard model and use of, say, dynamical mean-field theory (DMFT) to treat the latter. Our original approach can be reformulated within the DMFT.
The method can be also modified to calculate Dzialoshinskii-Moriya interactions which play a crucial role in the phenomenon of weak ferromagnetism, in physics of magnetic skyrmions, and in magnonics/spintronics in general.
I will discuss both general methods and their applications to electronic structure and magnetism of various groups of magnetic materials including elemental transition and rare-earth metals, half-metallic ferromagnets, transition metal oxides, molecular magnets, sp electron magnets based on adatoms on Si and SiC surfaces, and novel two-dimensional magnets.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.02.2022 - 15:00 German Time

Spins, Bits, and Flips: Essentials for High-Density Magnetic Random-Access Memory

Tiffany Santos, Western Digital Corporation, USA

The magnetic tunnel junction (MTJ), a device comprised of two ferromagnetic electrodes with a thin (about 1 nm) insulating tunnel barrier in between, was first proposed in a Ph.D. thesis by Michel Jullière in 1975 [1], and reached widespread commercialization nearly 30 years later as the read sensor in hard disk drives. MTJs became essential for data storage in consumer laptop and desktop computers, early-generation iPods, and now in data centers that store the information in “the Cloud.” The application of MTJs has expanded even further, becoming the storage element in non-volatile memory, first in toggle magnetic random-access memory (MRAM) used in automotive applications and outer space, and now in the production of spin-transfer torque MRAM as a replacement for embedded Flash memory. As computing capabilities advance and drive demand for high performance memory, innovation in MTJ continues in order to deliver faster, high-density MRAM that can support last-level cache, in-memory computing, and artificial intelligence.
In this talk, I will describe the seminal discoveries [2] that enabled MTJs for pervasive use in hard disk drives, MRAM, and magnetic sensors, such as the discovery of tunnel magnetoresistance (TMR) at room temperature, the invention of spin transfer torque as the means to flip magnetization without a magnetic field, and the prediction and realization of high TMR using MgO tunnel barriers. As the demand for faster and higher density memory persists, still more breakthroughs are needed for MTJs contained in device pillars (or bits) just tens of nanometers in diameter. These advances require tuning of the materials properties at the atomic scale as well as across arrays of millions of bits in a memory chip. I will describe the magnetic properties of MTJs that are essential for high performance MRAM, including perpendicular magnetic anisotropy, damping parameter, exchange constant, thermal stability factor, and TMR, and how to engineer these properties to deliver high spin-transfer torque efficiency and high data retention in spin-transfer torque MRAM devices [3],[4].

[1] M. Jullière, Ph.D. thesis, Rennes University, No. B368/217, Rennes, France, 1975; M. Jullière, “Tunneling between ferromagnetic films,” Phys. Lett. A, vol. 54, pp. 225-226, September 1975.
[2] J. S. Moodera, G.-X. Miao, and T. S. Santos, “Frontiers in spin-polarized tunneling,” Physics Today, vol. 63, pp. 46-51, April 2010.
[3] T. S. Santos, G. Mihajlović, N. Smith, J.-L. Li, M. Carey, J. A. Katine, and B. D. Terris, “Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM,” J. Appl. Phys. vol. 128, 113904, September 2020.
[4] G. Mihajlović, N. Smith, T. Santos, J. Li, B. D. Terris, and J. A. Katine, “Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers,” Appl. Phys. Lett., vol. 117, 242404, December 2020.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.12.2021 - 15:00 German Time

X-ray magnetization movies: Spin dynamics in reality

Gisela Schütz, Max Planck Institute for Intelligent Systems

The field of spintronics and magnonics has become a flourishing synonym of future low-power, ultra-fast and persistent advanced information technologies with fantastic promises. However, the relevant magnetization dynamics with spatial and temporal dimensions in the sub µm and sub ns range are hard to access experimentally. An effective (and maybe the only) magnetic imaging technique elucidating the material-related difficulties and principle limits is provided by time-resolved X-ray microscopy. We explain the physical and technical basics of this method, the potentials and difficulties and discuss several magnetization movies.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 01.12.2021 - 15:00 German Time

Spin-transport Mediated Single-shot All-optical Magnetization Switching of Metallic Films

Stéphane Mangin, CNRS

During the last decade all-optical ultrafast magnetization switching in magnetic material thin film without the assistance of an applied external magnetic field has been explored [1,2]. It has been shown that femto-second light pulses can induce magnetization reversal in a large variety of magnetic materials [3,4]. However, so far, only certain particular ferrimagnetic thin films exhibit magnetization switching via a single femto-second optical pulse. All optical helicity dependent switching of a ferromagnetic layer could be demonstrated for a low number of pulses [5]. We will present the single-pulse switching of various magnetic material (ferrimagnetic, ferromagnetic) within a magnetic spin-valve structure and further show that the four possible magnetic configurations of the spin valve can be accessed using a sequence of single femto-second light pulses. Our experimental study reveals that the magnetization states are determined by spin-polarized currents generated by the light pulse interactions with the GdFeCo layer [6]. A detail study showing how spin-polarized currents are generated and how they interact with a Ferromagnetic (FM) layer can lead to magnetization switching will be presented [7,8]. Finally, magnetization dynamics measurement show that the reversal of the FM layer happens in less than one picosecond which an be modelled[9].

[1] C. D. Stanciu, et al Phys. Rev. Lett. 2007, 99, 047601
[2] I. Radu et al, Nature 2011, 472, 207
[3] S. Mangin, et al, Nat. Mater. 2014, 13, 286
[4] C. -H. Lambert, et al Science 2014, 345, 1337
[5] G. Kichin, et al Phys. Rev. App. 12 (2), 024019 2019
[6] S. Iihama et al Adv Matter 1804004 2018
[7] Q. Remy, et al Adv. Sci. 2001996 2020
[8] J. Igarashi, et al Nano. Lett. 20, 12, 8654–8660 2020
[9] Q. Remy, et al to be published

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.11.2021 - 15:00 German Time

Antiferromagnetic Skyrmionics: generating and controlling topological textures

Hariom Jani, National University of Singapore

Whirling magnetic textures such as skyrmions, bimerons and their anti-particles could emerge as topologically-protected information bits for next-generation memory and logic. However, their practical exploitation has been inhibited by susceptibility to stray magnetic fields, strong internal dipolar fields, slow speeds or sideway motion. To alleviate these issues, there has been a surge of interest in antiferromagnetic (AFM) analogues, predicted to be robust, scalable and ultra-fast1,2. Even so, experimental progress in this field has been curtailed by magnetic compensation in AFM systems, which makes it difficult to visualize and control AFM textures via standard magnetic techniques.
To this effect, I will firstly discuss a recently-developed AFM vector-mapping technique3,4 exploiting angle-dependent dichroism to image spatial variations of the AFM order. Then, I will present a general field-free approach, employing the Kibble–Zurek transition, that we used to reversibly create a wide multichiral family of topological AFM textures, including exotic merons or antimerons and bimerons5. In the earth-abundant oxide (α-Fe2O3) these nanoscale textures can be nucleated and stabilized at room temperature. Particularly, the presence of widely tunable anisotropy6 and exchange4,5,7 interactions in this system enable unprecedented reversible control over the dimensions and orientation of AFM textures. I will then present how we may realize the hitherto undiscovered AFM skyrmions in α-Fe2O3, by introducing new symmetry breaking interactions8. Lastly, I will outline the path ahead for AFM skyrmionics, discussing how our results may be translatable to a broad class of AFM systems – including orthoferrites, orthochromites and layered-ferrates6,9, and sharing how electrical pathways can be exploited to control members of the topological family10-12.

[1] J Barker et al., Physical Review Letters 116, 147203 (2016)
[2] V Baltz et al., Review of Modern Physics 90, 015005 (2018)
[3] NW Price et al., Physical Review Letters 117, 177601 (2016)
[4] FP Chmiel et al., Nature Materials 17, 581 (2018)
[5] H Jani et al., Nature 590, 74 (2021)
[6] H Jani et al., Nature Communications 12, 1668 (2021)
[7] PG Radaelli et al., Physical Review B 101, 144420 (2020)
[8] J Harrison et al., arXiv:2111.15520 (2021)
[9] ZS Lim et al., MRS Bulletin (In Press, 2021), arXiv:2111.10562
[10] L Baldrati et al., Physical Review Letters 123, 177201 (2019)
[11] P Zhang et al., Physical Review Letters 123, 247206 (2019)
[12] Y Cheng et al., Physical Review Letters 124, 027202 (2020)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 10.11.2021 - 15:00 (CET)

Altermagnetism: spin-momentum locked phase protected by non-relativistic symmetries

Tomas Jungwirth, Institute of Physics of the Science Academy of the Czech Republic

 

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 17.11.2021 - 15:00 German Time

Magnetism and spin dynamics control by carrier doping in van der Waals magnet Cr2Ge2Te6

Hidekazu Kurebayashi, University College London

Two-dimensional (2D) van der Waals (vdW) materials have been intensively and extensively studied in the last two decades. A magnetic version of vdW systems has only gained attention since 2017 where a few mono-layers of exfoliated magnetic vdW ones were reported to sustain magnetism [1.2]. Since then, scientists started to seriously explore the physics and materials science of this new class of materials by applying their own research ideas and growth/measurement techniques. These material groups are ideal, for example, in studying magnetism and spin transport at the truly 2D limit, and in exploring how these materials can be responded by external stimuli such as current-induced torques and electric field. These experiments will also be enriched by an unlimited combination of their heterostructures that can be fabricated without significant lattice-matching constraints present in typical thin-film sample-growth techniques such as MBE and sputtering. Furthermore, inherent low symmetry nature of vdW materials will offer a wealth of spin-orbit Hamiltonians that are the backbone of current-induced magnetization switching research and future technologies [3].
We started to work on one of magnetic 2D vdW materials, Cr2Ge2Te6 (CGT), to study its spin dynamics and how to control the magnetism by any external stimuli. In this presentation, I will start with a brief introduction of magnetic 2D vdW materials and then move on to our latest work of controlling magnetism (Curie temperatures and magnetic anisotropies) in CGT by electric field [4] and chemical doping. Both doping techniques show the change of carrier density in CGT by orders of magnitude (from insulator to metallic). As a result, the exchange coupling strength has been greatly enhanced, leading to Curie temperature enhancement. The carrier doping also modifies the spin-orbit interaction within CGT which is measured by a significant change of the magnetic anisotropy parameters. These have been characterized by magneto-transport as well as spin dynamics techniques [5]. Furthermore, if time permits, I will also briefly show our photon-magnon coupling in CGT and on-chip resonator systems.

[1] Gong et al. Nature 546 265 (2017).
[2] Huang et al., Nature 546, 270 (2017).
[3] H. Kurebayashi et al., arXiv:2107.03763; Nat. Rev. Phys. (in-press).
[4] Verzhbitskiy et al., Nature Electron. 3, 460 (2020).
[5] For example, for undoped CGT, Khan et al., Phys. Rev. B 100, 134437 (2019);
arXiv:1903.00584.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.11.2021 - 15:00 German Time

Analytic and ab initio theory of magnetization dynamics

Peter Oppeneer, Uppsala University

The Landau-Lifshitz-Gilbert (LLG) equation forms a cornerstone of contemporary magnetism research, yet it was originally proposed on the basis of phenomenological considerations. To put the equation on a fundamental footing, we start from the relativistic Dirac-Kohn-Sham equation, consider the motion of spin angular momentum in an external electromagnetic field and show that it leads to the LLG equation with anisotropic damping, as well as to additional terms, such as the field-derivative torque, the optical spin-orbit torque, spin-transfer torque, and inertial term [1,2]. Besides providing a foundational basis for the LLG equation, our analytic theory predicts new effects that could be observed in experiments.
Electric field or current induced spin-orbit torques (SOTs) arising from the spin Hall effect or Rashba-Edelstein effect (REE) have recently emerged as promising tools to achieve efficient magnetization dynamics [3]. To explore the origin of SOTs on a materials’ specific level, we employ density functional and linear-response theory to calculated ab initio the electric field induced magnetic polarizations. For the noncentrosymmetric antiferromagnets CuMnAs and Mn2Au we compute the induced polarizations and find that there exists dominantly an orbital Rashba-Edelstein effect that is much larger than the spin REE and does not require spin-orbit coupling to exist [4]. The staggered, field-induced orbital polarization moreover exhibits Rashba-type symmetry in contrast to the induced spin polarization.
Considering typical bilayer systems consisting of Pt and 2 monolayers of a 3d element (Co, Ni, Cu) we compute in a layer-resolved manner the spin and orbital conductivities and spin and orbital moment accumulations. We identify the contributions that lead to the fieldlike SOT and the dampinglike SOT, which are mainly the spin REE and magnetic spin Hall effect, respectively. The current-induced orbital accumulation transverse to the electric field is always much larger than the corresponding spin accumulation and exist without spin-orbit interaction [5]. This exemplifies that the induced orbital polarization is the primary response to the electric field and suggests the possibility of utilizing large orbital effects in light-metal devices.

[1] R. Mondal, M. Berritta, A.K. Nandy, and P.M. Oppeneer, Phys. Rev. B 96, 024425 (2017).
[2] R. Mondal, M. Berritta, and P.M. Oppeneer, Phys. Rev. B 94, 144419 (2016); Phys. Rev. B 98, 214429 (2018).
[3] A. Manchon, J. Železný, I.M. Miron, T. Jungwirth, J. Sinova, A. Thiaville, K. Garello, and P. Gambardella, Rev. Mod. Phys. 91, 035004 (2019).
[4] L. Salemi, M. Berritta, A.K. Nandy and P.M. Oppeneer, Nature Commun. 10, 5381 (2019).
[5] L. Salemi, M. Berritta, and P.M. Oppeneer, Phys. Rev. Mater. 5, 074407 (2021).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.12.2021 - 15:00 German Time

Femto-magnetism meets spintronics: Towards integrated magneto-photonics

Bert Koopmans, Eindhoven University of Technology

Novel schemes for optically controlling ferromagnetic order at a femtosecond time scale [1] receive great scientific interest. In the strongly non-equilibrium regime, it has become possible not only to quench magnetic order, but even to deterministically switch the magnetic state by a single femtosecond laser pulses. Moreover, it has been shown that pulsed laser excitation can induce spin currents over several to tens of nanometers. This development triggered a merge of the fields of ‘femto-magnetism’ and spintronics – opening up a fascinating playground for novel physical phenomena. In this lecture I will discuss the underlying principles, but also envision their exploitation in THz magnonics and integrated spintronic-photonic memories.

After a brief review of the field, mechanisms for ultrafast loss of magnetic order upon fs laser heating [2] as well as all-optical switching will be explained. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer [3]. These fs spin currents are absorbed within a few nanometers, providing ideal conditions for exciting and exploring THz spin waves [4]. Finally, it will be argued that synthetic, layered ferrimagnets provide an ideal platform for combining fs optical control with advanced spintronic functionality. It will be shown how magnetic bits can be written ‘on-the-fly’ by fs laser pulses in a so-called magnetic racetrack, where they are immediately transported by a dc current [5]. Such schemes may lead to a novel class of integrated photonics, in which information is transferred back and forth between the photonic and magnetic domain without any intermediate electronic steps.

[1] E.E. Fullerton, H.A. Dürr, A.V. Kimel and B. Koopmans, Chapter VI “Interfacial effects in ultrafast magnetization dynamics”, in F. Hellman, et al., “Interface-induced phenomena in magnetism”, Rev. Mod. Phys. 89, 025006 (2017).
[2] B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Faehnle, T. Roth, M. Cinchetti, and M. Aeschlimann, “Explaining the paradoxical diversity of ultrafast laser-induced demagnetization”, Nature Materials 9, 259 (2010).
[3] A.J. Schellekens, K.C. Kuiper, R.R.J.C. de Wit, and B. Koopmans, “Ultrafast spin-transfer torque driven by femtosecond pulsed-laser excitation”, Nat. Commun. 5, 4333 (2014).
[4] M. L. M. Lalieu, R. Lavrijsen, R. A. Duine, and B. Koopmans, “Investigating optically excited terahertz standing spin waves using noncollinear magnetic bilayers”, Phys. Rev. B 99, 184439 (2019).
[5] M.L.M. Lalieu, R. Lavrijsen, and B. Koopmans, “Integrating all-optical switching with spintronics”, Nat. Commun. 10, 1038 (2019).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 20.10.2021 - 15:00 German Time

Archimedean screw and time quasi-crystals in driven chiral magnets

Achim Rosch, Institute for Theoretical Physics, University of Cologne

The Archimedean screw is one of the oldest machines of mankind. We show theoretically [1] how one can realize and drive such an Archimedean screw
in chiral magnets, where helical spin textures are realized. A small oscillating magnetic field at GHz frequencies induces a net rotation and screw-like motion of the magnetic texture. This effect arises from the coupling of the oscillating field to the Goldstone mode of the system. The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a large voltage in metallic systems. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing the oscillating field forming a `time quasicrystal' which oscillates in space and time for moderately strong drive.

[1] Nina del Ser, Lukas Heinen, Achim Rosch, SciPost Phys. 11, 009 (2021).

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