On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.01.2022 - 15:00 German Time

Iron garnet thin films for spintronic and photonic devices

Caroline A. Ross, Massachusetts Institute of Technology

Ferromagnetic insulator thin films provide unique functionality in spintronic, magnonic, and magnetooptical devices. Yttrium iron garnet has very low magnetic damping, and substitution of rare earth ions as well as the introduction of point defects such as antisite defects allows the anisotropy, magnetostriction, compensation temperature and optical properties to be tuned. We use pulsed laser deposition to produce single crystal films of rare earth garnets down to a thickness of 2.5 nm, about 2 unit cells. We show intriguing magnetic behavior in garnet/heavy metal bilayers including spin orbit torque-driven domain wall motion at room temperature at velocities exceeding 4 km/s, switching the magnetic state. Iron garnets also exhibit magnetooptical activity and high transparency in the infrared, and we show how garnets grown on silicon can be used in integrated magnetooptical isolators to control the flow of light in photonic circuits.

References: Nature Commun. 11 1090 (2020), Nature Nanotech. 14 561 (2019), Optica 6 473 (2019), ACS Photonics 5, 5010 (2018), Phys. Rev. Mater. 2, 094405 (2018), Nature Materials 16, 309–314 (2017), Adv. Electron. Mater. 3 1600376 (2017)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.02.2022 - 15:00 German Time

Three dimensional spintronics: “Faster, higher, stronger”

Amalio Fernández-Pacheco, CSIC-University of Zaragoza

The expansion of spintronics to three dimensions provides exciting opportunities to explore new physical phenomena, opening great prospects to create 3D magnetic devices for future technologies [1]. To get full access to the rich phenomenology predicted to emerge when moving to 3D, we have developed a new framework for the “3D nano-printing” of materials using focused electron beam induced deposition [2], which enables the fabrication of complex-shaped 3D magnetic structures with sub-100nm resolution. Making use of this tool, in combination with advanced magneto-optical and X-ray magnetic microscopy methods, we are studying the controlled motion of domain walls along the whole space in 3D magnetic interconnectors, either via external fields [3] or geometrical effects [4]. We are also studying the magnetoelectrical signals generated in these devices, where the non-collinear configuration of magnetic states and electrical currents results in deviations from standard angular dependences normally obtained in planar devices [5]. I will also present our recent work on chiral effects in 3D helical geometries formed by interlaced nanowires, where exchange and dipolar interactions are balanced to result in a very rich phenomenology. The freedom provided to control magnetic effects in this type of geometries has been exploited to form chiral interfaces between domain walls of opposite chirality, allowing us to imprint topological spin defects at localized regions [6]. Furthermore, helical structures may also form strongly coupled domain wall pairs, which result in complex stray magnetic field configurations with topological features [7].

[1] A. Fernández-Pacheco et al, Nature Comm. 8, 1 (2017)
[2] L. Skoric, Nano Letters 20, 184 (2020)
[3] D. Sanz-Hernández et al, ACS Nano 11, 11066 (2017)
[4] L. Skoric et al, arXiv:2110.04636
[5] F. Meng et al, ACS Nano 15, 6765 (2021)
[6] D. Sanz-Hernández et al, ACS Nano 14, 8084 (2020)
[7] C. Donnelly et al, Nature Nanotechnol. (2021). https://doi.org/10.1038/s41565-021-01027-7

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.02.2022 - 15:00 German Time

Detecting, imprinting and switching spin chirality in magnetic materials

Yuriy Mokrousov, JGU

Among magnetic materials, those which exhibit chiral non-collinear spin ordering are intensively explored these days from the viewpoint of basic properties and diverse applications. In this context, an ability to read out the exact chiral state from basic transport measurements is of great importance, since it allows for an educated design of spintronics devices based on spin chiral effects. In my talk I will attempt to outline a way which can be used to categorize various chiral contributions to charge and spin currents arising in non-collinear magnets. I will show that this gives an important ability to track the overall features and exact details of spin distribution in various classes of magnetic materials ranging from canted antiferromagnets [1] to smooth magnetization textures [2,3]. Moreover, I will demonstrate that chiral charge and spin currents are intrinsically related to the effect of spin-orbit torque in chiral spin systems, and they play a pivotal role for enabling chirality switching. Finally, I will show that chiral functionality can be activated even in intrinsically non-chiral materials either by thermal fluctuations or controlled optical pulses [3]. While the former type of incoherent chirality can give rise to unexpected manifestations in transport and magnetization dynamics, the optical control of chirality can be key to our ability to engineer chiral states and chiral dynamics in complex magnets.

[1] Kipp et al., Comm. Phys. 4, 99 (2021); Bac, Lux et al., arXiv:2103.15801
[2] Lux et al., Phys. Rev. Lett. 124, 096602 (2020)
[3] Kipp et al., Phys. Rev. Res. 3, 043155 (2021)
[4] Ghosh et al., arXiv:2011.01670; Zhang et al., Comm. Phys. 3, 227 (2020)

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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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