On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.03.2022 - 15:00 German Time

Ferrimagnetic Spintronics

Kyung-Jin Lee, KAIST

Compensated ferrimagnets combine the best features of antiferromagnets and ferromagnets [1]. Antiferromagnets are of considerable interest because the exchange torques between the two sublattices give a time scale that is much faster than that in ferromagnets and the lack of magnetization and net angular momentum lead to minimal perturbation by stray fields and eased constraints due to angular momentum conservation. A compensated ferrimagnet has all these virtues. At the same time, the lack of symmetry between the two sublattices in a compensated ferrimagnet means that quantities like average spin currents are not zero making the systems potentially easier to manipulate and detect the consequences. We will describe calculations and measurements of domain wall and skyrmion motion at the angular momentum compensation point. At this point with no net spin density, the rotational motion of the magnetic textures (domain walls and skyrmions) is absent. As a result, domain walls move fast [2,3], the skyrmion Hall effect vanishes [4], the magnon-photon coupling enhances [5], and a relativistic domain wall motion is realized [2,6]. We will also discuss the increased efficiency of spin torques due to the weakened dephasing in compensated ferrimagnets. Combining experiments with theoretical studies, Refs. [7] and [8] show large torques for ferrimagnetic multilayers and for ferrimagnetic domain walls, respectively.

[1] S. K. Kim, G. S. D. Beach, K.-J. Lee, T. Ono, Th. Rasing, and H. Yang, Ferrimagnetic spintronics, Nat. Mater. 21, 24 (2022).
[2] T. Shiino et al., Antiferromagnetic domain wall motion driven by spin-orbit torques. Phys. Rev. Lett. 117, 087203 (2016).
[3] K.-J. Kim et al., Fast domain wall motion in the vicinity of the angular momentum compensation temperature of ferrimagnets. Nat. Mater. 16, 1187 (2017).
[4] Y. Hirata et al., Vanishing skyrmion Hall effect at the angular momentum compensation temperature of a ferrimagnet. Nat. Nanotechnol. 14, 232 (2019).
[5] J. Shim, S.-J. Kim, S. K. Kim, and K.-J. Lee, Enhanced magnon-photon coupling at the angular momentum compensation point of ferrimagnets. Phys. Rev. Lett. 125, 027205 (2020).
[6] L. Caretta et al., Relativistic kinematics of a magnetic soliton. Science 370, 1438 (2020).
[7] J. Yu et al., Long spin coherence length and bulk-like spin-orbit torque in ferromagnetic multilayers. Nat. Mater. 18, 29 (2019).
[8] T. Okuno et al., Spin-transfer torques for domain wall motion in antiferromagnetically coupled ferrimagnets. Nat. Electron. 2, 372 (2019).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.03.2022 - 15:00 German Time

Towards coupling coherent femtosecond charge and spin dynamics in antiferromagnets

Davide Bossini, University of Konstanz

The pressing demand to develop novel information technology schemes has bolstered the research area of spintronics. In particular, the colossal amount of data to process and transfer, especially in the cloud, have pushed researchers to explore concepts for an ever faster and less energy-demanding method to record and handle information. A possible approach consists in employing femtosecond laser pulses to optically generate spin dynamics in magnetic materials. One of the possible outcomes of the light-spin interaction on the sub-picosecond time-scale is the generation of coherent magnons. This process has been demonstrated [1-2] even in the absence of absorption of light by the lattice [3]. Additionally both low- and high-energy magnons can be induced and coherently manipulated [4-5]. In my talk I will introduce first some aspects of the all-optical generation of coherent magnons on the ultrashort time-scale, discussing several possible mechanisms. Furthermore I will focus on the possibility to drive an hybrid electronic-magnonic transition, called exciton-magnon, which allows to arbitrarily amplify coherent THz spin oscillations. In addition, exploring this concept in a multi-domain antiferromagnet, an unprecedented coupling between different magnon modes was reported, as the ultrafast spin dynamics enters the nonlinear regime. This effect was proved both experimentally and theoretically to be due to the domain walls, which are effectively able to couple different magnon modes in different domains [6-7]. While this discovery enables perspectives of coherent transfer of energy on the picosecond time- and (sub)-micrometer length-scale, the implications for the electronic system are left open to investigate.

[1] Nature 435, 655 (2005)
[2] Physical Review Letters 105, 077402 (2010)
[3] Physical Review B 89, 060405(R) (2014)
[4] Nature Communications 7, 10645 (2016)
[5] Physical Review B 100, 024428 (2019)
[6] J. Phys. D: Appl. Phys. 54, 374004 (2021)
[7] Physical Review Letters 127, 077202 (2021)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.02.2022 - 15:00 German Time

Magneto-Acoustic Waves in Magnetic Thin Films

Mathias Weiler, TU Kaiserslautern

Spin waves form the basis for the field of magnonics, where they are used for information transport and processing [1]. Surface acoustic waves (SAWs) are widely employed as frequency filters in mobile communication technology. SAWs have group velocities comparable to that of spin waves and consequently can be generated with magnon-compatible wavelengths and frequencies. In magnetic media, spin waves can interact with SAWs which defines the field of magnetoacoustics. Magneto-acoustic phenomena can be used to excite and detect magnetization dynamics acoustically and control SAW propagation magnetically. Because of the ellipticity of the magneto-acoustic driving fields, as well as the spin-wave non-reciprocity due to dipolar coupling and the Dzyaloshinskii-Moriya interactions [2,3], magneto-acoustic waves are thereby generally chiral and non-reciprocal.
I will introduce the fundamentals of magnetoacoustics and then discuss the symmetry and non-reciprocity of magneto-acoustic waves in magnetically ordered thin films and heterostructures [4-6]. We quantitatively model the SAW-spin wave interaction based on the Kalinikos-Slavin equation and spin wave excitation by elliptically polarized coherent phonons to reveal that the magnon-phonon coupling is driven not only by magneto-elastic interactions [7] but also by magneto-rotation [4,8]. Our experiments furthermore demonstrate that SAW based spin-wave spectroscopy provides a sensitive measurement of the spin-wave dispersion and the Dzyaloshinskii-Moriya interaction. Non-reciprocal magneto-acoustic waves may be useful for the implementation of miniaturized on-chip microwave isolators.

[1] A. V. Chumak et al., Nat. Phys. 11, 453 (2015).
[2] J.-H. Moon et al., Phys. Rev. B 88, (2013).
[3] H. T. Nembach et al., Nat. Phys. 11, 825 (2015).
[4] M. Küß et al., Phys. Rev. Lett., 125, 217203 (2020).
[5] M. Küß et al., Phys. Rev. Applied, 15, 034060 (2021).
[6] M. Küß et al., Phys. Rev. Applied, 15, 034046 (2021).
[7] M. Weiler et al., Phys. Rev. Lett. 106, 117601 (2011).
[8] M. Xu et al., Sci. Adv. 6, eabb1724 (2020).

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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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