Oral Presentations Magnonics 2017

Magdalen College, Oxford August 7-10, 2017

It is a tremendous pleasure to welcome you to the 2017 Magnonics Conference. On behalf of the Local Committee, I would like to thank all those who have contributed so much time, energy, and scientiﬁc insight to the organization of the meeting and wish everyone stimulating and collegial stay here at Magdalen College.

Alexy Karenowska Magdalen College and Department of Physics, University of Oxford.

1 Organising Committee

Dr Alexy Karenowska (University of Oxford, UK), Co-chair Dr Hidekazu Kurebayashi (University College London, UK), Co-chair

Prof. Johan Akerman˚ (University of Gothenburg, Sweden) Prof. Christian Back (Universitt Regensburg, Germany) Dr Andrii Chumak (Technische UniversitätKaiserslautern, Germany) Prof. Sergej Demokritov (UniversitätMünster,Germany) Prof. Julie Grollier (CNRS/Thales, France) Prof. Ilya Krivorotov (University of California, Irvine, USA) Prof. Eiji Saitoh (IMR Tohuku University, Japan) Prof. Andrei Slavin (Oakland University, USA)

2 Oral Presentations

1 Monday, Session I: Opto-magnonics 5 1.1 Time-resolved imaging of photo-induced spin wave tunneling through an air gap (invited)...... 5 1.2 Exciting THz spin waves using femtosecond optical spin-transfer-torque . . . . . 6 1.3 Spin-wave tomography ...... 7

2 Monday, Session II: Spin-wave states and transport 8 2.1 Topological excitations in thin ﬁlm ferromagnets and artiﬁcial spin ices (invited) 8 2.2 Nano-scaled magnon transistor based on three-magnon splitting ...... 9 2.3 Spin dynamics in antiferromagnets and ferrimagnets (invited) ...... 10 2.4 Antiferromagnetic domain wall as a spin wave polarizer and retarder ...... 11

3 Tuesday, Session III: Spin-orbit torques 12 3.1 Excitation and ampliﬁcation of coherent spin waves by spin currents (invited) . 12 3.2 Optical detection of magnetization dynamics induced by spin-orbit torques (invited) ...... 13 3.3 Non-equilibrium spin generation in ferromagnets and antiferromagnets. Moving towards ultra-fast spintronics (invited) ...... 14

4 Tuesday, Session IV: Magnonic crystals 15 4.1 Localization of spin waves in magnonic crystals and quasicrystals ...... 15 4.2 Reconﬁgurable magnetic nanopatterns prepared by thermally assisted scanning probe lithography ...... 16 4.3 Spin waves in ferromagnetic periodic and quasiperiodic nanostructures (invited) 17

5 Tuesday, Session V: Skyrmions 18 5.1 Spin waves in nanostructure magnetic lattices for sub-100 nm magnonics (invited) 18 5.2 Simulations of skyrmions at ﬁnite temperature - annihilation mechanisms and lifetimes ...... 19 5.3 Skyrmion based microwave detectors and oscillators and stabilization, nucleation and manipulation of radial vortices ...... 20

6 Tuesday, Session VI, Quantum magnonics I 21 6.1 Non-local detection of magnon supercurrents (invited) ...... 21 6.2 Conservation of angular momentum, spin currents and spin-superﬂuidity in systems with spin-orbit torques ...... 22 6.3 Electrically driven Bose-Einstein condensation of magnons in antiferromagnets . 23 6.4 Bottleneck accumulation of hybrid magneto-elastic bosons ...... 24

7 Tuesday, Session VII: IEEE Distinguished Lecture 25 7.1 Spin current physics and applications ...... 25

8 Wednesday, Session VIII: YIG magnonics 26 8.1 Insulator magnon spintronics in YIG (invited)...... 26 8.2 Pulsed laser grown ultra-thin YIG ﬁlms for magnonic waveguides (invited) . . . 27 8.3 Magnon-magnon coupling in YIG/Co heterostructures ...... 28 8.4 The ﬁnal chapter in the saga of YIG ...... 29

3 9 Wednesday, Session IX: Spin current and magnons 30 9.1 Interaction between spin current and magnons: from magnon spin transport to black holes (invited) ...... 30 9.2 Phase-sensitive detection of inverse spin orbit torques at microwave frequencies (invited)...... 31 9.3 Phase-resolved detection of spin-orbit torques by optical ferromagnetic resonance in ultrathin perpendicularly magnetized ﬁlms ...... 32

10 Wednesday, Session X: Spin-torque oscillators and magnetization dynamics I 33 10.1 Imaging the magnetization dynamics of nano-contact spin-torque vortex oscillators (invited) ...... 33 10.2 Nonlinear magnetization dynamics in YIG thin ﬁlms and nanodiscs (invited) . . 34 10.3 Antiferromagnetic spin-Hall oscillator of THz-frequency radiation ...... 35 10.4 Droplet solitons in magnetic nanowires (invited)...... 36

11 Thursday, Session XI: Quantum magnonics II 37 11.1 Nonlinear interactions in a magnonic Bose-Einstein condensate (invited) . . . . 37 11.2 Spin transport by magneto-elastic bosons in yttrium-iron-garnet ﬁlms ...... 38 11.3 Spin superﬂuidity in antiferromagnetic insulators ...... 39

12 Thursday, Session XII: Spin-torque oscillators and magnetization dynamics II 40 12.1 Spin transfer due to zero-point magnetization ﬂuctuations (invited) ...... 40 12.2 Spin wave modes in ferromagnetic nano-disks, their excitation and auto-oscillations 41 12.3 Inﬂuence of interlayer coupling on spin torque driven excitations in nanopillar structures (invited)...... 42 12.4 Brain-inspired computing using the transient dynamics of spin-torque oscillators 43

13 Thursday, Session XIII: THz magnonics 44 13.1 Polaritons and eﬀects of propagation in terahertz magnonics ...... 44 13.2 Terahertz magnons above the Curie temperature ...... 45 13.3 Generation of THz-frequency signals using current-driven canted antiferromagnets 46

4 5 1 Monday, Session I: Opto-magnonics

1.1 Time-resolved imaging of photo-induced spin wave tunneling through an air gap (invited) Authors: Takuya Satoh Email: [email protected] Aﬃliations: Kyushu University, Department of Physics, Fukuoka, Japan

Spin wave reflection at the sample edge or transmission through an air gap has been reported in finite-size samples [1]. In the present study, we report on time- and phase-resolved imaging of photo-induced spin wave’s transmission through an air gap using pump-probe technique with a CCD camera. In the experiment, a bismuth-doped rare earth iron garnet crystal with a thickness of 110 µm was used as a sample. Circularly polarized pump pulses with a time duration of 150 fs were employed to excite the sample via the inverse Faraday effect [2-4]. Faraday rotation of time-delayed probe pulses was measured. We measured the transmission of spin wave excited in the left hand sample through an air gap to the right hand sample, where the gap width was 40 µm and the time delay was 1000 ps. The center wavelength of the spin waves was observed to be 100-200 µm meaning that the spin waves were dipolar- dominated magneto-static waves. The relation between transmission, phase and the gap width was analyzed. The experimental results were compared with simulation results, which suggest the tunneling nature of the transmission. [1] S. O. Demokritov et al, Phys. Rev. Lett. 93, 047201 (2004). [2] A. V. Kimel et al., Nature 435, 655 (2005). [3] T. Satoh et al., Nature Photon. 6, 662 (2012). [4] I. Yoshimine et al., J. Appl. Phys. 116, 043907 (2014).

5 6

1.2 Exciting THz spin waves using femtosecond optical spin-transfer- torque Authors: M.L.M. Lalieu∗, R. Lavrijsen and B. Koopmans Email: [email protected] Aﬃliations: Eindhoven University of Technology, Physics of Nanostruc- tures, Eindhoven, the Netherlands

In recent years it has been found that spin currents can be generated by femtosecond laser-pulse excitation of a ferromagnetic thin film. Here we show that in addition to its general importance in the field of spintronics, these optically generated spin currents could also be of high potential for THz magnonics. In this work it is demonstrated that femtosecond laser pulses can be used to excite THz standing spin waves within a non-collinear magnetic bilayer. The bilayer consists of two ferromagnetic thin films, a Co layer with an in-plane (IP) anisotropy and a [Co/Ni]N multilayer with an out-of-plane (OOP) anisotropy, which are separated by a conducting spacer. After irradiation a spin current is generated in the OOP generation layer, and injected into the IP absorption layer. The absorbed transverse spins result in a spin transfer torque on the IP magnetization, canting it out of plane. Thickness dependent studies reveal a spin absorption depth of approx. 2 nm. It will be shown that this very local absorption near the injection interface leads to the excitation of (multiple order) THz standing spin waves within the IP layer. By using a wedged absorption layer, the dispersion of these spin waves is investigated for thicknesses down to a few nanometers, showing frequencies above 1 THz. finally, it was found that the Gilbert damping of the higher order spin waves is almost an order of magnitude larger than for the homogeneous precession, and increases rapidly for decreasing layer thicknesses. Results will be compared with simple simulations, and mechanisms for the enhanced damping will be discussed.

6 7

1.3 Spin-wave tomography Authors: Y. Hashimoto∗,1, E. Saitoh1−3 Email: [email protected] Aﬃliations: 1WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 2Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 3Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan

Spin wave is the collective excitation of the magnetization precession in magnetic materials. The properties of spin waves are encoded in their dispersion relation. The characteristics of spin waves depend strongly on their wavenumber (k), and are classiﬁed into the exchange spin waves, dipole-exchange spin waves, and magnetostatic waves. The dispersion relations of the exchange spin waves and the dipole-exchange waves have been observed by neutron scattering and Brillouin light scattering experiments, respectively. The dispersion relation of magnetostatic waves has been theoretically and numerically described in previous studies, while their experimental observation, which needs an experimental system with high k-resolution, has been missing.In this presentation, we will report the observation of the band dispersion structures of pure-magnetostatic spin waves by developing a table-top all-optical spectroscopy named spin- wave tomography (SWaT). SWaT is based on the observation of the propagation dynamics of optically-excited spin waves by time-resolved magneto-optical imaging. The dispersion relation of magnetostatic waves is reconstructed by analysing the observed waveform of the propagating spin waves with model using Fourier transform. The result unmasks characteristics of pure-magnetostatic waves, so-called non-reciprocal surface mode of magnetostatic waves and backward volume magnetostatic waves. Moreover, we demonstrate the time-resolved measurements of the SWaT spectra and discuss the energy transfer from optically-generated phonons to spin waves via magnetoelastic coupling.

7 8

2 Monday, Session II: Spin-wave states and transport

2.1 Topological excitations in thin film ferromagnets and artificial spin ices (invited) Authors: E. Iacocca1,2 Email: [email protected], [email protected] Affiliations: 1Chalmers University of Technology, Department of Physics, Gothenburg, Sweden 2University of Colorado, Department of Applied Mathemat- ics, Boulder, USA

Topological structures in magnetic materials have been extensively studied due to their remarkable stability and potential use in diverse applications. Typically, topological structures are associated with localized objects such as vortices, domain walls, and skyrmions. However, there is a broader class of topological excitations that are spatially extended. Two examples of such states are uniform hydrodynamic states in thin film, planar ferromagnets [1] and topologically protected spin waves in artificial square ices [2]. Uniform hydrodynamic states (UHSs) have been recently described in the context of a dispersive hydrodynamic formulation of magnetization dynamics for thin film ferromagnets [1]. Arbitrarily large topology is conferred to UHSs by large-amplitude, spatially periodic precession about the normal-to-plane symmetry axis. Nu- merical simulations reveal that defects can give rise to topology-conserving vortex-antivortex trains at subsonic conditions and wavefronts and a Mach cone at supersonic conditions [3], similarly observed in superfluids. Artificial spin ices (ASIs) are metamaterials composed of geometrically-placed nanomagnets. Dipole-dipole mediated spin waves in ASIs are reconfigurable in which the band structure depends on the magnetization state [4]. Including interfacial Dzyaloshinskii-Moriya interaction e.g., from a neighboring heavy metal layer, bands can acquire topology depending on the magnetization state [2]. This suggests the possibility to actively toggle topologically protected edge modes in ASIs.These extended topological excitations ex- pose new physics in magnetic materials and suggest novel approaches towards applications. [1] E. Iacocca, T. J. Silva, and M. A. Hoefer, Phys. Rev. Lett. 118, 017203 (2017) [2] E. Iacocca and O. Heinonen, arXiv:1612.07203 (2016) [3] E. Iacocca and M. A. Hoefer, arXiv:1610.08940 (2016) [4] E. Iacocca, S. Gliga, R. L. Stamps, and O. Heinonen, Phys. Rev. B 93, 134427 (2016)

8 9

2.2 Nano-scaled magnon transistor based on three-magnon splitting Authors: A. Chumak, Qi Wang, Philipp Pirro Presenting Thomas Brächer Author: Email: [email protected] Affiliations: Fachbereich Physik and Landesforschungszentrum OPTI- MAS,Technische UniversitätKaiserslautern, Kaiserslautern, Germany

Spin waves and their quanta magnons open up a promising branch of high-speed and low- power information processing [1]. The realization of single-chip all-magnon information systems demands for the development of circuits in which magnon currents can be manipulated by magnons themselves. In Ref. 2 we presented and tested experimentally a proof-of-concept magnon transistor. The density of magnons flowing from the transistor’s source to its drain could be decreased three orders of magnitude by the injection of magnons into the gate. The operational principle is based on a nonlinear four-magnon scattering process. Here we use micromagnetic simulations [3] to propose a conceptually different approach for the realization of a nano-scaled magnon transistor. In this device, a three- rather than a four-magnon scattering process is utilized. The transistor is constructed from 50 nm-thick and 100 nm wide yttrium iron garnet (YIG) waveguides. Gate magnons of frequency 9.8 GHz are injected into the transistor’s gate. Source magnons of almost twice smaller frequency of 4.4 GHz are injected in the transistor’s source and propagate towards the gate. When the source magnons reach the gate region, they interact with the gate magnons boosting a three-magnon scattering process in which one gate magnon scatters into one new source magnon and into one idle magnon of frequency 5.4 GHz. As a result, the number of the source magnons at the drain is increased and the transistor acts as an amplifier of magnon signals. A gain factor of 6.3 is demonstrated. Our studies show that this type of magnonic transistor can be used for amplification of magnonic currents as well as for logic operations in future all-magnon magnonic circuits. Financial support by the ERC Starting Grant “MagnonCircuits” is gratefully acknowledged. [1] A. V. Chumak, et al., Nat. Phys. 11, 453-461 (2015) [2] A. V. Chumak, et al., Nat. Commun. 5, 4700, (2014) [3] A. Vansteenkiste, et al., AIP Adv. 4, 107133 (2014

9 10

2.3 Spin dynamics in antiferromagnets and ferrimagnets (invited) Authors: T. Ono Email: [email protected] Aﬃliations: Kyoto University, Institute for Chemical Research, Uji, Japan

Spin transfer torque (STT) has been an efficient and promising technique to control magnetizations of ferromagnetic materials in modern spintronics devices. This novel technique is based on an interaction between electron spin and local magnetic moments. Namely, the angular momentum of the electron spin is transferred to and exerts a torque on the magnetization. The same interaction should be conserved in antiferromagnets (AFMs), in which there are microscopic local magnetic moments that compensate each other to exhibit no net magnetization. As AFMs have been abandoned as an active material in spintronics in spite of their potential applications in the THz regime, it is of great interest to investigate the STT in AFMs. We prepare the epitaxial MgO(001)[100]/Pt(001)[100]/NiO(001)[100]/FeNi/SiO2 films to investigate the spin transport in the NiO antiferromagnetic insulator. The ferromagnetic resonance measurements of the FeNi under a spin current injection from the Pt by the spin Hall effect revealed the change of the ferromagnetic resonance linewidth depending on the amount of the spin current injection. The results can be interpreted that there is an angular momentum transfer through the NiO [1]. Antiferromagnets are expected to show much faster spin dynamics than ferromagnets because they have higher resonance frequencies than ferromagnets. However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored mainly because of the immunity of antiferromagnets to magnetic fields. Furthermore, this immunity makes field-driven antiferromagnetic DW motion impossible despite rich physics of field-driven DW dynamics as proven in ferromagnetic DW studies. We show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare-earth-3d-transition metal ferrimagnetic compounds where net angular moment is nonzero at TA, the field-driven DW mobility remarkably enhances up to 20 km s- 1T-1. The collective coordinate approach generalized for ferrimagnets and atomistic spin model simulations show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics. [1] Appl. Phys. Lett. 106, 162406 (2015) [2] arXiv:1703.07515 (2017)

10 11

2.4 Antiferromagnetic domain wall as a spin wave polarizer and retarder Authors: J. Lan1,2, W. Yu∗,1,2 and J. Xiao1,2 Email: [email protected] Aﬃliations: 1Department of Physics and State Key Laboratory of Surface Physics, Shanghai, China 2Collaborative Innovation Center of Advanced Microstruc- tures, Nanjing, China

As a collective quasiparticle excitation of the magnetic order in magnetic materials, spin wave, or magnon when quantized, can propagate in both conducting and insulating materials without Joule heating. Like the manipulation of its optical counterpart, the ability to manipulate spin wave polarization is not only important but also fundamental for magnonics, an emerging field in information processing using the low-dissipation spin wave as the information carrier. Due to the broken time reversal symmetry, ferromagnets can only accommodate the right-handed circularly polarized spin wave modes, which leaves no freedom for polarization manipulation. In contrast, antiferromagnets, with time reversal symmetry restored, have both left and right circular polarizations, as well as all linear and elliptical polarizations. Here we demonstrate theoretically and confirm by micromagnetic simulations that, in the presence of Dzyaloshinskii- Moriya interaction (DMI), an antiferromagnetic domain wall acts naturally as a spin wave polarizer or a spin wave retarder (wave plate). Our findings provide extremely simple yet flexible routes toward magnonic information processing by harnessing the polarization degree of freedom of spin wave.

11 12

3 Tuesday, Session III: Spin-orbit torques

3.1 Excitation and ampliﬁcation of coherent spin waves by spin currents (invited) Authors: V. E. Demidov Email: [email protected] Aﬃliations: Institute for Applied Physics, University of Muenster, Cor- rensstrasse 2-4, Muenster, 48149, Germany

In this talk, we review our recent experiments on utilization of pure spin currents created by the spin-Hall effect and the nonlocal spin injection for excitation and manipulation of coherent propagating spin waves in magnonic nano-structures based on metallic and insulating magnetic films. We show that spin currents enable novel functionalities of magnonic devices not achievable by using traditional approaches. In particular, spin currents allow highly efficient excitation of continuous spin waves and short spin-wave packets with the duration down to a few nanoseconds exhibiting nonlinear self-stabilization during propagation. These demonstra- tions open a route for implementation of high-speed magnonic devices characterized by high information flow capacity. Additionally, spin currents allow the control of the propagation length of spin waves. We show that this mechanism is particularly efficient in magnonic systems based on ultrathin YIG films. In contrast to all-metallic spin-Hall systems, where the current-induced variation of the propagation length typically does not exceed a factor of two, in YIG based systems, the demonstrated increase of the propagation length is about an order of magnitude. This highly efficient controllability allows an increase of the spin-wave intensity at the output of a 10 micrometer long transmission line by three orders of magnitude. The obtained results open new perspectives for the future-generation electronics using electron spin degree of freedom for transmission and processing of information on the nanoscale. [1] V. E. Demidov and S. O. Demokritov, IEEE Trans. Mag. 51, 0800215 (2015). [2] V. E. Demidov et al., Nat. Commun. 7, 10446 (2016). [3] M. Evelt et al., Appl. Phys. Lett. 108, 172406 (2016). [4] B. Divinskiy et al., Appl. Phys. Lett. 109, 252401 (2016). [5] V. E. Demidov et al., Phys. Rep. 673, 1 (2017).

12 13

3.2 Optical detection of magnetization dynamics induced by spin- orbit torques (invited) Authors: T. M. Spicer1, C. J. Durrant1, P. S. Keatley1, V. V. Kruglyak1, R. J. Hicken∗,1, Q. Hao2, G. Xiao2, P. D¨urrenfeld3, A. Houshang3, M. Ranjbar3, A. A. Awad3, R. K. Dumas3, M. Dvornik3, and J. Akerman˚ 3. Email: [email protected] Aﬃliations: 1Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK. 2Department of Physics, Brown University, Providence, Rhode Island 02912, USA. 3Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.

Current induced magnetization dynamics underlie the writing of data to spin transfer random access memory and the operation of the spin transfer oscillator (STO). Spin-orbit torques (SOT) provide freedom to manipulate the geometry and design of these devices. However, the dependence of SOT upon atomic scale structure is still being explored, and the action of Oersted field torques generated by the current can hinder quantitative characterisation of the SOT. Using scanning Kerr microscopy (SKM) we have shown that current induced switching in Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy is a stochastic domain wall process, in which the Oersted field plays an important role in nucleating and then pinning domains at the edges of a microscale Hall bar [1]. We have also observed the precessional dynamics induced by picosecond current pulses, and used their bias field dependence to infer the character of the SOT. In a separate study, time resolved SKM (TRSKM) was used to image the current induced dynamics within a spin Hall nano-oscillator (SHNO). The SHNO was formed by fabricating two Au (150nm) triangular nano-contacts on top of a 4 micron diameter bi-layer disk of Py (5nm)/Pt (6nm). The excitation of dynamics by AC and DC currents has been explored. In the AC case, ferromagnetic resonance (FMR) is induced through the combined action of the AC Oersted field and spin transfer-torque (STT). For DC currents, a localized spin wave mode (auto-oscillation) is excited between the noncontacts, with a frequency that lies below that of the FMR. We show how this mode can be injection locked to an applied AC current, and the effect that this current has on the magnetization of the disk and the oscillating mode. The spatial variation of the FMR and localised modes will be compared, and the contributions made by the different excitation mechanisms discussed. [1] C. J. Durrant, R. J. Hicken, Q. Hao and G. Xiao, Phys. Rev. B 93, 014414 (2016).

13 14

3.3 Non-equilibrium spin generation in ferromagnets and antiferromagnets. Moving towards ultra-fast spintronics (invited) Authors: Chiara Ciccarelli Email: [email protected] Aﬃliations: University of Cambridge

The transfer of the magnetic information via spin currents is one of the building blocks of spintronics. This ﬂow of angular momentum is enacted by spin-polarised charge currents in conductors.In my talk I will start by describing how inversion asymmetry in crystal magnets can induce a non-equilibrium spin-polarisation of the carriers, which leads to the generation of magnetic torques [1,2] and to new forms of magnetoresistance [3].In the second part of the talk I will discuss how an anti-ferromagnet can behave as a spin-current emitter despite having zero magnetization. In particular I will discuss how lattice sensitive excitation of carriers in a ferrimagnetic alloy with a compensation point leads to spin emission even when the overall magnetisation is zero [4]. [1] Fang et al., Nature Nanotechnology 6, 413 (2011). [2] Ciccarelli et al., Nature Physics 12, 855 (2016) [3] K. Olejnik et al., PRB 91, 180402(R) (2015) [4] T. Huisman et al., APL 110, 072402 (2017)

14 15

4 Tuesday, Session IV: Magnonic crystals

4.1 Localization of spin waves in magnonic crystals and quasicrystals Authors: J. Rych ly∗, J. W. K los,M. Krawczyk Email: [email protected] Aﬃliations: Faculty of Physics, Adam Mickiewicz University in Poznan, Umultowska 85, 61-614 Poznan, Poland.

In this work we present our latest investigations regarding localization of spin waves (SWs) in the ﬁnite magnonic crystals (MCs) and magnonic quasicrystals (MQs). To observe localized states in MC, the structural defect has to be present. These defects can be introduced in two ways: (i) by introducing the borders in the periodic structure (creating surface defects), and (ii) by introducing defects in the bulk region of the periodic structure [1]. We study here two types of surface/defect states that are known from electronic crystals and are referred as Shockley and Tamm states. Shockley states appear due to the breaking of the MC exactly at the symmetry point of symmetric unit cell, while Tamm states appear due to the presence of additional perturbation. The MC with only exchange interactions taken into account is a direct analog of electronic crystal, whereas the magnonic system in a dipolar-exchange regime shows distinct diﬀerences in comparison to the electronic case, which are due to longrange dynamic dipolar interactions [2]. For both MCs and MQs, the defect states in the bulk and at the surfaces (surface states) have frequencies placed in the band gap regions, which are forbidden for the bulk states found in an undisturbed systems. Moreover, quasicrystals, despite lack of periodicity, are the structures characterized by the long-range order, which is manifested by the appearance of complex system of band gaps. Due to uniqueness of the surroundings of elements building the MQs, we have observed localization of SWs in the bulk region of an undisturbed structure. This localization appear for higher SW bands (characterized by higher frequencies) [3]. This work was supported by: NSC grant UMO-2012/07/E/ST3/00538, Horizon2020-GA-No.644348 (MagIC). [1] J. W. K los,et al., J. Appl. Phys. 113, 133907 (2013). [2] J. Rych lyet al., J. Phys. D: Appl. Phys. 50, 164004 (2017). [3] J. Rych lyet al., Phys. Rev. B, 92, 054414 (2015).

15 16

4.2 Reconfigurable magnetic nanopatterns prepared by thermally assisted scanning probe lithography Authors: E. Albisetti1,2, D. Petti1, M. Madami3, S. Tacchi∗,4, P. Vavassori5,6, E. Riedo2,7, R. Bertacco1,8 Email: [email protected] Affiliations: 1Dipartimento di fisica, Politecnico di Milano, Milano, Italy 2CUNY-Advanced Science Research Center, New York, USA 3Dipartimento di fisica e Geologia, Universit di Perugia, Italy 4Istituto Officina dei Materiali del CNR (CNR-IOM), Sede Secondaria di Perugia, c/o Dipartimento di fisica e Geologia, Perugia, Italy 5CIC nanoGUNE, Donostia,-San Sebastian, Spain 6IKERBASQUE, Basque Foundation for Science, Bilbao, Spain 7Department of Physics, CUNY-City College of New York, New York, USA and Physics Program, CUNY-The Graduate Center, New York, USA 8IFN-CNR, c/o Politecnico di Milano, Milano, Italy

Reprogrammable magnonic crystals, whose functionality can be actively re-designed, are cur- rently the subject of an intense research activity. Up to now the control of the magnetic configuration and as a consequence of the dynamic response has been obtained mainly using locally generated magnetic fields. In this work, we propose ‘thermally assisted magnetic scanning probe lithography’ (tam-SPL) [1], for creating reconfigurable magnetic nanopatterns by crafting at the nanoscale the magnetic anisotropy landscape of a ferromagnetic layer exchange- coupled to an antiferromagnetic one. By performing a highly localized field cooling with the hot tip of a scanning probe microscope, magnetic structures, with arbitrarily oriented magnetization and tunable unidirectional anisotropy, are patterned without modifying the film chemistry and topography. Using tam-SPL, we patterned 2.5-µm-wide tracks with alternating 0◦ and 90◦ remanent magnetization in a continuous CoFeB/IrMn exchange bias bilayer. Micro-focused Brillouin Light Scattering (Micro-BLS) measurements were performed to map the spin-wave intensity distribution. We found that spin waves efficiently propagate (rapidly attenuate) in the tracks where the local magnetization is perpendicular (parallel) to the spin waves wavevector. In addition, we show that it is possible to reversibly “switch-off” the spin-wave excitation within the pattern by reorienting the magnetization with an external magnetic field. [1] E. Albisetti, D. Petti, M. Pancaldi, M. Madami, S. Tacchi, J. Curtis, W.P. King, A. Papp, G. Csaba, W. Porod, P. Vavassori, E. Riedo, and R. Bertacco, Nat. Nanotechnol. 11, 545 (2016).

16 17

4.3 Spin waves in ferromagnetic periodic and quasiperiodic nanostructures (invited) Authors: P. Gruszecki, J. Rychy, M. Zelent, J. W. Kos, M. Krawczyk∗ Email: [email protected] Aﬃliations: Faculty of Physics, Adam Mickiewicz University in Poznan, Umultowska 85, 61-614 Poznan, Poland

Spin waves in ferromagnetic thin films span the broad range of frequencies from hundreds of MHz up to tens of GHz with the respective wavelengths ranging from micrometers to nanometers. The spin wave spectrum can be tuned by external magnetic field and depends on the magnetization configuration. This offers suitable characteristics important for applications in microwave and information processing technology. We present our advances allowing for tailoring the spin wave dynamics in ferromagnetic thin films by periodic and quasiperiodic patterning in nanoscale. We explain formation of the spin wave band structure in magnonic crystals, with and without inversion symmetry. Opening of the magnonic band gaps and variation of the band structure resulting from changes of the magnonic crystal geometry and symmetry is ex- emplified. Quasiperiodicity in magnonics provides additional features, which can be relevant for technological applications. We show the structures characterized by the frequency spectra with multiple band gaps and localized eigenmodes, which have interesting dynamic respond on the spatially uniform microwave field. Further control of spin waves can be achieved by change of the spin wave refractive index. We present the influence of continuous variation of the internal magnetic field on the propagating spin wave beams and demonstrate the basic approach for explanation of the graded index materials properties useful in magnonics. We acknowledge the financial NCN project UMO-2012/07/E/ST3/00538 and the EU Horizon 2020 GA No 644348 (MagIC).

17 18

5 Tuesday, Session V: Skyrmions

5.1 Spin waves in nanostructure magnetic lattices for sub-100 nm magnonics (invited) Authors: D. Grundler Email: [email protected] Affiliations: Laboratory of Nanoscale Magnetic Materials and Magnon- ics, Institute of Materials, Ecole Polytechnique Fédéralede Lausanne, Station 17, 1015 Lausanne, Switzerland

Periodically modulated magnetic thin films have given rise to the development of one- and two-dimensional magnonic crystals, magnon transistors and grating couplers for exchange- dominated spin waves recently [1,2]. Such components are expected to form the basis of a magnonics technology that aims at exploiting short-wavelength spin waves for information processing. Here, reconfigurable device architectures are of particular interest as to optimize the functionality and power consumption. Reconfigurable architectures might involve periodic lattices of artificially tailored nanomagnets and skyrmion lattices with translational invariance or quasicrystalline designs [3,4,5]. Using broadband spin-wave spectroscopy and inelastic light scattering we explore the different approaches to tailor the band structures of magnonic crystals and optimize grating couplers for sub-100 nm spin waves based on nanostructured ferromagnetic metals and insulating ferrimagnets [2,6]. We report latest results. The work is supported by DFG via project GR1640/5-2, TRR80 and SNF via grant number 163016. [1] A.V. Chumak, A.A. Serha, and B. Hillebrands, arXiv:1702.06701. [2] H. Yu et al., Nat. Commun. 7, 11255 (2016). [3] L. Heyderman and R. Stamps, J. Phys.: Condens. Matter 25, 363201 (2013). [4] M. Krawczyk and D. Grundler, J. Phys.: Condens. Matter 26, 123202 (2014). [5] M. Garst, J. Waizner, and D. Grundler, arXiv:1702.03668. [6] V.S. Bhat, F. Heimbach, I. Stasinopoulos, and D. Grundler, Phys. Rev. B 93, 140401 (R) (2016).

18 19

5.2 Simulations of skyrmions at finite temperature - annihilation mechanisms and lifetimes Authors: P. Bessarab1, G. Muüller2, I. Lobanov3 , F. Rybakov4, S. Bluügel2, N. Kiselev2, L. Bergqvist5, A. Delin∗,5 Email: [email protected] Affiliations: 1University of Iceland, Reykjavik, Iceland. 2Forschungszentrum Juülich, Jülich, Germany. 3ITMO University, St. Petersburg, Russian Federation. 4Institute of Metal Physics, Ekaterinburg, Russian Federa- tion. 5KTH Royal Institute of Technology, Stockholm, Sweden.

Skyrmions transported on racetracks is a promising concept for future spin-based information technology. Quantitative prediction of the stability of individual skyrmions at a given temperature is important for a practical realization of skyrmion-based devices for information processing and storage. Here, we present our recent work in this direction. By means of har- monic transition state theory, we compute the lifetime of a skyrmion on a racetrack. We find that the lifetime is determined mainly by two annihilation mechanisms: escape through the boundary and radial collapse at the interior. The two mechanisms contribute asymmetrically to the skyrmion stability. At low external magnetic fields, the escape mechanism prevails, but a crossover field exists, above which the collapse mechanism becomes dominant. The two annihilation mechanisms acquire different temperature dependence due to the presence of Goldstone modes in the system. Our calculations are performed with experimental micromagnetic parameters extracted from a PdFe bilayer strip on an Ir(111) substrate. We find excellent agreement with experimental results reported for that system. The cross-over effect we predict should be a general feature for racetrack skyrmions.

19 20

5.3 Skyrmion based microwave detectors and oscillators and stabilization, nucleation and manipulation of radial vortices Authors: G. Finocchio Email: [email protected] Aﬃliations: University Messina, Department of Mathematical and Com- puter Sciences, Physical Sciences and Earth Sciences, Mess- ina, Italy

Solitons are very promising for the design of the next generation of ultralow power devices for storage and computation. In particular, Neélskyrmions stabilized by the interfacial Dzyaloshinskii- Moriya Interaction (IDMI) in out-of-plane materials offer a scalability beyond the limit of CMOS technology for storage. However, the interesting aspect of skyrmions is their possible use for ICT devices such as oscillators [1] and detectors[2]. The oscillators can be achieved in the IDMI region parameter where the a dynamical skyrmion is stabilized by a dc spin-polarized current, while the unbiased detectors exhibit sensitivities (output voltage/input power) as larger as 2000V/W. Here, we discuss in details the results of micromagnetic simulations and experiments of different solutions for the realization of skyrmion based microwave oscillators and detectors[3]. In conclusions, our findings show the potential of skyrmions for the development of a skyrmion based technology[4]. The second part of the presentation will focus on the proprieties of radial vortices that can stabilized and manipulated in materials with in-plane easy axis and IDMI. In particular, I will discuss how the IDMI is able to lift the energy de- generacy of a magnetic vortex state by stabilizing a topological soliton with radial chirality. It has a non-integer Skyrmion number S (0.5 ¡ S ¡ 1) due to both the vortex core polarity and the magnetization tilting induced by the IDMI boundary conditions. Micromagnetic simulations predict that a magnetoresistive memory based on the radial vortex state in both free and polarizer layers can be efficiently switched by a threshold current density smaller than 106 Acm−2. The switching processes occur via the nucleation of topologically connected vortices and vortex-antivortex pairs, followed by spin-wave emissions due to vortex-antivortex annihil- ations. [1] Carpentieri M., et al, Topological, non-topological and instanton droplets driven by spin- transfer torque in materials with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya Interaction, Sci. Rep. 5, 16184 (2015). [2] finocchio G., et al, Skyrmion based microwave detectors and harvesting, Appl. Phys. Lett. 107, 262401 (2015). [3] finocchio G., Zeng Z., et al, in preparation. [4] finocchio G., et al, Magnetic skyrmions: from fundamental to applications, J. Phys. D. Appl. Phys. 49, 423001 (2016). [5] Siracusano G., Tomasello R., et al, Magnetic Radial Vortex Stabilization and Efficient Ma- nipulation Driven by the Dzyaloshinskii-Moriya Interaction and Spin-Transfer Torque, Phys. Rev. Lett. 117, 87204 (2016).

20 21

6 Tuesday, Session VI, Quantum magnonics I

6.1 Non-local detection of magnon supercurrents (invited) Authors: Burkard Hillebrands Email: [email protected] Aﬃliations: Technische Universit¨at Kaiserslautern, Fachbereich Physik and Landesforschungszentrum OPTIMAS, 67663 Kaiserslaut- ern, Germany

Finding new ways for fast and efficient processing and transfer of data is one the most challen- ging tasks nowadays. Elementary spin excitations - magnons (spin wave quanta) - open up a very promising direction of high-speed and low-power information processing. Magnons are bosons, and thus they are able to form spontaneously 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 this macroscopic quantum phenomenon for the information transfer and processing. Recently we have succeeded to demonstrate the possibility to set a condensate in motion by introducing a time-dependent spatial phase gradient into its wavefunction [1]. In the present work, I demonstrate non-local probing of a magnon supercurrent, which provides direct evidence of the condensate propagation driven by a thermal gradient. The results were obtained by means of time- and wavevector-resolved Brillouin light scattering spectroscopy. By utilizing a separate blue laser for heating purposes, we are able to control the thermal gradient, while a low-power green laser is used for local probing of the condensate area. A thermally induced phase shift in the condensate wavefunction drives a bidirectional flow of a magnon supercurrent from the center of the hot spot. The supercurrent pulse is detected on an undisturbed background of the slowly decaying magnon BEC. The occurrence of the supercurrent directly confirms the phase coherency of the magnon condensate and opens door to studies in the general field of magnonic macroscopic quantum transport phenomena at room temperature as a novel approach in the field of information processing. The work is supported by the ERC within the ERC AdG “Supercurrents of Magnon Condens- ates for Advanced Magnonics” and the DFG within the SFB/TR49 (project INST 161/544-3). [1] D.A. Bozhko et al. Nature Physics 12, 1057 (2016)

21 22

6.2 Conservation of angular momentum, spin currents and spin- superﬂuidity in systems with spin-orbit torques Authors: S.O. Demokritov1,2 Email: [email protected] Aﬃliations: 1Institute for Applied Physics and Center for Nanotechnology, University of Muenster, Corrensstrasse 2-4, 48149 Muenster, Germany 2Institute of Metal Physics, Ural Division of RAS, Yekaterin- burg 620041, Russia

Spin currents, the flow of angular momentum without the simultaneous transfer of electrical charge, play an enabling role in magnonics. Recently a lot of efforts has been made to implement the concept of spin current for description of different dynamic phenomena in magnetic systems, including spin-troque- and spin-Hall nano-oscillators, domain wall motion, and spin wave propagation. In parallel, different magnetic systems demonstrating dissipationless spin currents have been suggested theoretically. Most of these works, however, ignore relativistic spin-orbit interactions, including the magnetic dipole interaction. Unlike the charge, the spin is not a conservative quantity within the magnetic subsystem in the presence of the spin orbit torques that couples the spin of the magnetic moments to angular momentum in the lattice. In this contribution, I analyze the flow of the angular momentum between the spin subsystem and the lattice and show that the common definition of the spin current in electrically insulating systems, that takes into account the exchange interaction only, results in qualitatively false statements in some important cases. As an example, I analyze the flow of angular momentum in magnon condensate and show that the dipole interaction plays a vital role in this process. Moreover, I show that the definition of spin superfluidity based on dissipationless spin currents should be revised. In this connection, I present and discuss a trivial, but pictorial system with a persistent, dissipationless spin current, which has nothing to do with spin superfluidity.

22 23

6.3 Electrically driven Bose-Einstein condensation of magnons in antiferromagnets Authors: E. Løhaugen Fjærbu∗, N. Rohling, and A. Brataas Email: [email protected] Aﬃliations: Norwegian University of Science and Technology, Department of Physics, Trondheim, Norway

We explore routes to realize electrically driven Bose-Einstein condensation of magnons in insulating antiferromagnets. Even in insulating antiferromagnets, the localized spins can strongly couple to itinerant spins in adjacent metals via spin-transfer torque and spin pumping. We describe the formation of steady-state magnon condensates controlled by a spin accumulation polarized along the staggered ﬁeld in an adjacent normal metal. Two types of magnons, which carry opposite magnetic moments, exist in antiferromagnets. Consequently, and in contrast to ferromagnets, Bose-Einstein condensation can occur for either sign of the spin accumulation. This condensation may occur even at room temperature when the interaction with the normal metal is fast compared to the relaxation processes within the antiferromagnet. In antiferromagnets, the operating frequencies of the condensate are orders of magnitude higher than in ferromagnets.

23 24

6.4 Bottleneck accumulation of hybrid magneto-elastic bosons Authors: D. A. Bozhko1, G. A. Melkov2, V. S. L’vov3, A. Pomyalov3, V. I. Vasyuchka∗,1, A. V. Chumak1, B. Hillebrands1, and A. A. Serga1 Email: [email protected] Aﬃliations: 1Technische Universit¨atKaiserslautern, Fachbereich Physik and Landesforschungszentrum OPTIMAS, Kaiserslautern, Germany 2Taras Shevchenko National University of Kyiv, Faculty of Radiophysics, Electronics and Computer Systems, Kyiv, Ukraine 3Weizmann Institute of Science, Department of Chemical Physics, Rehovot, Israel

Macroscopic quantum states-Bose-Einstein condensates (BECs) can be created in overpopu- lated gases of bosonic quasiparticles (excitons, polaritons, magnons, photons, etc.). However, interactions between quasiparticles of different nature [1], for example between magnons and phonons in a magnetic medium, can significantly alter the properties of these gases and thus modify the condensation scenarios. Here, we report on the discovery of a novel condensation phenomenon mediated by the magnon-phonon interaction: A bottleneck accumulation of hybrid magneto-elastic bosons. We have found that the transfer of quasiparticles toward a BEC state is almost fully suppressed near the intersubsection point between the magnon and phonon spectral branches. Such a bottleneck leads to a strong spontaneous accumulation of the quasiparticles trapped near the semi-linear part of the magnon-phonon hybridization area [2]. As opposed to BEC, which is a consequence of equilibrium Bose statistics, the bottleneck accumulation is determined by varying interparticle interactions.The developed theoretical model describes the experimentally observed peak of hybrid magneto-elastic bosons. Moreover, it proves the saturation effect in accumulation of quasi-particles: An increase in the pumping power leads to the increase of the magnon BEC population and a following reduction of the bottleneck effect. The bottleneck accumulation phenomenon discovered for a magnon-phonon gas is not unique for this particular system and can occur in any multicomponent gas-mixture of interacting quasiparticles with significantly different scattering amplitudes. Financial support from the Deutsche Forschungsgemeinschaft (project INST 161/544-3 within the Transregional Collaborative Research Centre SFB/TR49 “Condensed Matter Systems with Variable Many-Body Interactions”) is gratefully acknowledged. [1] L. Venema et al., Nat. Phys. 12, 1085 (2016) [2] D. A. Bozhko et al., arXiv: 1612.05925 (2016)

24 25

7 Tuesday, Session VII: IEEE Distinguished Lecture

7.1 Spin current physics and applications Authors: E. Saitoh1,4 Email: [email protected] Aﬃliations: [1] ERATO-SQR, JST, Japan 2WPI-AIMR, Tohoku University, Japan 3Institute for Materials Research, Tohoku University, Japan 4ASRC, JAEA, Japan

Generation and utilization of a flow of spin angular momentum of electrons in condensed matter, called spin current, are the key challenge of today’s nano-scale magnetism and spintronics. The discovery of the inverse spin Hall effect (ISHE) [1-3], the conversion of spin current into electric voltage via spin-orbit interaction, has allowed researchers to detect and utilize spin current directly, and, since then, many spin-current driven effects have been discovered by exploiting the ISHE. Here, such newly discovered spin-current effects will be outlined, including light-spin conversion [1,4], plasmon-spin conversion, sound-spin conversion, and heat-spin conversion [5- 6], and their common mechanism and future possible application will be discussed. Among them, a typical conversion effect is the spin Seebeck effect (SSE) [5], spin current generation from a temperature gradient. SSE has attracted a great deal of interest since it may realize new type thermo-electric convertors which make full use of the characteristic feature of spins: the non-reciprocal dynamics. This non-reciprocity allows a spin to rectify thermal fluctuation into unidirectional spin current via the spin pumping mechanism, which can be converted into electric power via the ISHE. Spins, working as a natural rectifier in magnets, may thus provide a versatile mechanism of energy conversion in condensed matter. I will show also that the rectification mechanism underlies various spin related phenomena which were found recently. At the end of my talk, spin current generation from mechanical motion of condensed matter will be discussed. [1] E. Saitoh et al., Applied Physics Letters 88 (2006) 182509. [2] J. Wunderlich et al., Physical Review Letters 94 (2005) 047204. [3] T. Kimura et al., Physical Review Letters 98 (2007) 156601. [4] K. Uchida et al., Nature communications 6 (2015) 5910. [5] K. Uchida et al., Nature materials 9 (2010) 894. [6] T. An et al., Nature materials 12 (2013) 549.

25 26

8 Wednesday, Session VIII: YIG magnonics

8.1 Insulator magnon spintronics in YIG (invited) Authors: B. van Wees Email: [email protected] Aﬃliations: Zernike Institute of Advanced Materials, University of Groningen, The Netherlands

I will give an overview of our recent work on magnon spin injection, transport and detection in lateral devices based on YIG. A Pt strip is used as an magnon injector, either electrically by the spin Hall effect, or thermally by the spin Seebeck effect . Magnon spins diffuse laterally and are detected by the inverse spin Hall effect by a Pt detector strips [1]. In this we have determined a magnon relaxation length of about 10 micrometers [2]. We investigated the temperature and YIG thickness dependence of this length as well as of the magnon spin conductivity [2,3]. A comparison with a model based on diffusive magnon transport allows to extract the magnon spin transport parameters, but also shows discrepancies which may be due to the nature of the magnon injection and detection [3,4]. The magnon transport is slightly dependent on the magnitude of the magnetic field [5] , and shows anisotropic magnetoresistance as well as planar Hall effect [6]. finally we showed that we can change the magnon conductance of the YIG by injecting additional magnons by a DC current biased additional Pt electrode in between injector and detector [7], thus realizing an analogue of an electrical field effect transistor. [1] L.J. Cornelissen, J. Liu, R.A. Duine, J. Ben Youssef, and B.J. van Wees, ”Long-distance transport of magnon spin information in a magnetic insulator at room temperature”, Nature Phys. 11, 1022 (2015) [2] L.J. Cornelissen, J. Shan, and B.J. van Wees, ”Temperature dependence of the magnon spin diffusion length and magnon spin conductivity in the magnetic insulator yttrium iron garnet”, Phys. Rev. B 94, 180402(R) (2016) [3] J. Shan, L.J. Cornelissen, N. Vlietstra, J. Ben Youssef, T. Kuschel, R.A. Duine, and B.J. van Wees, ”Influence of yttrium iron garnet thickness and heater opacity on the nonlocal transport of electrically and thermally excited magnons”, Phys. Rev. B 94, 174437 (2016) [4] L.J. Cornelissen, K.J.H. Peters, G.E.W. Bauer, R.A. Duine, and B.J. van Wees, ”Magnon spin transport driven by the magnon chemical potential in magnetic insulators”, Phys. Rev. B 94, 014412 (2016) [5] L.J. Cornelissen and B.J. van Wees, ”Magnetic field dependence of the magnon spin diffusion length in the magnetic insulator yttrium iron garnet”, Phys. Rev. B 93, 020403(R) (2016) [6] J. Liu, L.J. Cornelissen, J. Shan, T. Kuschel, and B.J. van Wees, ”Magnon Planar Hall Effect and Anisotropic Magnetoresistance in a Magnetic Insulator”, Phys. Rev. B 95, 140402(R) (2017) [7] L.J. Cornelissen et al., in preparation.

26 27

8.2 Pulsed laser grown ultra-thin YIG films for magnonic waveguides (invited) Authors: A. Anane Email: [email protected] Affiliations: UnitéMixte de Physique, CNRS, Thales, Univ. Paris-Sud, UniversitéParis-Saclay, 91767 Palaiseau, France

The use of spin waves (SWs) as a “Beyond CMOS” paradigm for analog or digital signal processing is a key aim for the field of magnonics[1]. Spin waves propagate best in magnetic materials with low Gilbert damping and when it comes to Gilbert damping Yttrium Iron Garnet (Y5F3O12 or YIG) is the absolute benchmark. Within the last few years The advent of ultrathin YIG films has made it possible to fabricate nanostructured and microstructured waveguides that lead to spin-orbit torque induced magnetization auto-oscillation[2-5]. The talk will be about the latest progresses in the growth an the tailoring of magnetization properties of pulsed laser deposited thin YIG films. Epitaxial strain induced anisotropy will be presented. In a second part SWs propagation in an assembly of microfabricated 20 nm thick, 2.5 µm wide Yttrium Iron Garnet (YIG) waveguides using propagating spin-wave spectroscopy (PSWS) and phase resolved micro-focused Brillouin Light Scattering (µ-BLS) spectroscopy. On the same waveguides, using phase resolved µ-BLS, direct mapping of spin waves allows us to reconstruct the spin-wave dispersion relation and to confirm the multi-mode propagation in the waveguides. [1] Chumak, A. V., Vasyuchka, V. I., Serga, A. A. & Hillebrands, B. Magnon spintronics. Nature Physics 11, 453-461, doi:10.1038/nphys3347 (2015). [2] Demidov, V. E. et al. Magnetization oscillations and waves driven by pure spin currents. Physics Reports 673, 1-31, (2017). [3] Collet, M. et al. Generation of coherent spin-wave modes in yttrium iron garnet microdiscs by spin-orbit torque. Nat Commun 7, doi:10.1038/ncomms10377 (2016). [4] Demidov, V. E. et al. Direct observation of dynamic modes excited in a magnetic insulator by pure spin current. Scientific Reports 6, doi:10.1038/srep32781 (2016). [5] Evelt, M. et al. High-efficiency control of spin-wave propagation in ultra-thin yttrium iron garnet by the spin-orbit torque. Applied Physics Letters 108, doi:10.1063/1.4948252 (2016). [6] M. Collet et al. Spin-wave propagation in ultra-thin YIG based waveguides. Appl. Phys. Lett. 110, 092408 (2017); http://doi.org/10.1063/1.4976708

27 28

8.3 Magnon-magnon coupling in YIG/Co heterostructures Authors: S. Klingler∗,1,2, S. Geprägs1, V. Amin3,4, M.D. Stiles3, H. Huebl1,2,5, R. Gross1,2,5, S.T.B. Goennenwein6,7 and M. Weiler1,2 Email: [email protected] Affiliations: [1] Bayerische Akademie der Wissenschaften, Walther- Meißner-Institut, Garching, Germany 2Technische Universität München, Physik-Department, Garching, Germany 3National Institute of Standards and Technology, Center for Nanoscale Science and Technology, Gaithersburg, Maryland, USA 4Maryland NanoCenter, University of Maryland, College Park, Maryland, USA 5Nanosystems Initiative Munich, Munich, Germany 6Technische Universität Dresden, Institut für Festkörperphysik, Dresden, Germany 7Technische UniversitätDresden, Center for Transport and Devices of Emergent Materials

The fields of spintronics and magnonics aim to employ the electron spin angular momentum as information carrier. In a magnetic material, spin angular momentum can be transported by spin waves. For the application in magnonic devices, the use of exchange dominated spin waves with wavelengths substantially below 1µm is desirable, as exchange-spin waves have isotropic dispersion relations and high group velocities. However, the excitation of exchange spin waves by a conventional microwave antenna requires lithography with a feature size comparable to the magnon wavelength. Here, we present a broadband ferromagnetic resonance (FMR) study of magnetization dynamics in unpatterned Yttrium Iron Garnet (YIG)/Cobalt (Co) thin film bilayers excited by a coplanar waveguide (CPW) with a center conductor width of about 100µm. Perpendicular standing spin waves (PSSWs) with wavelengths down to 50nm can be excited in the YIG film whenever the resonance conditions of the FMR of the Co film and the PSSW in the YIG coincide. We attribute the excitation of the PSSWs to a dynamic interfacial coupling mechanism, similar to that observed in [1] for an all-metallic system. This dynamic coupling of the YIG PSSWs to the Co FMR mode results in avoided crossing of the YIG PSSW and Co FMR dispersions. The coupling strength can be tuned by insertion of a Cu spacer in between the YIG and Co layers. We use a phenomenological model based on coupled Landau-Lifshitz- Gilbert equations to model our broadband magnetic resonance data and discuss the origin of the coupling in terms of field-like and damping-like interfacial torques. [1] Heinrich, B. et al. Phys. Rev. Lett. 90, 187601 (2003).

28 29

8.4 The ﬁnal chapter in the saga of YIG Authors: A. J. Princep∗,1, R. A. Ewings2, S. Ward3, S. T´th3, C. Dubs4, A. T. Boothroyd1 Email: [email protected] Aﬃliations: 1Department of Physics, University of Oxford, Clarendon Laboratory, Oxford OX1 3PU, United Kingdom. 2ISIS Facility, STFC Rutherford Appleton Laboratory, Har- well Campus, Didcot OX11 0QX, United Kingdom 3Laboratory for Neutron Scattering and Imaging, Paul Scher- rer Institut, CH-5232 Villigen, Switzerland. 4INNOVENT e.V., Technologieentwicklung, Pruessingstrasse. 27B, D-07745 Jena, GERMANY

The magnetic insulator Yttrium Iron Garnet can be grown with exceptional quality, has a ferrimagnetic transition temperature of nearly 600 K, and is used as the basis for microwave and spintronic devices that can operate at room temperature. The most accurate prior measurements of the spinwave spectrum date back nearly 40 years, but cover only 3 of the lowest energy modes out of 20 distinct magnon branches. Here we have used time-of-flight inelastic neutron scattering to measure the entire spinwave spectrum in a large number of different Brillouin zones with high momentum and energy resolution. We find that the existing model of the excitation spectrum, well known from an earlier work titled “The Saga of YIG”, fails to describe the optical magnon modes. These modes have a substantial thermal population at room temperature and must be taken into account to model, for example, the spin-Seebeck effect. Using a very general spinwave hamiltonian we show that the magnetic exchange interactions are both longer-ranged, and more complex, than was previously understood. This result provides the basis for accurate microscopic models of the finite temperature magnetic properties of Yttrium Iron Garnet, necessary for next-generation electronic devices.

29 30

9 Wednesday, Session IX: Spin current and magnons

9.1 Interaction between spin current and magnons: from magnon spin transport to black holes (invited) Authors: R.A. Duine Email: [email protected] Aﬃliations: Institute for Theoretical Physics, Utrecht University, and Physics of Nanostructures, Department of Applied Physics, Eindhoven University of Technology.

Spin currents carried by magnons have recently gained attention both for fundamental studies and for pursuing the goal of nearly dissipationless spin transport. In this talk I will discuss how magnons interact with electronic spin currents. In particular, I will discuss this interaction near interfaces between metals and magnetic insulators, and describe the resulting local and non-local transport phenomena. I will also discuss how spin currents can be used to implement black holes for magnons.

30 31

9.2 Phase-sensitive detection of inverse spin orbit torques at microwave frequencies (invited) Authors: M. Weiler1,2 Email: [email protected] Affiliations: 1Walther-Meißner-Institut, Bayerische Akademie der Wis- senschaften, Garching, Germany 2Physik-Department, Technische Universität München, Garching, Germany

Direct and inverse spin-orbit torques (SOT) at ferromagnet/normal-metal (FM/NM) interfaces with strong spin-orbit coupling allow for electrical control and detection of magnetization dynamics. The SOT scheme finds magnonic applications in the form of spin-torque oscillators and magnetic random access memory. Quantifying and understanding SOTs is thus key to developing scalable and efficient spintronic devices. Here, we employ a powerful microwave spectroscopy method for quantitative measurement of inverse SOTs in FM/NM bilayers by a vector network analyzer. Our method is contactless, self-calibrated and does not require any patterning of the samples. Owing to phase-sensitive detection and a quantitative evaluation scheme, we can disentangle field-like and damping-like contributions to the total torque. We extract the inverse spin-orbit torques in NiFe/Pt bilayers in a frequency range up to 40 GHz [1]. The total inverse SOT in these bilayers is compatible with our previous experimental findings using patterned devices and a complimentary spectroscopy technique [2]. In particular, we observe a large field-like contribution to the SOT, which opposes the voltage generated by Faraday’s law of induction in the NiFe/Pt bilayers. We attribute this field-like SOT to Rashba spin-orbit coupling at the NiFe/Pt interface. finally, we find evidence for the theoretically predicted [3] common origin of the Dzyaloshinskii-Moriya interaction (DMI) and Rashba SOTs in this material system by relating the extracted field-like SOT to our previously measured value for the interfacial DMI in very similar samples [4]. [1] Berger et al., arXiv: 1611.05798 (2016) [2] Weiler et al., Physical Review Letters 113, 157204 (2014) [3] Kim et al., Physical Review Letters 111, 216601 (2013) [4] Nembach et al., Nature Physics 11, 825 (2015)

31 32

9.3 Phase-resolved detection of spin-orbit torques by optical ferromagnetic resonance in ultrathin perpendicularly magnetized ﬁlms Authors: A. Capua∗,1−3, T. Wang2,4, S.H Yang2, C. Rettner2, T. Phung2, S. Parkin2,3 Email: [email protected] Aﬃliations: 1The Hebrew University of Jerusalem, Applied Physics De- partment, Jerusalem, Israel 2Almaden Research Center, IBM Research Division, San Jose, California, USA 3Max Planck Institute for Microstructure Physics, Halle, Ger- many 4Peking University, School of Physics, Beijing, China

Significant conversion of charge to spin current is made possible through the spin Hall effect (SHE). Following its discovery in semiconductors and metals, the conversion rate of charge currents into spin-polarized currents, known as the spin Hall angle, was studied extensively using various techniques such as the cavity ferromagnetic resonance (FMR), spin pumping, electrically and optically sensed spin-torque FMR, anomalous and planar Hall effect and by nonlocal spin transport. A majority of these techniques are limited to metallic ferromagnets that display magnetoresistance except for cavity FMR which has limited sensitivity for measurement of ultrathin magnetic layers. While generally the characterization of the spin Hall angle is not straightforward, its quantification in the presence of the considerable magnetic anisotropy adds difficulties of its own.Here we demonstrate a hybrid phase-resolved optical-electrical FMR (OFMR) method that we show can be used to reliably quantify the spin Hall angle in heavy-metal/ultrathin-ferromagnet bilayer systems exhibiting significant magnetocrystalline anisotropy. We derive a general analytical model for the ferromagnetic resonance spectrum in the presence of spin current. Our model and the experimental results readily illustrate that the amplitude information is more sensitive to the spin currents induced by the spin-orbit torques compared to the linewidth information. Moreover, we demonstrate a remarkable enhancement of the sensitivity to the injected spin current when the measurements are conducted under the frequency softening conditions often referred to as the “Smit point” for which the magnetic potential is shallow and the magnetic stiffness is minimal. The principles of our experiments will serve to unveil the underlying physics of spin orbit torques (SOT).

32 33

10 Wednesday, Session X: Spin-torque oscillators and magnetization dynamics I

10.1 Imaging the magnetization dynamics of nano-contact spin-torque vortex oscillators (invited) Authors: P.S. Keatley∗,1, S.R. Sani2−4 , G. Hrkac5, S.M. Mohseni6,P D¨urrenfeld7, T.H.J. Loughran1, J. Akerman˚ 2,3,7, R.J. Hicken1 Email: [email protected] Aﬃliations: 1University of Exeter, Department of Physics and Astronomy, Exeter, United Kingdom 2KTH Royal Institute of Technology, Materials and Nano Physics, School of ICT, Kista, Sweden 3NanOsc AB, Kista, Sweden 4Uppsala University, Department of Physics & Astronomy, Uppsala, Sweden 5University of Exeter, College of Engineering, Mathematics and Physical Science, Exeter, United Kingdom 6Shahid Beheshti University, Department of Physics, Tehran, Iran 7University of Gothenburg, Physics Department, Gothenburg, Sweden

The operation of nano-contact (NC) spin-torque vortex oscillators (STVOs) is underpinned by vortex gyration in response to spin torque delivered by high density current passing through the magnetic layers of a spin valve. Gyration directly beneath the NC yields radio frequency (RF) emission through the giant magnetoresistance (GMR) effect, which can be readily detected electronically. The magnetization dynamics that extend beyond the NC perimeter contribute little to the GMR signal, but are crucial for synchronization of multiple NC-STVOs that share the same spin valve film. In this work time-resolved scanning Kerr microscopy (TRSKM) was used to directly image the extended dynamics of STVOs phase-locked to an injected RF current. In this talk the dynamics of single 250-nm diameter NCs, and a pair of 100-nm diameter NCs, will be presented. In general the Kerr images reveal well-defined localized and far-field dynamics, driven by spin-torque and RF current Oersted fields respectively. The RF frequency, RF Oersted field, direction of an in-plane magnetic field, and equilibrium magnetic state, all influenced the spatial character of the dynamics observed in single NCs. In the pair of NCs, two modes were observed in the RF emission. Kerr images revealed that a vortex was formed beneath each NC and that the mode with enhanced spectral amplitude and line quality appeared to be correlated with two localized regions oscillating with similar amplitude and phase, while a second weaker mode exhibited amplitude and phase differences. This suggests that the RF emission was generated by collective modes of vortex gyration dynamically coupled via magnetization dynamics and dipolar interactions of the shared magnetic layers. Within the constraints of injection locking, this work demonstrates that TRSKM can provide valuable insight into the spatial character and time-evolution of magnetization dynamics generated by NC-STVOs and the conditions that may favor their synchronization.

33 34

10.2 Nonlinear magnetization dynamics in YIG thin films and nanodiscs (invited) Authors: Grégoirede Loubens Email: [email protected] Affiliations: SPEC, CEA, CNRS, UniversitéParis-Saclay, France

The equation of motion of magnetization is strongly nonlinear, yielding a series of interesting phenomena. For instance it is well known that in ferromagnetic resonance of extended films, where one uses a microwave field to pump the spin system at a particular frequency, spin- wave instabilities quickly develop as the excitation power is increased, preventing to achieve large angle of uniform precession. Due to its unmatched low damping, YIG has been for long the material of choice to investigate the deeply nonlinear regime. In this talk, I will discuss two different experiments, allowed by the recent development of ultra-thin YIG films of high dynamical quality. In the first one, we perform nonlinear ferromagnetic resonance in the out- of-plane configuration of individual YIG nanodiscs with thickness 20 nm and varying diameters below 1 µm. Due to the geometric confinement, spin-wave modes are highly quantized, and the instabilities described above are far less effective. As a result, a very large foldover of the main resonance line can be observed: for relatively weak microwave excitation fields, in the mT range, the nonlinear field shifts are as large as 0.15 T, corresponding to a reduction of the longitudinal magnetization by up to 80%. In the second one, we report on the nonlinear regime of spin transport in a 18 nm thick extended film of YIG. Propagating spin-waves are generated and detected using direct and inverse spin Hall effects in two Pt wires deposited on top of the YIG film. The non-local resistance measured in the lateral devices clearly deviates from a linear behavior above a current density in the Pt of about 5 · 1011 A/m2, where the dissipation of long wavelength spin-wave modes is compensated. The data provides evidence for a gradual spectral shift from thermal to subthermal magnon transport as spin orbit torque is increased. I would like to thank all my colleagues of CEA Saclay, CEA Grenoble, CNRS/Thales, CNRS/UBO, and Uni. Münsterwho contributed to this work.

34 35

10.3 Antiferromagnetic spin-Hall oscillator of THz-frequency radiation Authors: V. Tyberkevych∗,1, R. Khymyn2, I. Lisenkov3,4, O. Sulymenko5, O. Prokopenko5, B. Ivanov5,6, and A. Slavin1 Email: [email protected] Aﬃliations: 1Oakland University, Department of Physics, Rochester, MI, USA 2University of Gothenburg, Department of Physics, Gothen- burg, Sweden 3Oregon State University, Department of Electrical Engineer- ing and Computer Science, Corvallis, OR, USA 4Kotelnikov Institute of Radio-Engineering and Electronics of RAS, Moscow, Russia 5National Taras Shevchenko University of Kyiv, Department of Radiophysics, Kyiv, Ukraine 6Institute of Magnetism, National Academy of Sciences of Ukraine, Kyiv, Ukraine

An absence of compact and reliable generators and receivers of coherent signals in the THz- frequency range has been identified as a fundamental physical and technological problem, which restricts widespread use of THz technology. Here, we demonstrate theoretically that a bi-layer structure consisting of a heavy metal (e.g., Pt) and a bi-axial antiferromagnetic (AFM) dielectric (e.g., NiO) can operate as a tunable source of coherent THz-frequency signals. The structure is driven by a DC electric current flowing in the Pt layer, which creates a flow of spin current into the adjacent AFM layer via the spin Hall effect. When the spin-polarization of the spin current is parallel to the AFM hard axis and its magnitude is large enough, the spin current excites precession of the magnetization sublattices in the AFM. The frequency of the AFM precession is determined by the balance of spin torque and damping and can be tuned from sub-THz to several THz by varying the density of the driving DC current. The output THz signal can be picked up either by the inverse spin Hall effect, or, in a case of a canted AFM with non-zero net magnetization, by a resonant electromagnetic structure. In the first case, the output electric signal is proportional to the weak AFM anisotropy in the easy plane of the AFM and vanishes for AFM materials with pure uniaxial anisotropy. The theoretical estimations for a Pt/NiO bi-layer show that the generated frequency of the proposed AFM oscillator can be tuned from 0.1 to 2.0 THz with the driving current in the Pt layer from 108 A/cm2 to 109 A/cm2. The output power in this frequency range varies from 1.5 µW to 40 nW, which is comparable to the output power of cryogenic THz-frequency oscillators based on superconducting Josephson junctions. These characteristics make the proposed AFM device a promising candidate for a room-temperature tunable oscillator for future THz electronics.

35 36

10.4 Droplet solitons in magnetic nanowires (invited) Authors: M. Ahlberg∗,1, M. Ranjbar1,2, P. D¨urrenfeld1, S. M. Mohseni3, S. Chung4, S. R. Sani4, S. N. Piramanayagam2, J. Akerman˚ 1,4 Email: [email protected] Aﬃliations: 1University of Gothenburg, Department of Physics, Gothen- burg, Sweden 2Nanyang Technological University, School of Physical and Mathematical Sciences, Singapore 3Shahid Beheshti University, Department of Physics, Tehran, Iran 4KTH Royal Institute of Technology, Materials Physics, School of ICT, Kista, Sweden

The magnetic droplet is a localized excitation found in uniaxial ferromagnets where a polarized current provides sufficient spin transfer torque to counteract the inherent damping. This dis- sipative soliton was first predicted in 2010 [1] and experimentally verified a couple of years later using nanocontact spin torque oscillators (NC-STO) [2]. The droplet is created underneath the contact and consists of a reversed core where the spins precess at angles almost antiparallel to the initial state. Once formed, it can propagate away from the NC. It has been shown that this soliton is a necessary precursor to skyrmion injection in race track memories [3], but also that the boundaries of a confined geometry can attract a droplet and transform it to an edge mode or a quasi-1D droplet, depending on the distance from the NC to the edge [4]. The transport of droplets through a nanowire (NW) will thus be influenced by its width. We have fabricated STO:s with nanocontacts placed on top of nanowires made of orthogonal spin valves. The magnetodynamics of a 200 nm wide nanowire reveals two distinct drops in the frequency - one at a field of about 0.4 T and a second at 0.75 T. A shift to a lower frequency is consistent with a larger footprint of the droplet and we identify the first enlargement as a conversion to an edge mode, while the second transition is attributed to the formation of a quasi-1D droplet. This conclusion is corroborated by micromagnetic simulations. Our results not only serve as an experimental verification of the theoretically predicted modes, but also open up a route for studies of droplets in nanostructures. [1] M. A. Hoefer, T. J. Silva, and Mark W. Keller (2010). Phys. Rev. B. 82, 054432 [2] S. M. Mohseni, et al. (2013). Science 339, 1295 [3] J. Sampaio, V. Cros, S. Rohart, A. Thiaville, and A. Fert (2013). Nat. Nanotechnol. 8, 839 [4] E. Iacocca, et al. (2014). Phys. Rev. Lett. 112, 047201

36 37

11 Thursday, Session XI: Quantum magnonics II

11.1 Nonlinear interactions in a magnonic Bose-Einstein condensate (invited) Authors: I. Lisenkov1,2, V. Tiberkevich1, S. Demokritov3,4, A. Slavin∗,1 Email: [email protected] Aﬃliations: 1Oakland University, Department of Physics, Rochester, Michigan, 48309, USA 2Oregon State University, Department of Electrical Engineer- ing, Corvallis, Oregon, 97331, USA 3University of Muenster, Institute for Applied Physics, Muen- ster, 48149 Germany 4Institute of Metal Physics, Ural Division of RAS, Yekaterin- burg, 620041, Russia

Stability of a Bose-Einstein condensate (BEC) is determined by the character of the nonlinear interaction between the particles or quasi-particles forming a BEC. If the interaction between the quasi-particles is attractive - an untrapped BEC experiences explosive collapse [1]. The untrapped BEC of magnons (mBEC) at room temperature has been observed experimentally in in-plane magnetized yttrium-iron garnet (YIG) films in [2], and was found to be stable, in spite of the fact that the interaction between the magnons in in-plane magnetized magnetic films (backward volume magnetostatic wave geometry (BVMSW) [2]) is attractive. To resolve this contradiction we propose here a mechanism of mBEC collapse avoidance which involves interaction between the two mBECs having opposite wave vectors. It is well known that in the spectrum of BVMSWs in an in-plane magnetized magnetic film there are two minima of energy corresponding to the opposite wave vectors kBEC and −kBEC, and, therefore, two mBECs having opposite wave vectors are formed at the same spatial location in a YIG film by a microwave pumping. It is demonstrated that the interaction between the magnons having the same wave vectors is, indeed, attractive, but near the energy minima it is negligibly small, so that the magnon gas, existing near each particular energy minimum, can be considered approximately ideal. In contrast, the interaction between the magnons having the opposite wave vectors (kBEC and −kBEC) is two orders of magnitude higher and repulsive. Thus, the system of the two mBECs with opposite wave vectors is a mixture of two nearly ideal magnon gases with a strong mutual repulsive interaction, and, therefore, is immune from an explosive collapse. These theoretical conclusions are supported by the results of an experiment performed using the advanced Brillouin light scattering technique and demonstrating a positive nonlinear frequency shift of the mBEC. [1] F. Dalfovo, S. Giorgini, L. P. Pitaevskii, and S. Stringari, Rev. Mod. Phys. 71, 463 (1999). [2] S. O. Demokritov et al., Nature 443, 430-433 (2006).

37 38

11.2 Spin transport by magneto-elastic bosons in yttrium-iron-garnet films Authors: A.A. Serga∗,1, D.A. Bozhko1, G.A. Melkov2, V.S. L’vov3, A. Pomyalov3, and B. Hillebrands1 Email: [email protected] Affiliations: 1Technische UniversitätKaiserslautern, Fachbereich Physik and Landesforschungszentrum OPTIMAS, Kaiserslautern, Germany 2Taras Shevchenko National University of Kyiv, Faculty of Radiophysics, Electronics and Computer Systems, Kyiv, Ukraine 3Weizmann Institute of Science, Department of Chemical Physics, Rehovot, Israel

With the fast growth in the volume of information being processed, researchers are charged with the task of finding new ways for fast and energy efficient computing. The use of magnons allows the implementation of wave-based computing technologies free from the drawbacks inherent to modern electronics, such as Ohmic losses [1]. Moreover, macroscopic magnetic quantum phenomena, like magnon condensates [2] and supercurrents [3], offer additional opportunities for processing and transfer of data. Recently we have discovered a novel condensed magnon state, which occurs in a spectral point whose position is determined by the passage from the magnon to the phonon spectral branches in yttrium-iron-garnet films [4]. As opposed to the classical magnon BEC, the condensed magneto-elastic quasiparticles possess significantly nonzero group velocities and, thus, can be directly used for data transfer purposes. According to the developed theory and in contrast to a BEC this condensation occurs in a narrow but finite spectral region. The possibility of a coherent dynamics of magneto-elastic quasiparticles constitutes an intriguing problem of fundamental importance for physics in general and for forthcoming magnonic applications. An evidence in favor of such a coherent state is given by our direct measurements of spin transport: we observed two separate magnon-phonon groups with velocities of 200 and 1100 m/s. The faster group could be associated with a coherent bosonic state separated by a narrow energy gap from the gaseous cloud of accumulated quasiparticles. The support of DFG within the SFB/TR 49 is acknowledged. [1] A.V. Chumak, V.I. Vasyuchka, A.A. Serga, B. Hillebrands, Nat. Phys. 11, 453 (2015) [2] S.O. Demokritov et al., Nature 443, 430 (2006) [3] D.A. Bozhko et al., Nat. Phys. 12, 1057 (2016) [4] D.A. Bozhko et al., arXiv: 1612.05925 (2017)

38 39

11.3 Spin superﬂuidity in antiferromagnetic insulators Authors: A. Qaiumzadeh∗, H. Skarsvg, C. Holmqvist, and A. Brataas Email: [email protected] Aﬃliations: Norwegian University of Science and Technology (NTNU), Department of Physics, Trondheim, Norway

Achieving long-range spin transport is essential in spintronics. In metals, conduction electrons can carry spin information. The spin-diffusion length is generally less than a few hundred nanometers and often as short as a couple of nanometers. However, in magnets, there are additional transport channels via spin excitations, typically in the form of spin waves. In magnetic insulators, the absence of noisy itinerant carriers implies less dissipation but still the spin diffusion length decays exponentially. In this talk, I introduce a new way for spin transport through the so-called spin superfluidity in magnetic insulators in which the spin currents decays algebraically and consequently the spin transport can persist across many micrometers, even in dirty samples.

39 40

12 Thursday, Session XII: Spin-torque oscillators and magnetization dynamics II

12.1 Spin transfer due to zero-point magnetization ﬂuctuations (invited) Authors: S. Urazhdin∗, A. Zholud, and R. Freeman Email: [email protected] Aﬃliations: Emory University, Department of Physics, Atlanta, GA, USA

Most of the phenomena in magnetism are well described by the semiclassical approximation for the magnetization, while quantum effects such as magnetization tunneling are limited to molecular systems at cryogenic temperatures. For example, the dynamical states of magnetization induced by spin-polarized electric currents due to the spin transfer effect are generally described by the classical vector fields, even though the scattered electrons are treated quantum-mechanically. I will present magnetoelectronic data for the “standard” magnetic multilayer nanopillars demonstrating that quantum zero-point magnetization fluctuations provide a dominant contribution to spin transfer at cryogenic temperatures. The net effect of quantum fluctuations is large because the entire spectrum of the dynamical magnetic modes is involved in spin transfer. The demonstrated quantum spin transfer results in a singular piecewise-linear dependence of the dynamical magnetic energy on current, qualitatively distinguishing it from the previously established “classical” spin transfer effect characterized by a smooth dependence. I will present a toy model of quantum spin transfer that provides a remarkable agreement with the data, and predicts that the quantum effects remain non-negligible even at room temperature. I will also report thermal broadening effects at increased temperatures, indicative of the importance of the orbital energy of scattered electrons, which until now has been neglected in the analysis of spin transfer phenomena. I will present additional evidence for such energy dependence based on the electronic point contact spectroscopy of magnetic multilayers. finally, I will discuss the implications of the demonstrated quantum phenomena for our understanding of current-induced dynamics and spin transport in nanomagnetic systems.

40 41

12.2 Spin wave modes in ferromagnetic nano-disks, their excitation and auto-oscillations Authors: D. Mancilla-Almonacid and R. Arias∗ Email: [email protected] Aﬃliations: Universidad de Chile, Departamento de F´ısica, Santiago, Chile

The excitation of the linear spin wave modes of a soft ferromagnetic free layer of a nano-pillar structure through dc-ac currents that traverse the structure is studied by different means, in configurations magnetised in-plane (IP) and out of plane (OP). We also study the stability of a uniform auto-oscillation that has attained a large amplitude (IP) determining the thresholds in parameter space of exponential linear growth of non uniform modes. There is interest in understanding the magnetization dynamics in these structures since they may be used as microwave sources when these nano-oscillators enter into auto-oscillatory regimes. The free layer is a soft ferromagnet, like Permalloy, in the shape of a circular disk, with a very small thickness in the range of the exchange length. Using a description of the magnetization dynamics in terms of a Hamiltonian for weakly interacting waves we determine the spin wave modes of the structure and their excitation under two approximations: a very thin film limit, and under a model that includes the effect of the full magnetostatic interaction. DC-AC currents traverse this structure, become spin polarized by a fixed layer and excite the magnetization dynamics of the free layer through the transfer of spin angular momentum. At critical values of the DC current the spin transfer torque, that has an anti-damping effect, starts auto-oscillation regimes of different spin wave modes. For dc currents below the critical value we determined the ac current threshold for auto-oscillations under ac parametric resonance excitation: we find that an out of plane component of spin transfer is necessary for low ac current thresholds. Our study of the large auto-oscillation of a quasi-uniform mode reveals that a practical applied magnetic field threshold of operation in this IP configuration is approximately πMs, since over this field there is a high sensitivity to changing current and instabilities develop promptly.

41 42

12.3 Inﬂuence of interlayer coupling on spin torque driven excitations in nanopillar structures (invited) Authors: M. Romera1, B. Lacoste2, N. Monteblanco1, F. Garcia- Sanchez1, A. Jenkins1, A. Purbawati1, J. Hem1, D. Gusakova1, L. D. Buda-Prejbeanu1, B. Dieny1, U. Ebels∗,1 Email: [email protected] Aﬃliations: 1Univ. Grenoble Alpes, CEA/INAC - SPINTEC, CNRS - SPINTEC, F-38000 Grenoble, France 2International Iberian Nanotechnology Laboratory, Braga, Portugal

Spin torque driven auto-oscillations of nanopillar spin valve or tunnel junction structures are described in most models by considering the oscillations of the free layer only, neglecting coupling (such as dipolar or via spin momentum transfer) to the polarizing layer. However to understand excitation spectra of real devices, these interactions need to be taken into account. Here we summarize our recent experimental, simulation and analytical studies on the influence of coupling on the microwave emission properties such as the non-linear frequency shift with current or the linewidth. Different configurations are considered: a synthetic ferrimagnet without and with an external polarizer and the coupling of the free layer to a synthetic antiferromagnetic polarizer. These studies will be important to understand experimental results and to define configurations of improved microwave performances.

42 43

12.4 Brain-inspired computing using the transient dynamics of spin- torque oscillators Authors: M. Riou∗,1, J. Torrejon1, F. Abreu Araujo1, G. Khalsa2, M. Stiles2, S. Tsunegi3, A. Fukushima3, H. Kubota3, S. Yuasa3, D. Querlioz4, V. Cros1, J. Grollier1 Email: [email protected] Aﬃliations: 1CNRS/Thales Palaiseau, France 2NIST, Center for Nanoscale Science and Technology, Gaith- ersburg, USA 3 AIST, Spintronic Research Center, Tsukuba, Japan 4Univ. Paris-Sud, Institut d’Electronique´ Fondamentale, Or- say, France

For many tasks such as face or speech recognition, the brain processes information in a much faster and much more power efficient way than any computer. Neurons in the brain behave as non-linear oscillators, which develop rhythmic activity and interact to process information[1]. Developing a chip which takes inspiration from this behavior would require a huge amount of interacting oscillators. Therefore very small, energy efficient and stable non-linear oscillators are needed. Here we show for the first time that complex pattern recognition tasks can be performed experimentally with a nanoscale oscillator[2]. We use the transient dynamics of a single vortex spin-torque oscillator to emulate the behavior of a whole neuron network. The oscillator is excited by an input signal corresponding to the data to process, and the resulting voltage variations are recorded. The separation of different patterns in the input waveforms is allowed by the intrinsic memory and non-linearity of the oscillator magnetization dynamics. The classification is performed by sampling the recorded time traces and linearly recombining these data points in a single output. By tuning the applied current or magnetic field, we were able to tune the signal over noise ratio and optimize the recognition performances. We performed successfully speech recognition task, recognizing digits said by 5 different speakers with a state of the art success rate of 99.6%. This work demonstrates that spin-torque oscillators can emulate collections of neurons and are stable enough to achieve complex cognitive tasks. Coupled with their nanometric size, their low power consumption and their ability to interact, it opens the path to building energy efficient brain-inspired chip with dense array of interconnected spin torque oscillators. [1] G. Buzsaki, Rhythms of the Brain. (OUP USA, 2011) [2] J. Torrejon, M. Riou et al., ArXiv : 1701:07715 (2017)

43 44

13 Thursday, Session XIII: THz magnonics

13.1 Polaritons and eﬀects of propagation in terahertz magnonics Authors: R. V. Mikhaylovskiy∗,1, K. A. Grishunin2, G. Li1, Th. Rasing1 and A. V. Kimel1,2 Email: [email protected] Aﬃliations: 1Radboud University, Institute for Molecules and Materials, Nijmegen, Netherlands 2Moscow Technological University (MIREA), Moscow, Russia

The antiferromagnetic materials appeal to spintronics and magnonics because of their very high (terahertz) frequencies of spin dynamics and unique functionalities in comparison to conventional ferromagnets [1]. Recently the freely propagating terahertz electromagnetic radiation has been suggested as the most direct interface to the antiferromagnets [2] able to detect [3] and control [4] spin motion in them. Here we show that due to the strong coupling of the propagating THz electromagnetic fields with magnons, the hybrid magnon-polariton modes start to play a significant role. For instance, by measuring the terahertz emission from an archetypical antiferromagnet TmFeO3 we found a clear beating between the frequencies just below and above the frequency of antiferromagnetic resonance in this crystal. Our theoretical analysis indicates that the beating arises due to the energy exchange between the higher and lower polariton branches formed in vicinity of the antiferromagnetic magnon frequency. Our finding opens new opportunities for spin manipulation bringing the fields of magnonics and polaritonics together. [1] T. Jungwirth, et al. Nature Nanotech. 11, 231 (2016). [2] T. Kampfrath, et al. Nature Photon. 5, 31(2011). [3] R. V. Mikhaylovskiy, et al. Nature Comm. 6, 8190 (2015). [4] S. Baierl, et al. Nature Photon. 10, 715 (2016).

44 45

13.2 Terahertz magnons above the Curie temperature Authors: Kh. Zakeri∗,1,2, H.J. Qin2, A. Ernst2, and J. Kirschner2 Email: [email protected] Aﬃliations: 1Karlsruhe Institute of Technology, Heisenberg Spin-dynamics Group, Physikalisches Institut, Karlsruhe, Germany 2Max-Planck-Institut f¨urMikrostrukturphysik, Halle, Ger- many

Utilizing spin-polarized high resolution electron energy loss spectroscopy we investigate the temperature dependence of high-energy (terahertz) magnons, excited in an ultrathin ferromagnet. Both the energy and lifetime of terahertz magnons are measured as a function of temperature and across the magnetic transition temperature TC. Similar to the magnons’ energy, their lifetime decreases with temperature. The observed temperature-induced damping of terahertz magnons is discussed in terms of the multi-magnon scattering mechanism. Our results indicate that the damping resulted from this mechanism can be comparable to the intrinsic Landau damping of the system. We demonstrate that although at TC terahertz magnons are affected by different damping mechanisms, they still behave as well-defined collective excitations. We argue that the effects associated with the collective properties of terahertz magnons should sustain at TC and even beyond that. For instance the spin-Seebeck effect can eventually exist also in the paramagnetic state, where the long range magnetic order is absent. This means that spin caloritronics is not restricted to the ferromagnetic materials. Our results shall bring the topics of “paramagnonics” and “spin caloritronics using paramagnets” into discussion.

45 46

13.3 Generation of THz-frequency signals using current-driven canted antiferromagnets Authors: R.Khymyn1, O. Prokopenko2, O. Sulymenko∗,2,V. Tyberkevych3, and A. Slavin3 Email: [email protected] Aﬃliations: 1University of Gothenburg, Department of Physics, Gothen- burg, Sweden 2Taras Shevchenko National University of Kyiv, Department of Radio Physics, Electronics and Computer Systems, Kyiv, Ukraine 3Oakland University, Department of Physics, Rochester, MI, USA

A possibility to use canted antiferromagnets driven by DC current to generate coherent THz- frequency signals is studied theoretically. A novel THz-frequency source of coherent signals with power exceeding 1µW based on a bi-layer of a canted antiferromagnet (AFM) and Platinum (Pt) attached to a high-Q dielectric resonator is proposed.The proposed THz-frequency signal generator consists of a α-F e2O3 (Hematite)/Pt bi-layered structure placed into a THz-frequency resonator. The magnetization vectors M1, M2 of the AFM sublattices are canted inside the AFM easy plane by a bulk Dzyaloshinskii-Moriya interaction, creating a small net magnetization MDMI . When a DC electric current flows in the Pt layer it injects (due to the spin-Hall effect) a pure spin current into the adjacent AFM layer. If the spin current is polarized perpendicular to the easy plane of the AFM, the spin-transfer torque created by the spin current lifts the magnetizations M1 and M2 out of the AFM easy plane. M1 and M2 along with MDMI , start to rotate in the strong internal exchange field existing inside the AFM layer, and this rotation is modulated by the relatively weak anisotropy existing in the direction perpendicular to the easy-plane of the AFM. This non-uniform rotation creates an AC spin pumping back into the Pt layer, as well as the magnetic dipolar radiation caused by the rotation of MDMI , which can be extracted using an adjacent high-Q resonator.Several types of resonators (rectangular, microstrip, dielectric) were considered, and it was obtained that for a 5 nm thick Hematite disk of the 10 µm radius attached to a high-Q dielectric resonator the estimated generated AC power could reach 1 µW at the frequency of 0.5 THz.