York-Tohoku-Kaiserslautern Research Symposium on New-Concept Spintronics Devices. Abstract Book June 2017

Transcription

1 York-Tohoku-Kaiserslautern Research Symposium on New-Concept Spintronics Devices Abstract Book June 2017 Departments of Physics and Electronics, University of York

2 Session 1: Magnonics and Spin Dynamics Wednesday, 21 st June 09:30-11:00 Chair: Gonzalo Vallejo-Fernandez Prof. Burkard Hillebrands (Technical University of Kaiserslautern) "Physics and applications of focused spin-wave beams and caustics" Dr. Stuart Cavill (University of York) "Effect of symmetry on strain induced vortex core translation" Prof. Evangelos Papaioannou (Technical University of Kaiserslautern) "Efficient spintronic terahertz emitters based on expitaxially grown Fe/Pt bilayers"

3 Physics and applications of focused spin-wave beams and caustics F. Heussner, P. Pirro, A. A. Serga, and B. Hillebrands Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern, Kaiserslautern, Germany Currently, the application of magnons, the quanta of spin waves (SW), in wave-based logic networks is widely discussed due to its potential as CMOS complementary or even subsequent technology with extended functionality and improved performance [1]. The advantages arise from the possibility to use the phase as an additional degree of freedom. By utilizing interference effects, footprints of logic devices can be reduced, which allows for the realization of innovative and energy efficient ways of information processing. An instructive example of the unique possibilities to realize novel concepts for data processing is the transport of SW encoded information in unstructured 2D media via focused SW beams and caustics. These beams occur due to the anisotropic nature of the spin-wave dispersion relation, which can lead to (nearly) parallel directions of the group velocity of spin waves excited with a broad angular wave-vector spectrum. This provides outstanding design opportunities such as beams with sub-wavelength narrow apertures [2]. The excitation of such beams can be realized by opening of a 1D waveguide into a 2D medium, or via nonlinear emission from localized edge modes [3]. In the presentation, we will discuss and experimentally demonstrate the generation mechanisms of focused spin-wave beams and caustics. Subsequently, we will show how these focused beams can be employed to build pivotal devices for magnonic data processing circuits. Via micromagnetic simulations, we study the concept of a frequency-division (de-)multiplexer for spin waves, promising a multiplication of the throughput in magnonic networks. In addition, the design of a switchable SW signal splitter will be presented, which is exploiting the strong dependence of the direction of energy flow on the local magnetic field direction. [1]A. V. Chumak, V. I. Vasyuchka, A. A. Serga, and B. Hillebrands, Nat. Phys. 11, 453 (2015). [2] T. Schneider, A. A. Serga, A.V. Chumak, C.W. Sandweg, S. Trudel, S. Wolff, M. P. Kostylev, V. S. Tiberkevich, A. N. Slavin, and B. Hillebrands, Phys. Rev. Lett. 104, (2010). [3] T.Sebastian,T. Brächer, P. Pirro, A. A. Serga, B. Hillebrands, T. Kubota, H. Naganuma, M. Oogane, and Y. Ando, Phys. Rev. Lett. 110, (2013).

4 Manipulation of magnetic vortex cores in planar magnetic materials using a voltage induced strain I. Azaceta 1 and S.A.Cavill 1,2 1Department of Physics, University of York, Heslington, York, YO10 5DD, UK Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK 2 The high stability of magnetic vortices makes them very interesting for applications in magnetic resonators and magnetic memories. Such textures can be found in square planar microstructures of Galfenol, which exhibit the Landau flux-closure state due to the lateral confinement. It is already known that the polarization of a magnetic vortex can be switched by means of a dynamic field [1], leading to the possibility of using vortices for data storage applications. Moreover, the magnetic vortex adds a gyrotropic mode to these planar structures, which can be used as a source of microwave signals in spin-torque vortex oscillators (STVOs) [2]. Manipulation of the core using external magnetic fields has the drawback of high power dissipation and as an alternative, voltage induced strain has been shown to modify the magnetic domain pattern in structures containing vortex cores [3]. However the core position remains fixed due to the symmetry of the strain induced anisotropy. Here we present an investigation on the effects of inducing a strain gradient on micron-sized Galfenol structures containing vortex cores. Unlike previous studies [3] we demonstrate the ability to displace the vortex core itself without the need of magnetic fields, see Fig.1. FIG.1. z component of the magnetization in a simulated 2 µm square Galfenol structure for a) dks/dy = 0 and b) dks/dy = 30 (kj/m4), showing the displacement of the vortex core when a strain induced anisotropy gradient is applied along the [100] direction. [1] Matthias Kammerer et al. Nat. Commun., 2, 277 (2011) [2] P. S. Keatley et al. Phys. Rev. B 94, (2016) [3] D. E Parkes et al. Appl. Phys. Lett. 105, (2014)

5 Efficient Spintronic Terahertz Emitters based on Epitaxial Grown Fe/Pt Layer Structures S. Keller 1, G. Torosyan 2, L. Scheuer 1, B. Hillebrands 1, R. Beigang 1,2, E. Th. Papaioannou 1 1 University of Kaiserslautern, Research Center Optimas, Kaiserslautern, 67663, Germany 2 Photonic Center Kaiserslautern, Kaiserslautern, 67663, Germany Spin Hall effect and its reciprocal, the inverse spin Hall effect (ISHE) provide the means for conversion between the spin and charge currents. Recently, the decisive role of the ISHE effect on extending the field of spintronics in the terahertz regime was revealed [1, 2]. THz spintronics has the potential of application in ultra-fast current and computer technologies [3]. We report on efficient generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The optimized emitter provided a bandwidth of up to 8 THz. Low average pump powers have been used for terahertz generation. The latter and the general performance make the spintronic Fe/Pt terahertz emitter compatible with established emitters based on nonlinear generation methods. [1] T. Seifert et al., Efficient metallic spintronic emitters of ultrabroadband terahertz radiation. Nat Photon 10, (2016). [2] T. Kampfrath et al., Terahertz spin current pulses controlled by magnetic heterostructures. Nat. Nano 8, (2013). [3] J. Walowski & M. MŸnzenberg, Perspective: Ultrafast magnetism and THz spintronics. Journal of Applied Physics 120, (2016).

6 Session 2: Magnetic Recording Wednesday, 21 st June 11:30-13:00 Chair: Keith McKenna Dr Sameh Hassan (Seagate Technology) "Technology future of HDD industry"-abstract not available Prof. Markus Meinert (University of Bielefeld) "Exchange Bias with antiferromagnetic MnN" Dr Andrew Grier (Seagate Technology) "HDD design for heat assisted magnetic recording"-abstract not available

7 Exchange Bias with antiferromagnetic MnN Markus Meinert, Bielefeld University, Germany Exchange bias, the shift of the magnetization loop observed in ferromagnet / antiferromagnet bilayers, plays a vital role for spintronic devices, as it provides a means the fix the direction of a ferromagnetic layer s magnetization, which can be used as a reference layer in spin valves. We recently discovered that stacks of Ta / MnN / CoFe / TaOx can provide exchange bias values up to 1800 Oe at room temperature, with very little effort in preparation. As a proof-of-principle, we demonstrated a giant magnetoresistance multilayer system exchange biased with MnN. Detailed investigations of thickness and temperature dependencies showed that the blocking temperature of MnN is around 160 C and that the critical thickness for room temperature exchange bias is 10nm, both of which limit possible applications to cases, where ultimate down-scaling is not mandatory, but the use of expensive antiferromagnets like IrMn or PtMn shall be avoided. In further experiment, we prepared stacks of Ta / MnN / CoFeB / MgO / Ta with perpendicular magnetization. Also in the perpendicular case, we obtained significant exchange bias with a maximum of 3600 Oe at room temperature for a CoFeB thickness of 0.65nm. This constitutes the largest ever reported value for a perpendicularly magnetized exchange bias system. An intimate relation between exchange bias and perpendicular anisotropy field was found in detailed film thickness and annealing temperature studies.

8 Session 3: New Materials for Spintronics Wednesday, 21 st June 14:00-15:30 Chair: Seiji Mitani Prof. Koki Takanashi (Tohoku University) "Advanced spintronic materials based on ordered alloys" Dr. Vlado Lazarov (University of York) The role of defects on functionality of Heusler based heterostructures Dr. Andrew Pratt (University of York) "Effect of molecular orientation on spin transport in hybrid magnetic tunnel junctions"

9 Advanced spintronic materials based on ordered alloys Koki Takanashi Institute for Materials Research & Center for Spintronics Research Network, Tohoku University Materials used for spintronic devices should satisfy the following requirements like high spin polarization, leading to high efficiency in spin injection and high magnetoresistance, high magnetic anisotropy, leading to perpendicular magnetization and thermal stability of magnetization at reduced dimension and proper damping constant, leading to the optimization of the influence of spin transfer torque. Ordered alloys are promising for the application to spintronics, because some of them show excellent functionalities such as high spin polarization and high magnetic anisotropy. Our group has been working on half-metallic Heusler alloys with high spin polarization, and demonstrated high CPP-GMR [1-3], which will be promising for the application to read heads in next-generation HDD. CPP-GMR devices with half-metallic Heusler alloys also show high performance as spin torque oscillators (STOs) [4-6] because of their low magnetic damping. The present talk rather focuses on antiferromagnetic Heusler alloys than half-metallic ones. Among a plenty of Heusler alloys with a chemical formula of X 2 YZ, we can choose noble metal free antiferromagnets which may replace a commonly used antiferromagnet, Ir-Mn, used for the exchange bias in spin valves. The material development for replacing Ir-Mn is an important issue because Ir is one of the scarcest elements on the earth. We have demonstrated the exchange bias using antiferromagnetic Ni 2 MnAl [7] and Mn 2 VAl [8]. I will show an overview on our recent studies on the antiferromagnetic Heusler alloys for exchange bias effects. This work was partly supported by HARFIR under the SICORP program from JST and the NMP3-SL program by EC. [1] T. Iwase et al., Appl. Phys. Exp., 2, (2009). [2] Y. Sakuraba et al., Appl. Phys. Lett. 101, (2012). [3] T. Kubota et al., Appl. Phys. Exp., 8, (2015). [4] T. Seki et al., Appl. Phys. Lett. 105, (2014). [5] T. Yamamoto et al., Appl. Phys. Lett. 106, (2015). [6] T. Yamamoto et al., Phys. Rev. B 94, (2016). [7] T. Tsuchiya et al., J. Phys. D: Appl. Phys. 49, (2016). [8] T. Tsuchiya et al., submitted.

10 The role of the defects on functionality of Heusler based heterostructures V. Lazarov Atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy combined with ab initio electronic calculations are used to determine the structure and properties of the Fe 3 O 4 (111)/SrTiO 3 (111) polar interface. The interfacial structure and chemical composition are shown to be atomically sharp and of an octahedral Fe/SrO 3 nature. Band alignment across the interface pins the Fermi level in the vicinity of the conduction band of SrTiO 3. The high spin-polarization of Fe 3 O 4 preserved even at the interface suggests that this system may be excellent candidate for spintronic applications. Next we discuss the existence of a stable twin defect in Fe 3 O 4 thin films. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains.

11 Control of Molecular Orientation for Extended Spin Transport in Hybrid Magnetic Tunnel Junctions Yu Jeong Bae 1, Nyun Jong Lee 1, Pham Thi Kim Hang 1, Tae Hee Kim 1, Yasushi Yamauchi 2, and Andrew Pratt 3 1 Department of Physics, Ewha Womans University, Seoul, Republic of Korea 2 National Institute for Materials Science, Tsukuba, Japan 3 Department of Physics, University of York, York, U.K. Hybrid interface states, molecular geometry, and - stacking disorder complicate an understanding of spin transport in organic spintronic devices, to the detriment of their performance 1-3. Here, we show that such factors can be somewhat ameliorated by using an ultrathin oxide interlayer and a low-temperature setting procedure to control molecular orientation in hybrid Fe/MgO/CuPc/Co magnetic tunnel junctions (MTJs). Increased order at both organic-ferromagnetic interfaces opens up a long-range spin transport channel and leads to a magnetoresistance (MR) ratio that exceeds 100% at 77 K for a CuPc barrier of 1.2 nm. Significantly, the spin-dependent MR effect persists even across a 6.6-nm-thick barrier (1.6 nm MgO/5.0 nm CuPc), measuring approximately 4% at 77 K. The strong dependence of spin transport behaviour on barrier thickness, temperature, bias voltage and underlayer indicates that CuPc molecular orientation plays a key role in the magnetoresistive response of the MTJs, as clarified by the surface sensitive technique of metastable helium de-excitation spectroscopy and DFT calculations. Controlling this orientation for tailored spinterface design and enhanced spin transport is prerequisite to the development of an efficient device. [1] M. Cinchetti et al., Nature Mater. 16, 507 (2017) [2] G. Szulczewski et al., Nature Mater. 8, 693 (2009) [3] S. Sanvito, Nature Phys. 6, 562 (2010) [4] A. Pratt et al., Phys. Rev. B 85, (R) (2012)

12 Session 4: Electrically Controlled Magnetisation Wednesday, 21 st June 16:00-17:30 Chair: Evangelos Papaioannou Prof. Josep Fontcuberta (Institute of Materials Science of Barcelona) "Electric-field controlled metal-insulator transition in half-doped Manganites enhances electroresistance in BaTiO 3 ferroelectric tunnel junctions" Prof. Stefano Sanvito (Trinity College Dublin) "First principles multi-scale theory for current-driven magnetization dynamics" Prof. Seiji Mitani (National Institute for Materials Science) "Interface perpendicular magnetic anisotropy and its voltage control in Fe-based magnetic heterostructures"

13 Enhanced Electroresistance in Ferroelectric Tunnel junctions with Slave Half-doped Manganite La 0.5 Sr 0.5 MnO 3 Inserted Layers Radaelli G 1,2, Qian, M 1, Fina I 1, Liu F 1, Gutiérrez D 1, Sánchez F 1, Bertacco R 2, and Fontcuberta1 J1 1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193, Bellaterra, Catalonia, Spain 2 LNESS Center - Dipartimento di Fisica del Politecnico di Milano, Como 22100, Italy * fontcuberta@icmab.cat Insertion of layers displaying field-induced metal-to-insulator (M/I) transition in ferroelectric tunnel junctions (FTJs) has received attention as a potentially useful way to enlarge junction tunnel electroresistance (TER). Half-doped manganites being at the verge of metal-insulator character are thus good candidates to be slave layers in FTJs. However, the phase diagram of these oxides is extremely sensitive to strain and thus it can be radically different when integrated in epitaxial FTJs. Here we report a systematic study of Pt/La0.5Sr0.5MnO3/BaTiO3/La0.7Sr0.3MnO3 (Pt/HD/BTO/LSMO) FTJs, having different thicknesses of the ferroelectric (2-3nm) and HD layers (1-2nm), grown on substrates imposing either tensile (SrTiO3) or compressive (LaAlO3) strains. Room-temperature electric characterization of the FTJs shows polarization-controlled ON/ OFF states. It is found that the difference between junction resistance in OFF and ON states is increased by more than one order of magnitude compared to bare FTJs without slave HD layers. This observation suggests a field-induced M/I transition. However, this enhancement is only observed in junctions prepared on SrTiO3 but not on LaAlO3. The distinct response is rationalized on the basis of the phase diagram of HD under strain and it gives a clear hint on ways to optimize the TER response. Moreover we noticed that the measured TER has a remarkable dependence on polarization writing time. Aiming at elucidating a possible contribution of ionic motion to the observed TER, temperature-dependent experiments up to above the Curie temperature of the ferroelectric barriers have been conducted. [1] G. Radaelli, et al Adv. Mater. 27, 2602 (2015) [2] D. Gutierrez et al. Phys Rev B 89, (2014) [3] D. Pesquera, et al. Phys. Rev. Applied 6, (2016)

14 First principles multi-scale theory for current-driven magnetization dynamics Stefano Sanvito School of Physics, AMBER and CRANN, Trinity College, Dublin 2, Ireland Many devices and device concepts are based on driving the dynamics of the magnetic order parameter, either the magnetization of the Néel vector, with a spin-polarized current. This, in general, produces a torque, which enables switching and dynamics. The modelling of such dynamics is a complex task because the Physics involved spans over different time and space scales, and it is strictly materials and devices specific. On the one hand, one can model the current-induced torques by using first principles methods, which can be applied very generally to any materials class without the need of external parameters, but can deal with static or at best steady-state quantities. On the other hand, spin-dynamics can be very efficiently computed by using a vast range of micromagnetic techniques. These can tackle the time-dependent problem, but unfortunately depends on parameters, usually extracted from experiments or some other level of theory. Here we present a computational scheme, which combines such two worlds and aims at providing a general tool for materials-specific current-induced spin dynamics. Our method computes the spin-transfer torques by using a combination of density functional theory (DFT) and steady-state transport theory, implemented within the non-equilibrium Green s function formalism. The torques are derived from time-dependent DFT and can be extracted from the transport calculations 1. Then such torques are used as an input for atomistic spin-dynamics calculations 2, with a net result that a fully ab initio theory for spin dynamics can be implemented. Together with the theory backbone I will show a few examples of such computational scheme. Im particular I will look at dynamics in Fe/Co-based magnetic tunnel junctions and look at the possibility of constructing and all-antiferromagnetic current-driven device 3. REFERENCES 1 Y. Xie, I. Rungger, K. Munira, M. Stamenova, S. Sanvito and A.W. Ghosh, Spin transfer torque: A multiscale picture, in Nanomagnetic and Spintronic Devices for Energy-Efficient Memory and Computing, John-Wiley & Sons, (2016). 2 R. F. L. Evans, W. J. Fan, P. Chureemart, T. A. Ostler, M. O. A. Ellis, and R. W. Chantrell, Atomistic spin model simulations of magnetic nanomaterials, J. Phys. Condens. Matter 26, (2014). 3 M. Stamenova, R. Mohebbi, J. Seyed-Yazdi, I. Rungger and S. Sanvito, First-principles spin-transfer torque in CuMnAs/GaP/CuMnAs junctions, Phys. Rev. B 95, (R) (2017).

15 Interface perpendicular magnetic anisotropy and its voltage control in Fe-based magnetic heterostructures S. Mitani 1,2,* 1 National Institute for Materials Science (NIMS), Tsukuba , Japan 2 Grad. School of Pure & Appl. Sci., Univ. Tsukuba, Tsukuba , Japan Voltage control of magnetic anisotropy (VCMA) is of particular interest in the emerging technologies of magnetic random access memories. Large interface perpendicular magnetic anisotropy and its voltage effect were reported in monocrystalline Fe/MgO heterostructures [1,2], suggesting that monocrystalline systems can be a good playground in the research. In this study, interface perpendicular magnetic anisotropy and its voltage effect were investigated for various monocrystalline ferromagnetic metal/oxide layered structures such as Cr/Fe/MgO and Ru/Co 2 FeAl/MgO. We found relatively large voltage effect of magnetic anisotropy in the Co 2 FeAl/MgO system [3] and observed temperature-independent nonlinear behaviors in the applied electric field dependence of magnetic anisotropy in the Fe/MgO system [4]. The detailed results of VCMA will be presented as well as the current status in development of new materials for ferromagnetic metal/oxide layered structures and magnetic tunnel junctions. This work was performed in collaboration with J.W. Koo, Z.C. Wen, Q.Y. Xiang, H. Sukegawa, T. Niizeki, S. Kasai, K. Inomata, T.T. Sasaki, T. Ohkubo, K. Hono, T. Seki, T. Kubota, K. Takanashi and J. Okabayashi, and was partly supported by the ImPACT Program of Council for Science, Technology and Innovation, Japan, the JSPS-EPSRC Tohoku-Cambridge-CNRS Core-to-Core Program, and JSPS KAKENHI 16H References: [1] J. W. Koo, S. Mitani, T. T. Sasaki, H. Sukegawa, Z. C. Wen, T. Ohkubo, T. Niizeki, K. Inomata and K. Hono, Appl. Phys. Lett. 103, (2013). [2] T. Nozaki, A. Koziol-Rachwal, W. Skowronski, V. Zayets, Y. Shiota, S. Tamaru, H. Kubota, A. Fukushima, S. Yuasa and Y. Suzuki, Phys. Rev. Applied 5, (2016). [3] Z.C. Wen, H. Sukegawa, T. Seki, T. Kubota, K. Takanashi, and S. Mitani, Sci. Rep. 7, (2017). [4] Q.Y. Xiang, Z.C. Wen, H. Sukegawa, S. Kasai, T. Seki, T. Kubota, K. Takanashi and S. Mitani, in preparation. *S. Mitani, mitani.seiji@nims.go.jp

16 Session 5: Spintronic Memories Thursday, 22 nd June 09:00-10:30 Chair: Stuart Cavill Prof. Jian-Ping Wang (University of Minnesota) "Spintronics: A potential pathway to enable an exponential scaling for beyond-cmos era" Prof. Masafumi Shirai (Tohoku University) "Enhancement of voltage effect on magnetic anisotropy in ferromagnetic thin films" Prof. Yoshinobu Nakatani (University of Electro-Communications) "Reducing the switching current with a Gilbert damping constant in nanomagnets with perpendicular anisotropy"

17 Spintronics: A Potential Pathway to Enable an Exponential Scaling for the Beyond-CMOS Era Jian-Ping Wang Many key technologies of our society, including artificial intelligence and big data, have been enabled by the invention of transistor and its ever-decreasing size and ever-increasing integration at a large scale. There is a clear scaling limit to the conventional transistor technology, however, and many recently proposed advanced transistors are having an uphill fight in the lab because of necessary performance tradeoffs and limited scaling potential. In this talk, I argue for a new pathway, which involves layering multiple technologies that are beyond the available functions of conventional and newly proposed transistors. Within this context, several successful C-SPIN advances in logic-in-memory, cognitive computing, probabilistic computing and reconfigurable information processing will be briefly discussed. Then, I will introduce my group s two recent demonstrations. First, I will report the growth of ultra-smooth Bi x Se (1-x) films on a large silicon wafer with the largest spin Hall angle at room temperature (50 times larger than the best heavy metal W) ever reported using a semiconductor industry compatible sputtering process. I will also report the switching of a perpendicular CoFeB multilayer using spin-orbit torque (SOT) from this Bi x Se (1-x) with the lowest-ever switching current density reported in a bilayer system: A/cm 2 at RT. The giant SHA, smooth surface, ease of growth of the films on silicon substrate, successful growth and switching of a perpendicular CoFeB multilayer on Bi x Se (1-x) film opens a path for SOT-based memory and logic devices. Then, I will review a longstanding challenge for SOT devices and discuss the limits of previous efforts, by following up with a report on our very recent demonstration of an external-field-free switching SOT device, which is both compatible with a full MTJ stack and scalable down to sub 10 nm. Bio: Jian-Ping Wang is the Robert F. Hartmann Chair and a Distinguished McKnight University Professor of Electrical and Computer Engineering and a member of the graduate faculty in Physics and Chemical Engineering and Materials Science at the University of Minnesota. He received his PhD degree in 1995 from Institute of Physics, Chinese Academy of Sciences, where he performed research on nanomagnetism. He pursued his post-doctoral research at the National University of Singapore ( ). He established and managed the Magnetic Media and Materials program at Data Storage Institute, Singapore, from 1998 to He joined the faculty of the Electrical and Computer Engineering department at the University of Minnesota in 2002 and was promoted to full professor in He is the director of the Center for Spintronic Materials, Interfaces and Novel Architectures (C-SPIN), one of six STARnet program centers. He received the information storage industry consortium (INSIC) technical award in 2006 for his pioneering experimental work in exchange coupled composite magnetic media and the outstanding professor award for his contribution to undergraduate teaching in His group is also known for several important experimental demonstrations and conceptual proposals including the perpendicular spin transfer torque device, the magnetic tunnel junction based logic device and random number generator, ultra-fast switching of thermally stable MTJs, topological insulator spin pumping at room temperature, and a computation architecture in random access memory. He is an IEEE fellow.

18 Enhancement of voltage effect on magnetic anisotropy in ferromagnetic thin films Masafumi Shirai Research Institute of Electrical Communication (RIEC), Tohoku University, Sendai, Japan Center for Spintronics Research Network (CSRN), Tohoku University, Sendai, Japan Voltage-control of magnetic anisotropy energy (MAE) is a possible solution reducing the energy consumption required for magnetization reversal in magnetic tunnel junctions (MTJ). Indeed, the magnetization reversal by the voltage pulse was demonstrated in MgO-based MTJ, where the variation of MAE with respect to the electric field is fj/vm. However, further enhancement of the voltage effect on MAE is required for the magnetization reversal in the MTJ with reduced dimensions. Thus we explored the ferromagnetic thin films exhibiting huge voltage effect on MAE. We investigated the voltage effect on MAE for Fe(001) and Co(0001) thin films covered by various 5d transiton-metal monolayers by using first-principles calculations. As a result, we found that Ir/Fe, Os/Fe and Ir/Co films possess both the variation of MAE larger than 100 fj/vm and the perpendicular MAE greater than 10 mj/m 2. In particular, the Ir/Fe film has the largest variation of MAE, 263 fj/vm, which is about 8 times larger than that of the bare Fe surface. The huge perpendicular MAE and its variation with respect to electric field are originated predominantly from the Ir monolayer. We also evaluated the variation of MAE with respect to electric field in vacuum region for MgO/Ir/Fe film and obtain -298 fj/vm, which correspond to -2,920 fj/vm by taking the relative dielectric constant of MgO. Thus we expect that the insertion of Ir atoms into the MgO/Fe interface could enhance the voltage effect on MAE considerably. This work was funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan) and has been accomplished in collaboration with Masahito Tsujikawa of RIEC/CSRN, Tohoku University.

19 Reducing the switching current with a Gilbert damping constant in nanomagnets with perpendicular anisotropy K. Yamada 1, and Y. Nakatani 2 1Faculty of Engineering, Gifu University, Gifu , Japan Graduate School of Informatics and Engineering. University of Electro-Communications, Tokyo, Japan 2 We report on current-induced magnetization switching in a nanomagnet with perpendicular anisotropy, and investing the effects of the damping constant (α) on the switching current (Isw) by varying the nanosecond-scale pulse current duration (tp), the saturation magnetization (M s ), and the magnetocrystalline anisotropy (K u ). The results show that reduction of α below a certain threshold (α c ) is ineffective in reducing Isw for short Tp. When tp is short, it is necessary to reduce both α and M s simultaneously until α c is reached to reduce Isw. The results presented here offer a promising route for the design of ultrafast information storage and logic devices using current-induced magnetization switching. [1] J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, and D. C. Ralph, Phys. Rev. Lett. 84, 3194 (2000). [2] R. H. Koch, J. A. Katine, and J. Z. Sun, Phys. Rev. Lett. 92, (2004). [3] S. Mangin. D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris, and E. E. Fullerton, Nat. Mater. 5, 210 (2006). [4] S. Mangin, Y. Henry, D. Ravelosona, J. A. Katine, and E. E> Fullerton, Appl. Phys. Lett. 94, (2009). [5] K. Yamada, K. Oomaru, S. Nakamura, T. Sato, and Y. Nakatani, Appl. Phys. Lett. 106, (2015).

20 Session 6: Antiferromagnetic Spintronics Thursday, 22 nd June 11:00-12:00 Chair: Gerrit Bauer Dr. Vincent Baltz (Spintec) "Spin transport in antiferromagnets" Prof. Takahiro Moriyama (Kyoto University) "Spin torque and magnetoresistance in antiferromagnets"

21 Spin transport in antiferromagnets L. Frangou 1, O. Gladii 1, G. Forestier 1, S. Auffret 1, S. Gambarelli 2, and V. Baltz 1,* 1 SPINTEC (Univ. Grenoble Alpes / CNRS / INAC-CEA), F Grenoble, France 2 SYMMES (Univ. Grenoble Alpes / INAC-CEA), F Grenoble, France *vincent.baltz@cea.fr Antiferromagnetic materials, through their robustness against perturbation due to magnetic fields, absence of production of parasitic stray fields, ultrafast dynamics and generation of large magneto-transport effects, have a number of interesting properties. Intense research efforts over the past decade have been invested in unraveling spin-dependent transport properties in antiferromagnetic materials. Whether and how spin currents can be injected, transmitted and converted in antiferromagnetic materials, how subsequent variations can be detected, and what is the actual influence of the magnetic order are some of the thrilling challenges currently being addressed [1 3]. This talk focuses on electronic and magnonic spin transport in thin antiferromagnetic films through spin pumping experiments. Spin pumping results from the magnetization dynamics of a ferromagnetic spin injector, which pumps a spin current into an adjacent spin sink. This spin sink filters, absorbs and converts the current to an extent which depends on its interface and bulk spin-dependent properties [4]. This can be recorded either through the changes induced in ferromagnetic damping or through direct electrical means by measuring the inverse spin Hall effect. Whether the transport regime is electronic or magnonic depends on the electrical nature of the spin-sink and how strongly injector and sink are coupled. Due to magnetic coupling transfer/sink and propagation of spin angular momentum involves spin waves from the oscillating ferromagnet feeding into the antiferromagnet. Measurements of the spin penetration depth were obtained for several antiferromagnetic metals and insulators [3,5 8]. Interestingly, spins propagate more efficiently in layers where the magnetic order is fluctuating rather than static [3,6]. Magnonic spin transport in antiferromagnetic materials is also more efficient than its electronic counterpart [3,8]. The experimental data were compared to some of the recently developed theories [9 11]. [1] H. V. Gomonay and V. M. Loktev, Low Temp. Phys. 40, 22 (2014). [2] T. Jungwirth, X. Marti, P. Wadley, and J. Wunderlich, Nat. Nanotechnol. 11, 231 (2016). [3] V. Baltz, A. Manchon, M. Tsoi, T. Moriyama, T. Ono, and Y. Tserkovnyak, Arxiv: (2016). [4] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, Rev. Mod. Phys. 77, 1375 (2005). [5] P. Merodio, A. Ghosh, C. Lemonias, E. Gautier, U. Ebels, M. Chshiev, H. Béa, V. Baltz, and W. E. Bailey, Appl. Phys. Lett. 104, (2014). [6] L. Frangou, S. Oyarzun, S. Auffret, L. Vila, S. Gambarelli, and V. Baltz, Phys. Rev. Lett. 116, (2016). [7] L. Frangou, G. Forestier, S. Auffret, S. Gambarelli, and V. Baltz, Phys. Rev. B (2017). [8] L. Frangou et al [9] Y. Ohnuma, H. Adachi, E. Saitoh, and S. Maekawa, Phys. Rev. B 89, (2014). [10] R. Khymyn, I. Lisenkov, V. S. Tiberkevich, A. N. Slavin, and B. A. Ivanov, Phys. Rev. B 93, (2016). [11] K. Chen, W. Lin, C. L. Chien, and S. Zhang, Phys. Rev. B 94, (2016).

22 Magnetic moment orientation dependent spin dissipation in antiferromagnets Takahiro Moriyama Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto, , Japan. Spin current interaction in antiferromagnetic materials is a central interest in recently emerged antiferromagnetic spintronics. In this work, we explored the spin current interaction in antiferromagnetic FeMn by the spin pumping effect. Exchange biased FeNi/FeMn films, in which the Neel vector can be presumably controlled via the exchange spring effect, were employed to investigate the damping enhancement depending on the relative orientation between the Neel vector and the polarization of the pumped spin current. The correlation between the enhanced damping and the strength of the exchange bias suggests that the twisting of the Neel vector induces an additional spin dissipation, which evidences that the Slonczewski-type spin torque is effective in antiferromagnetic materials.

23 Session 7: Spin Caloritronics Thursday, 22 nd June 14:00-15:30 Chair: Burkard Hillebrands Prof. Günter Reiss (University of Bielefeld) "Thermally driven spin currents: spin Seebeck and tunneling magneto Seebeck effect" Dr. Ken-ichi Uchida (National Institute for Materials Science) "Thermographic measurements of spin-caloritronic phenomena (tentative)" Prof. Hiromi Yuasa (Kyushu University) "Enhancement of spin mixing conductance and utilizing large spin Hall angle in spin Seebeck effect"

24 Thermally driven spin currents: spin Seebeck and tunneling magneto Seebeck effect G. Reiss 1, A. Böhnke 1, M. Glas 1, D. Meier 1, T. Kuschel 1,2, Ch. Klewe 1,3, H.W.Schumacher 4, S. Serrano-Guisan 5, M. Walter 6, M. Münzenberg 6 1 Department of Physics, Universität Bielefeld, Bielefeld, Germany 2 Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, 9747AG Groningen, The Netherlands 3 Advanced Light Source, 6 Cyclotron Rd, Berkeley, CA 94720, USA 4 Physikalisch-Technische Bundesanstalt, Braunschweig, Germany 5 International Iberian Nanotechnology Laboratory, Braga, Portugal 6 Institut für Physik, Ernst-Moritz-Arndt-Universität Greifswald, Greifswald, Germany Spin-electronics and -caloritronics are rapidly developing and many new effects have been observed such as the Spin-Seebeck effect (SSE) the Tunneling Magneto Seebeck Effect (TMS) or spin transfer torque switching (STT) at ultralow current density. The first part will discuss the TMS in magnetic tunnel junctions. We present results on the TMS obtained for CoFeB. Based on band structures and the physical origin of Seebeck voltages, we demonstrate both strongly enhanced thermally generated voltages as well as TMS ratios when replacing the CoFeB by Heusler compounds. The second part will discuss the SSE in transverse and longitudinal geometry. We demonstrate the LSSE and its dependence on the base temperature in insulating and semiconducting ferromagnets and use X-ray resonant magnetic reflectivity to exclude proximity induced magnetization in Pt. Finally, we show that the voltage detected at a Pt stripe on NiFe 2 O 4 or YIG that was attributed to a transverse SSE is most probably generated by unintended out-of-plane temperature gradients.

25 Thermographic measurements of spin-caloritronic phenomena Ken-ichi Uchida 1-3* 1 National Institute for Materials Science, Tsukuba , Japan 2 Center for Spintronics Research Network, Tohoku University, Sendai , Japan 3 PRESTO, Japan Science and Technology Agency, Saitama , Japan * UCHIDA.Kenichi@nims.go.jp The Peltier effect modulates the temperature of a junction comprising two different conductors in response to charge currents across the junction, which is used in solid-state heat pumps and temperature controllers in electronics. Recently, in spintronics, a spin counterpart of the Peltier effect was observed [1]. The spin Peltier effect modulates the temperature of a magnetic junction in response to spin currents. Here we report thermal imaging of the spin Peltier effect; using active thermography technique, we visualize the temperature modulation induced by spin currents injected into a magnetic insulator from an adjacent metal [2-4]. The thermal images reveal characteristic distribution of spin-current-induced heat sources, resulting in the temperature change confined only in the vicinity of the metal/insulator interface, which is different from thermal distribution expected from standard heat sources. From systematic experiments and numerical calculations, we found that this seemingly counterintuitive result is attributed to dipolar heat sources generated by spin currents. The finding of this anomalous temperature distribution allows us to estimate the actual magnitude of the temperature modulation induced by the spin Peltier effect, which is more than one order of magnitude greater than previously believed. [1] J. Flipse et al., Phys. Rev. Lett. 113, (2014). [2] S. Daimon, R. Iguchi, T. Hioki, E. Saitoh, and K. Uchida, Nature Communications 7, (2016). [3] K. Uchida, R. Iguchi, S. Daimon, R. Ramos, A. Anadon, I. Lucas, P. A. Algarabel, L. Morellon, M. H. Aguirre, M. R. Ibarra, and E. Saitoh, Physical Review B 95, (2017). [4] S. Daimon, K. Uchida, R. Iguchi, T. Hioki, and E. Saitoh, arxiv:

26 Enhancement of spin mixing conductance and spin Hall angle in spin Seebeck effect Hiromi Yuasa 1, 2 1Graduate School and Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan 2PRESTO, Japan Science and Technology Agency, Kawaguchi, Japan Spin Seebeck effect and inverse spin Hall effect generate voltage by temperature gradient in the ferrimagnetic oxide and nonmagnetic metal systems, which has potential of thermoelectric generation by a uniform film [1, 2]. However, the voltage is too small to use for application. The important physical parameters determining the voltage are the spin Hall angle of the nonmagnetic metal and the spin mixing conductance at the interface between ferrimagnetic oxide and nonmagnetic metal. Although -W have large spin Hall angle [3], the Spin Seebeck voltage of YIG/W are smaller than that of YIG/Pt [4]. A considerable reason is the small spin mixing conductance due to W oxidation at the interfaces YIG/W. In order to obtain the high quality interface without oxidation, we tried two approaches; Ta 50 W 50 alloy as nonmagnetic layer and Ru insertion into the interface since Ta 50 W 50 alloy and Ru are known for the higher oxidation resistance than W. The combination of Ta 50 W 50 alloy and Ru insertion, successfully increased the spin Seebeck coefficient, for example, bulk-yig (1mm)/Ru (0.5nm)/Ta 50 W 50 (4.5nm) showed 2.4 times magnitude of spin Seebeck coefficient S of the conventional bulk-yig (1mm)/Pt (5nm). Furthermore, S of bulk-yig (1mm)/Ru (0.5nm)/Ta 50 W 50 (4.5nm) was higher than that of bulk-yig (1mm)/Ru (0.5nm)/W (4.5nm),which indicates that the Ta 50 W 50 has the higher spin Hall angle than W. Additionally, bulk-yig (1mm)/Ru (0.5nm)/Pt (4.5nm) showed the higher S than the bulk-yig (1mm)/Pt (5nm), which suggests that the YIG/Ru interface possesses the higher spin mixing conductance than YIG/Pt. It is important to select materials with the high spin Hall angle and the high spin mixing conductance simultaneously for the spin Seebeck voltage improvement. This work was supported by PRESTO-JST (JPMJPR15R8), and Tateisi Science and Technology Foundation. References [1] K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa and E. Saitoh: Nature 455, 778(2008). [2] A. Kirihara, K. Uchida, Y. Kajiwara, M. Ishida, Y. Nakamura, T. Manako, E. Saitoh and S. Yorozu: Nature materials 11, 686 (2012). [3] Chi-Feng Pai, Luqiao Liu, Y. Li, H. W. Tseng, D. C. Ralph, and R. A. Buhrman: Appl. Phys. Lett. 101, (2012). [4] M. Ishida, A. Kirihara, H. Someya, K. Uchida, S. Kohmoto, E. Saitoh, T. Murakami: arxiv: (Submitted on 12 Jul 2013).

27 Session 8: Oxides for Spintronics Thursday, 22 nd June 16:00-17:30 Chair: Andrew Pratt Prof. Gerrit Bauer (Tohoku University) "Spintronics with magnetic insulators" Dr. Dmytro Bozhko (Technical University of Kaiserslautern) "Direct observation of dipolar-exchange magnon and phonon spectra in arbitrary magnetized yttrium-iron-garnet films" Dr. Keith McKenna (University of York) "Understanding defects in metal oxide materials"

28 Spin Caloritronics of YIG Gerrit E.W. Bauer a,b a Institute for Materials Research and WPI-AIMR, Tohoku University, Sendai , Japan b Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands Magnetic insulators are a class of versatile materials with great technological importance. The most important magnetic insulators are arguably the man-made yttrium iron garnets, ferrimagnets with Curie transitions far above room temperature and record magnetic quality [1-2]. The discovery by K. Uchida, E. Saitoh c.s. that magnetic insulators can be actuated thermally and electrically by metallic contacts. attracted much interest since it allows for their integration into conventional electronic and thermoelectric devices. The discovery of entirely new phenomena, such as the spin Seebeck effect, raises the hope for a new and green spintronics based on YIG or other magnetic insulators. The transport of magnons from the bulk of the magnet to its interface or between contacts can dominate the experimental observations of thermal and electric magnon injection. There is much evidence that magnon transport is strongly affected by interaction with phonons. While a general theory is not yet available, several experiments can be well described by ad-hoc models for diffuse and ballistic transport that give insights into the nature of the interaction. This talk will address the evidence for the formation of hybrid magnon-phonon quasiparticles or magnon polarons [3,4]. Magnons also hybridize with electromagnetic waves, forming magnon polaritons with microwaves and scatter light inelastically. The coupling can be enhanced by confining the photons in cavities. The talk will introduce new games that can be played in spin cavitronics with YIG [5,6]. [1] V. Cherepanov, I. Kolokolov, V. L'vov, The Saga of YIG: Spectra, Phys. Rep. 229, 81 (1993). [2] M. Wu and A. Hoffmann (eds.), Solid State Physics 64, 1 (2013). [3] K. Shen and G.E.W. Bauer, Phys. Rev. Lett. 115, (2015). [4] B. Flebus, K. Shen, T. Kikkawa, K. Uchida, Z. Qiu, E. Saitoh, R.A. Duine, and G. E. W. Bauer, Phys. Rev. B 95, (2017). [5] S. Sharma, Y. Blanter, and G.E.W. Bauer, to be published. [6] B. Zare Rameshti and G.E.W. Bauer, to be published.

29 Dipolar-exchange magnon and phonon spectra in arbitrarily magnetized yttrium-iron-garnet films Dmytro A. Bozhko Fachbereich Physik and Landesforschungszentrum OPTIMAS Technische Universität Kaiserslautern, Kaiserslautern, Germany Spin waves (or their quantum mechanical representatives magnons) demonstrate a diversity of dispersion characteristics in comparison with well-known electromagnetic or acoustic waves. Variety of linear and relatively low-threshold nonlinear effects make magnons not only interesting for physics in general, but also a promising data carrier for wave-based information processing. For this purposes materials with ultra-low magnon decay rate are of particular interest. So far, a magnetic insulator yttrium-iron-garnet (YIG) possesses the lowest known value of magnon damping. Magnon spectrum in YIG films can be controlled via a wide range of parameters such as saturation magnetization, shaping and structuring of a sample, or the orientation and value of an applied bias magnetic field, et cetera. Although the behavior of a magnon spectrum for in-plane and out-of-plane configurations magnetization conditions has been studied in detail, the spectra of dipolar-exchange magnons in obliquely magnetized thin films remained terra incognita for researchers. In the present work, I demonstrate results of experimental and theoretical investigation of thermal spectra of dipolar-exchange magnons, which provides evidences of wavelength-dependent transformation of magnon modes in obliquely magnetized YIG films. The developed new theoretical approach, which is based on the model presented in [1], fully describes the observed phenomenon. Magnon and transversal phonon dispersions were probed by wavevector-resolved Brillouin light scattering (BLS) spectroscopy. The used scattering geometry allows for detection of wavevectors directed along the projection of the bias magnetic field (µ 0 H=250 mt) on the film surface. The sample constitutes a YIG film of 5.6 µm thickness, which was grown on a gadolinium-gallium-garnet substrate. It has been found that in the case of oblique magnetization the spectra of long-wavelength dipolar magnons split into spin-wave modes with forward and backward dispersions. It is remarkable, that in the experiment only the modes with quasi-uniform redistribution across the film thickness are detected due to the back-scattering geometry used in the BLS experiments. The important consequence of this experimental limitation is that the visibility of the modes in the measured BLS spectra strongly depends on the homogeneity of the spin-wave mode profiles. In the case of an obliquely magnetized film, profiles of higher modes become more homogeneous than the profile of the lowest mode, which becomes non-homogeneous during a transition to exchange area of spectrum. We show that by taking into account calculated spin-wave mode thickness profiles one can perfectly fit the experimentally observed dispersion relations. Financial support by the European Research Council (ERC) within the ERC Advanced Grant Supercurrents of Magnon Condensates for Advanced Magnonics is gratefully acknowledged. [1] B. A. Kalinikos and A. N. Slavin, J. Phys. C: Solid State Phys. 19, 7013 (1986)

30 Grain boundaries and dislocations in metal-oxide materials K. P. McKenna Department of Physics, University of York, York, UK Metal oxide materials exhibit a wide range of electronic, optical, chemical and magnetic properties and find diverse technological applications in areas such as electronics, energy generation, catalysis and medicine. Whether in the form of thin crystalline films, nanoparticles or bulk polycrystals extended defects such as grain boundaries and dislocations are ubiquitous in metal oxides. These defects often significantly perturb structural and electronic properties affecting functionality and performance for many applications. In recent work we have shown how first principles materials modelling in synergy with electron microscopy can provide invaluable insights into the role of extended defects in oxide materials. In this talk, I will present a number of examples relevant to applications in spintronics including grain boundaries and dislocations in the oxides MgO and Fe3O4 [1-6]. This includes a study of the structure and electronic properties of grain boundaries inside the MgO barrier of a magnetic tunnel junction which demonstrates how shunting of current through grain boundaries imposes fundamental limits on the maximum magnetoresistance that can be achieved [5]. I will also present the results of an investigation into the structure and magnetic properties of antiphase boundary defects in Fe3O4 which helped elucidate the role they play in spin transport and magnetoresistance [6]. [1] Z. Wang et al, Nature 479, (2011) [2] K. P. McKenna, Journal of the American Chemical Society 135, (2013) [3] Z. Wang et al, Nature Communications 5, 3239 (2014) [4] Z. Wang et al, Nano Letters 16, 1530 (2016) [5] J. J. Bean et al, Scientific Reports 7, (2017) [6] K. P. McKenna et al, Nature Communications 5, 5740 (2014)

31 Session 9: MRAM and Beyond Friday, 23 rd June 09:00-10:30 Chair: Atsufumi Hirohata Prof. Hideo Ohno (Tohoku University) "Analog spintronics memory" Dr. Johan Swerts (IMEC) "Perpendicular magnetic tunnel junction stacks with high annealing tolerance for high density memory applications" Dr. Lucian Prejbeanu (Spintec) "Ultrafast sub-ns MRAM concepts for cache applications"

32 Three-terminal Spintronics Devices for VLSI Integration Hideo Ohno1-5 1Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Sendai, Japan 2Center for Spintronics Integrated Systems, Tohoku University, Sendai, Japan 3Center for Innovative Integrated Electronics, Tohoku University, Japan 4WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan 5Center for Spintronics Research Network, Tohoku University, Sendai, Japan I review our recent studies on realizing three-terminal spintronics devices for integration with CMOS VLSI. Three-terminal devices separate the current paths one for magnetization switching and the other for reading the state of magnetization, thereby, in principle, allow a relaxed operation window resulting in high speed swtching compared to their two-terminal counterpart [1]. Of particular current interest are devices that utilize spin-orbit torque (SOT) for magnetization switching, which consist of a heavy metal layer and a target ferromagnetic structure placed on top of it. The first topic I discuss is a high-speed operation of an SOT switching with a target ferromagnetic pillar having an in-plane magnetic easy axis collinear with the current flow direction in the underneath heavy-metal [2, 3]. We show that one can switch magnetization as fast as 500 ps in this structure without significant increase in switching current; this switching speed is not readily available in two-terminal devices utilizing spin-transfer torque (STT) switching without applying considerably large current - STT requires switching current inversely proportional to the switching speed in this speed range. We also show that there is a device configuration to avoid application of static magnetic field otherwise needed to observe the switching. The second topic I will discuss is to use an antiferromagnetic layer as a source of spin flow as well as the source of an exchange field: The former is for the switching and the latter is for the switching in the absence of external magnetic field. It was shown in a (Co/Ni)-multilayer/PtMn structure one can switch magnetization in the absence of external magnetic field [4]. The use of antiferromagnet led us to realize an analog memory, which we recently used to demonstrate an associative memory operation in a spintronics-device based artificial neural network [5]. I thank all my collaborators especially Shunsuke Fukami. Work supported in part by ImPACT from JST and by the R & D for Next-Generation Information Technology of MEXT. [1] H. Ohno, Int. Elect. Dev. Meeting (IEDM) (invited) (2010). [2] S. Fukami et al. Nature Nanotechnology doi: /nnano (2016). [3] S. Fukami et al Symp. on VLSI Tech., T06-5 (2016). [4] S. Fukami et al. Nature Materials 15, 535 (2016); doi: /nmat4566. [5] W. A. Borders et al., Appl. Phys. Express 10, (2017).