This PDF file contains the front matter associated with SPIE Proceedings Volume 10732, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Generation, manipulation and detection of spin currents are important issues in the operation spintronic devices because a spin current plays an important role in spin-dependent transport and spin-transfer switching. Especially, pure spin current which is the spin current without accompanying the charge current is an attractive quantity for utilizing the spin current efficiently. Nonlocal spin valve measurements in laterally configured ferromagnetic metal (FM)/nonmagnetic metal (NM) hybrid nanostructures is a powerful means for evaluating the intriguing properties of pure spin current precisely. In this talk, I will introduce materials for the efficient generation and detection of the pure spin current and a structure for efficient control of the absorption property of the pure spin current. In the first part, I will introduce the results on the efficient generation of pure spin current using CoFeAl. We show that CoFeAl alloy is an excellent material not only for the electrical spin injection but also thermal spin injection because of its favorable band structure[1]. Further optimization for the thermal spin injection has been demonstrated by adjusting the Co and Fe composition.[2] In the second part, I will introduce the thermal spin injection based on the ferromagnetic resonance (FMR) heating effect. We show that the heat dissipation due to the FMR produces the temperature change over 10 K[3]. This FMR heating is found to induce sufficiently large electrical voltage by combining with inverse spin Hall effect in the ferromagnetic/nonmagnetic bilayer system[4]. The device structure for the efficient thermal spin injection is also presented. Reference [1] S. Hu, H. Itoh, and T. Kimura, NPG Asia Mater. 6, e127 (2014). [2] T Nomura, T Ariki, S Hu, T Kimura, J. Phys. D: Appl. Phys. 50, 465003 (2017) [3] K. Yamanoi, Y. Yokotani and T. Kimura, Appl. Phys. Lett. 107, 182410 (2015); [4] K. Yamanoi, Y. Yokotani and T. Kimura, Phys. Rev. Appl. 8, 054031 (2017).

We experimentally investigate the dynamic properties of Co25Fe75 thin films grown by dc magnetron sputtering. Using ferromagnetic resonance spectroscopy, we demonstrate an ultralow total damping parameter in the out-of-plane configuration of < 0.0013, whereas for the in-plane configuration we find a minimum total damping of < 0.0020. In both cases, we observe low inhomogeneous linewidth broadening in macroscopic films. We observe a minimum full-width half-maximum linewidth of 1 mT at 10 GHz resonance frequency for a 12nm thick film. We characterize the morphology and structure of these films as a function of seed layer combinations and find large variation of the qualitative behavior of the in-plane linewidth vs. resonance frequency. Finally, we use wavevector-dependent Brillouin light scattering spectroscopy to characterize the linewidth of spin-wave modes and their group velocity at wave vectors up to 23 µm-1.

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both damping-like and field-like spin-orbit torques have been observed in the forward process of current-driven spin torque and damping-like inverse spin-orbit torque has been well-studied via spin pumping into heavy metal layers. Here we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the AC charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly-extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all AC charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Permalloy/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques. While the magnitudes of both the damping-like and field-like inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the damping-like magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the damping-like spin-to-charge conversion. Spin memory loss (SML) [1] and proximity-induced magnetic moments at the FM/NM interface [2] have been invoked to explain the large damping enhancement caused by thin NM films even when the NM thickness is less than its spin diffusion length. In this model, spin loss at the FM/NM interface acts as an additional parallel spin relaxation pathway to that of spin pumping and diffusion into the Pt bulk. From damping measurements alone, the relative contributions of these mechanisms are not resolvable. In this work, we show that a self-consistent fit of Gilbert damping and damping-like iSOT versus Pt thickness—where both sets of data are described by the same spin diffusion length—makes it possible to separate these sources of damping. Furthermore, this data analysis methodology allows for unambiguous determination of the spin-mixing conductance at the FM/NM interface. We therefore can determine the spin Hall conductivity (or spin Hall angle) without having to refer to spin transport parameters, e.g. the spin-mixing conductance and spin diffusion length, as determined from measurements performed on dissimilar samples or theoretical idealized values. For our samples of Pt deposited on Permalloy, only 37 ± 6% of the total damping enhancement from the Pt film is attributable to spin pumping into the Pt layer when the Pt thickness is much greater than the spin diffusion length. The self-consistent fit also results in a spin diffusion of length of (4.2 ± 0.1) nm, and a spin mixing conductance of (130,000 ± 20,000) 1/(μΩ cm^2), which is in good agreement with the maximum theoretical value for Pt of 107,000 1/(μΩ cm^2) [3], given the estimated error, and σ_SH = (2.36 ± 0.04) 1/(μΩ m). This corresponds to a spin Hall angle of 0.387 ± 0.008. While this θ_SH is among the largest reported for Pt [4, 5], it is a necessary logical conclusion that with less spin current driven into the NM (on account of SML), a larger spin-to-charge conversion efficiency is required to fit the data than would be otherwise obtained if the SML were negligible. We furthermore stress that the phenomenological value for the damping-like spin orbit torque is comparable to that measured with other techniques [5-7]. This indicates that the Pt layer in our samples behaves conventionally, and stresses the importance of characterizing spin loss mechanisms to optimize SOT for magnetic switching. REFERENCES [1] J.-C. Rojas-Sánchez, N. Reyren, P. Laczkowski, et al., "Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces," Phys. Rev. Lett., vol. 112, p. 106602 (2014). [2] M. Caminale, A. Ghosh, S. Auffret, et al., "Spin pumping damping and magnetic proximity effect in Pd and Pt spin-sink layers," Physical Review B, vol. 94, p. 014414 (2016). [3] Y. Liu, Z. Yuan, R. J. H. Wesselink, et al., "Interface Enhancement of Gilbert Damping from First Principles," Physical Review Letters, vol. 113, p. 207202 (2014). [4] W. Zhang, W. Han, X. Jiang, et al., "Role of transparency of platinum-ferromagnet interfaces in determining the intrinsic magnitude of the spin Hall effect," Nat Phys, Article vol. 11, pp. 496-502 (2015). [5] C.-F. Pai, Y. Ou, L. H. Vilela-Leão, et al., "Dependence of the efficiency of spin Hall torque on the transparency of Pt/ferromagnetic layer interfaces," Phys. Rev. B, vol. 92, p. 064426 (2015). [6] K. Garello, I. M. Miron, C. O. Avci, et al., "Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures," Nat. Nano., vol. 8, pp. 587-593 (2013). [7] M.-H. Nguyen, D. C. Ralph, and R. A. Buhrman, "Spin Torque Study of the Spin Hall Conductivity and Spin Diffusion Length in Platinum Thin Films with Varying Resistivity," Physical Review Letters, vol. 116, p. 126601 (2016).

Although there has been enormous development in the field of spintronics, it is a challenge to interpret the experimental results and estimate the key parameters e.g., spin diffusion length. While designing functional devices, it creates a severe issue since an inaccurate estimation of one parameter also affects the estimation of other parameters concomitantly. The spin diffusion length of a giant spin-orbit material platinum (Pt) has been reported in literature in a wide range of 0.5 - 14 nm, and it is usually treated as a constant value independent of Pt's thickness. For an accurate estimation of spin diffusion length, noting that circuit theory has been tremendously successful in translating physical equations into circuit elements in organized form, we construct the spin-circuit representation of spin pumping. Thereby it is shown clearly that a thickness-dependent conductivity and spin diffusion length is necessary to simultaneously match the experimental results of effective spin mixing conductance and inverse spin Hall voltage due to spin pumping. Such thickness-dependent spin diffusion length is tantamount to Elliott-Yafet spin relaxation mechanism and it bodes well for transitional metals. It is also shown that this conclusion is not altered when there is a significant interfacial spin memory loss.

Characterizing the dynamic magnetic properties of nanoscale magnetic thin films, multilayers, and nanostructures is crucial for exploiting their potential for practical applications such as in logic and microwave devices operating in the GHz region. Among the various techniques suitable for high-frequency characterization, ferromagnetic resonance spectroscopy (FMR) is widely considered as one of the gold standards. In its most advanced version, broadband vector network analyzer (VNA) FMR, it represents the perfect tool for detailed and accurate analysis of magnetic damping processes. In the first part of this talk, a compact introduction to FMR including underlying physics as well as technical details will be given, followed by a short presentation of a recently built VNA-FMR setup at UVSQ. In the second and longer part, both static and dynamic magnetic properties of two selected material systems will be discussed in depth, with focus on data obtained from VNA-FMR measurements. The first study involves Fe/Ag continuous films and nanodot arrays of various sizes prepared for elucidating the mechanisms of a potentially existing magneto-plasmonic coupling. The second sample system consisting of trilayers of Py/Al/NdCo is investigated with support from micromagnetic simulations with the purpose of studying the influence of competing anisotropies, in-plane for Permalloy (Py) and out-of-plane for NdCo, on its magnetic properties.

The ability to control magnetic domain patterns with high frequency strain waves could result in a novel, fast method for moving domain walls. We examine the motion of magnetic domain walls in Co/Pt multilayers when subject to a high frequency strain standing wave generated by a focused surface acoustic wave transducer. Our ability to map focused strain waves using optical methods allow us to make subsequent magnetic measurements without losing the location of strain nodes and antinodes. Our results indicate that domain walls move preferentially towards strain antinodes and away from the nodes.

Spin-orbit coupling effects in materials with broken inversion symmetry are responsible for peculiar spin textures. Among them, ferroelectric materials allow for non-volatile control of the spin degree of freedom through the non-volatile electrical inversion of the spin texture, through to their reversible spontaneous polarization. Such functionality holds potential for technological applications exploiting spin effects controlled by electric fields. The ferroelectric Rashba semiconductor Germanium Telluride stands out as material for Spin-Orbitronics: its ferroelectricity provides a nonvolatile state variable able to generate and drive a giant bulk Rashba-type spin splitting of the electronic bands, while its semiconductivity would allow for the realization of spin-based transistors. The ferroelectric control of the bands topology and of the spin texture is expected to reflect in the tunability of the spin transport properties. Here we exploit the unidirectional spin Hall magnetoresistance of Fe/GeTe heterostructures to characterize charge-to-spin conversion in GeTe. Our preliminary results indicate a sizable conversion efficiency at low temperature (120 K), which promotes ferroelectric Rashba semiconductors as promising candidates for the implementation of non-volatile electrically reconfigurable computing devices based on spin transport in semiconductors.

The ability to control and manipulate magnetic anisotropy in the colossal magnetoresistive (CMR) oxide (La,Sr)MnO₃ (LSMO) is critical for its implementation in magnetic memory applications. In this work, we employ the planar Hall effect (PHE) as a powerful tool to probe the magnetic anisotropy in LSMO thin films and nanostructures, where the magnetization is too small to be detected by conventional magnetometry techniques. By analyzing the angular- and magnetic field-dependences of the PHE, we deduced an in-plane biaxial magnetocrystalline anisotropy (MCA) energy of ~1.2x10⁵ erg/cm² in LSMO thin films fully strained on (001) SrTiO₃ substrates. Creating nanoscale periodic depth modulation in LSMO establishes a uniaxial anisotropy with substantially enhanced MCA energy density, which is attributed to a high strain gradient sustained in the nanostructure. The energy competition between the biaxial and uniaxial MCA leads to multi-level resistance switching behavior in properly engineered LSMO nanostructures, which can be utilized to design the switching dynamics in magnetic memory devices. Our work points to the critical role of epitaxial strain in determining the MCA in CMR oxides, and provides an effective material strategy for engineering the magnetic properties of LSMO for novel spintronic applications with high thermal stability and high density data storage.

Rare earth iron garnets (REIG’s) are important component materials in magnetic insulator based spintronics due to their low spin wave damping and electrically insulating properties. Yttrium iron garnet (YIG) has been the mainstay material because of its unusually low spin damping. However, YIG thin films thus far have in-plane magnetization. Recent studies on thulium iron garnet (TIG) thin films have demonstrated robust perpendicular magnetic anisotropy (PMA), however, spin damping in TIG films is significantly higher compared to YIG. It would be useful to have an insulating magnetic material that exhibits both low spin damping and robust, tunable PMA because of its potential for novel device configurations. In this study, we synthesized YIG-TIG solid solution powders across the compositional phase diagram and with different particle sizes using the polymeric steric entrapment technique in order to begin to decouple compositional effects from size and morphological effects. Powder characterization, including XRD, VSM, SEM and FMR techniques, was also performed to understand their magnetic behavior.

Here we show spin transport and spin relaxation phenomena in germanium (Ge) up to room temperature. Using four-terminal nonlocal voltage and Hanle-effect measurements in lateral spin-valve devices with ferromagnet/Ge Schottky-tunnel contacts, we detected reliable spin transport from low temperatures to room temperature in n-Ge and discussed the spin relaxation mechanism in the conduction band of Ge [1-4]. We also demonstrated the detection of the spin transport in p-Ge by using vertically stacked structures including ferromagnet/Ge interfaces on Si [5,6]. From the magnitude of the spin signals, we also discussed the spin relaxation in p-Ge up to room temperature. From the viewpoint of low contact resistance for applications in the field of group-IV semiconductor spintronics, our technologies are more suitable than ferromagnet/MgO tunnel contacts. [1] Y. Fujita, KH et al., Phys. Rev. B 94, 245302 (2016). [2] M. Yamada, KH et al., Phys. Rev. B 95, 161304(R) (2017). [3] Y. Fujita, KH et al., Phys. Rev. Appl. 8, 014007 (2017). [4] M. Yamada, KH et al., Appl. Phys. Exp. 10, 093001 (2017). [5] M. Kawano, KH et al., Appl. Phys. Lett. 109, 022406 ¬(2016). [6] M. Kawano, KH et al., Phys. Rev. Mater. 1, 034604 (2017). The author appreciates good collaborative research with Prof. K. Sawano, Prof. V. Lazarov, and the colleagues of our group in Osaka University. This work was partly supported by KAKENHI (No. 25246020, 16H02333, 17H06120, 26103003) from JSPS/MEXT.

Ferromagnetic semiconductors (FMSs) with high Curie temperature (TC) are highly desired for spintronics device applications. So far, the mainstream study of FMSs is the Mn-doped III-V FMSs;, however they are only p-type and TC is much lower than 300K. To search for new FMSs with high-performance, there have been world-wide efforts that were mainly concentrated on wide-gap materials. However, reliable and systematic results have not yet been presented. In this study, we present an alternative approach by using Fe instead of Mn as the magnetic dopants in narrow-gap III-V semiconductors; InAs, GaSb, and InSb. In these Fe-based FMSs, because the Fe atoms are in the isoelectronic Fe3+ state, the carrier type (electrons or holes) can be controlled independently by co-doping non-magnetic dopants. These carriers reside in the conduction band (CB) or the valence band (VB) of the host semiconductors and thus move faster with higher coherency. Using low-temperature molecular beam epitaxy, we have successfully grown single-phase thin-film crystals of both p-type FMS [(Ga,Fe)Sb [2]] and n-type FMSs [(In,Fe)As [3], (In,Fe)Sb [4]]. TC increases monotonically with the Fe content; and there is a tendency that TC is higher as the bandgap is narrower. Intrinsic room-temperature ferromagnetism has been observed in (Ga1-x,Fex)Sb with x > 23% [2] and (In1-x,Fex)Sb with x > 16% [4]. In n-type FMS (In,Fe)As, large spontaneous spin splitting in the CB was observed, which is the first in all FMSs [5]. These results indicate that the Fe-doped III-V FMSs are promising for spintronic devices operating at room temperature. [1] N. T. Tu et al., APL 108, 192401 (2016). [2] P. N. Hai, APL 101, 182403 (2012). [3] N. T. Tu et al., arXiv:1706.00735 (2017). [4] L. D. Anh et al., PRB 92, 161201(R) (2015). [5] L. D. Anh et al., Nat. Commun. 7, 13810 (2016).

A spin-dependent and electric-field tunable magnetoresistance (MR) of a semiconducting (SC) channel placed between two ferromagnetic (FM) contacts is a key ingredient in many novel spin-based device concepts. Whereas successful realization of such devices requires a large magnetoresistance signal, the signals measured in semiconductor-based devices are usually very low, well below 1\%, because of highly resistive tunnel FM/SC interfaces. In this talk, we will discuss how the finite electric fields effects in lateral FM/SC/FM devices lead to enhancement of the measured magnetoresistance by increasing the efficiency of the spin transport in the channel and boosting spin-to-charge conversion at the FM/SC interface. We will illustrate this discussion with the results of our recent experiments on lateral all-semiconductor spin valve devices with a transport channel formed in the two-dimensional electron system embedded in GaAs/(Al,Ga)As interface and with ferromagnetic (Ga,Mn)As/GaAs Esaki diodes as source and drain contacts [1]. We have measured very large two-terminal spin valve signals, in order of 1 kOhm in such devices, with MR reaching even up to 80\% in the nonlinear regime of the current-voltage characteristic [2]. We will also demonstrate that the MR signal can be additionally tuned by means of an electric gate, with the gating scheme based on switching between uni- and bidirectional spin diffusion, without resorting to spin–orbit coupling. The work has been supported by Deutsche Forschungsgemeinschaft (DFG) through SFB689. [1] M. Oltscher et al., Phys. Rev. Lett. 113, 236602 (2014) [2] M. Oltscher et al., Nature Commun. 8, 1807 (2017)

Detection of spin diffusion length in different materials has been launched tremendously so far. But due to the difficulty of obtaining a high-quality semiconductor (SC) layer on ferromagnetic metals, until now most of the studies of spin diffusion transport in SC were only limited on lateral structure devices. Here, by using ultra-high vacuum wafer-bonding technique, a vertical structure of CoFeB/MgO/Si/Pt is fabricated successfully and based on which, the spin diffusion transport is demonstrated at room temperature by spin pumping. With the Pt layer on top to detect the inverse spin hall voltage for different thickness of n-Si layer, the spin diffusion length is determined to be 3.8 μm, which is comparable with the value reported in lateral devices. Furthermore, our experiments reveal the existence of interface state at MgO/Si interface, which is essentially important to build the model of spin-mixing conductance for spin-pumping into Si.

The electrons’ spin-momentum locking of the topological surface states is an intriguing property in the development of spintronics. We investigate the topological insulators in conjunction with magnetic materials to study the static and dynamic magnetic properties. When the ferromagnetic layer is metallic NiFe, there is an effective magnetic field in the ferromagnetic resonance measurements at low temperatures. We attribute these extra fields to the exchange coupling to the surface states. When ferrimagnetic insulator yttrium iron garnet (YIG) replaces NiFe, we see evidence of strong interfacial coupling manifested as large interfacial anisotropy. Magnetic proximity effect are observed from magnetoresistance and temperature-dependent ferromagnetic resonance measurements. Our results show that the topological insulators can play an essential role for the development of spintronic devices.

In solid state conductors, linear response to a steady electric field is normally dominated by Bloch state occupation number changes. Recently it has been realized that, for a number of important physical observables, the dominant response is electric-field induced coherence between Bloch states in different bands. Examples include the anomalous and spin-Hall effects, spin torques in magnetic conductors, and the minimum conductivity and chiral anomaly in Weyl and Dirac semimetals. Here we first discuss the framework of a general quantum kinetic theory of linear response to an electric field which can be applied to solids with arbitrarily complicated band structures and includes the inter-band coherence response and the Bloch-state repopulation responses on an equal footing. We demonstrate that the inter-band response in conductors consists primarily of two terms: an intrinsic contribution due to the entire Fermi sea that captures, among other effects, the Berry curvature contribution to wave-packet dynamics, and an anomalous contribution caused by scattering that is sensitive to the presence of the Fermi surface. Next we discuss an important interband coherence effect on Hall transport. Classical charge transport, such as longitudinal and Hall currents in weak magnetic fields, is usually not affected by quantum phenomena. Yet relativistic quantum mechanics is at the heart of the spin-orbit interaction, which has been at the forefront of efforts to realize spin-based electronics, new phases of matter and topological quantum computing. In this work we demonstrate that quantum spin dynamics induced by the spin-orbit interaction are directly observable in classical charge transport. We determine the Hall coefficient R_H of two-dimensional hole systems at low magnetic fields and show that it has a sizable spin-orbit contribution, which depends on the density p, is independent of temperature, is a strong function of the top gate electric field, and can reach 30% of the total. We provide a general method for extracting the spin-orbit parameter from magnetotransport data, applicable even at higher temperatures where Shubnikov-de Haas oscillations and weak antilocalisation are difficult to observe. Our work will enable experimentalists to measure spin-orbit parameters without requiring large magnetic fields, ultralow temperatures, or optical setups.

Due to their large bulk band gap, bismuthene, Sb and As on SiC offer intriguing new opportunities for room-temperature quantum spin Hall (QSH) applications. Although edge states have been observed in the local density of states (LDOS) of bismuthene/SiC [1], there has been no experimental evidence until now that they are spin-polarized and topologically protected. We predict experimentally testable fingerprints of these properties originating from magnetic fields, such as changes in the LDOS and in ballistic magnetotransport [2]. We show that the edge termination, zigzag versus armchair, and large Rashba SOC result in fundamental differences of the helical edge states and their protection in bismuthene/SiC compared to those in the better studied HgTe quantum wells [3,4]. In particular, for armchair edges we find a distinctive behavior for out-of-plane fields (gap of a few meV between the QSH states) and fields along the edge direction (no gap). While we focus on bismuthene/SiC, our main findings are also applicable to other honeycomb-lattice-based QSH systems, revealing an unexpected robustness of the QSH states in these systems against magnetic fields due to the interplay between topology and geometry. [1] F. Reis, G. Li, L. Dudy, M. Bauernfeind, S. Glass, W. Hanke, R. Thomale, J. Schäfer, R. Claessen, Science 357, 287 (2017) [2] F. Dominguez, B. Scharf, G. Li, J. Schäfer, R. Claessen, W. Hanke, R. Thomale, E. M. Hankiewicz, preprint [3] G. Tkachov, E. M. Hankiewicz, Phys. Rev. Lett. 104, 166803 (2010) [4] B. Scharf, A. Matos-Abiague, J. Fabian, Phys. Rev. B 86, 075418 (2012); B. Scharf, A. Matos-Abiague, I. Zutic, J. Fabian, Phys. Rev. B 91, 235433 (2015)

Dynamics of domain wall (DW) nucleation, relaxation and switching is studied in a series of GaMnAsP films with anisotropy perpendicular to the film plane using ac susceptibility measurements. Measurements of dc and ac magnetization along the [001] crystal axis (the easy axis of GaMnAsP film) were performed by a superconducting quantum interference device (SQUID). The ac driving field was applied at various frequencies, from 0.1 Hz to 1000 Hz at an amplitude of 3.77 Oe. Characteristic dynamic modes in the ac spectra were mapped in all GaMnAsP films near the Curie temperature T_C, where the coercive field is comparable to the ac driving field. More significantly, we were able to identify DW relaxation, sliding, and switching from Cole-Cole plots. We found that DW segmental relaxation becomes dominant in GaMnAsP films with higher P concentration, where the coercive fields are enlarged as P concentration increases.

Using solid-state spin qubits for quantum information processing requires a detailed understanding of the decoherence mechanisms. For electron spins in quantum dots (QDs), considerable progress has been achieved in strong external magnetic fields; however, decoherence at very low magnetic fields remains puzzling when the magnitude of the Zeeman energy becomes comparable with intrinsic couplings. Phenomenological models of decoherence currently recognize two types of spin relaxation; fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼2ns) and a much slower process (>1μs) of irreversible relaxation of the spin polarization due to nuclear spin co-flips with the central spin. Here, we demonstrate that not only two but three distinct stages of decoherence can be identified in the relaxation. Measurements and simulations of the spin projection without an external field clearly reveal an additional decoherence stage at intermediate timescales (∼750ns) [1]. The additional stage corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath itself induced by quadrupolar coupling of nuclear spins to strain driven electric field gradients, leading to a relatively fast but incomplete non-monotonic relaxation of the central spin. For hole spins we observe a more than two-orders of magnitude slower dephasing due to the reduced hyperfine interaction of the p-like wavefunction. In addition, time domain measurements of T2* and T2 (via spin-echo) are discussed. [1] A. Bechtold et al., Nature Physics 11, 1005-1008 (2015) [2] T. Simmet et al., in preparation

Strong quantum confinement in semiconductors compresses 
the wavefunctions of band carriers to nanometer-scale volumes, significantly enhancing their interactions with dopants. In magnetically doped 
semiconductors, where paramagnetic dopants couple to band carriers via strong sp–d spin 
exchange, giant magneto-optical effects can be 
realized using few, or even single, 
impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit: the statistical N^1/2 fluctuations of N 
spins are expected to generate giant effective magnetic fields 
B_eff, which dramatically impacts carrier spin dynamics, 
 even in the absence of an applied field. Here, I present measurements of both the initial and final stages of exciton magnetic polarons (EMPs) in lightly doped Cd_1-xMn_xSe colloidal nanocrystals. In the first section, I show our ultrafast spectroscopic investigations of the large B_eff in this system. At B_applied= 0T, extremely rapid 
(300–600 GHz) electron spin precession is 
observed, indicating B_eff∼15−30 T [1]. In the second part of the talk, I show our work on fully-formed EMPs. Using the highly sensitive technique of resonant photoluminescence, we directly measure the EMP binding energy to be tens of meV. Temperature- and field-dependent studies reveal that the exchange field is approximately 10 T, which agrees with theoretical estimates [2]. Taken together, the measurements of initial, ultrafast coherent dynamics and final, static formation states, give us a comprehensive picture of EMPs in doped nanocrystals. [1] W. D. Rice et al. Nature Nanotech. 11, 137 (2016). [2] W. D. Rice et al. Nano Lett. 17, 3068 (2017).

Magnetically doped topological insulators (TIs) attract a great deal of interest for both fundamental scientific studies and potential applications. These systems are promising for the realization of quantum anomalous Hall effect, and more generally, for the potentially controllable magnetism, which can underlay many useful technological applications. Here, we reveal the first photoinduced magnetization dynamics in a 40 nm thin film of magnetically doped TIs, CrxBi2-xTe3 with x=0.29 using femtosecond time-resolved magneto-optical Kerr effect (MOKE) spectroscopy. The ultrafast spin dynamics of the sample at low temperature 5 K is characterized by two demagnetization dynamics, attributed to spin-spin (~0.5 ps) and spin-phonon (~10 ps) scatterings, respectively, followed by a slow recovery process with 100s of ps time scale. While it gets faster at elevated temperature and finally vanishes above ~ 90 K, much higher than the reported Curie temperature Tc~23.8 K, due to strong Van Vleck magnetic susceptibility in the magnetically doped TIs system, distinct from the case of GaMnAs. In addition to providing implications for deeper understanding of spin dynamics in magnetically doped TI systems, the study will potentially benefit the development of magnetic TIs-based spintronic devices.

We report on epitaxial crystal growth and spin Hall effect in BiSb topological insulator thin films. We show that BiSb thin films with conductivity as high as σ ~ 2.5∗10⁵ Ω^-1m^-1 can be epitaxially grown on GaAs(111)A substrate. Meanwhile, evaluation of spin-orbit-torque in Bi_0.9Sb_0.1/MnGa bi-layers reveals a colossal spin Hall angle of θ_SH ~ 52 and a spin Hall conductivity σ_SH ~ 1.3∗10⁷ ℏ/2e Ω^-1m^-1 at room temperature. We demonstrate that BiSb thin films can generate a colossal spin-orbit field of 2.3 kOe/(MA/cm²) and a critical switching current density as low as 1.5 MA/cm² in Bi_0.9Sb_0.1/MnGa bi-layers. BiSb is the best candidate for the first industrial application of topological insulators.

The transport properties of topological insulator nanostructures prepared by selective-area molecular beam epitaxy is investigated. For the nanocolumn structures based on Sb₂Te₃/Bi₂Te₃-heterostructures pronounced universal conductance fluctuations are observed in the magnetoconductance, indicating phase-coherent transport. Furthermore, angle-dependent measurements indicate that the phase coherent loops are mainly oriented parallel to the substrate plane. Measurements on nanoribbons based on (Bi_0.57Sb_0.43)₂Te₃ revealed a resistance dip due to weak antilocalization as well as universal conductance fluctuations. Here, we also found indications, that the phase-coherent loops are predominantly oriented parallel to the quintuple layers forming the topological insulator.

Molecular spintronics is made possible by the coupling between electronic configuration and magnetic po- larization of the molecules. For control and application of the individual molecular states it is necessary to both read and write their spin states. Conventionally, this is achieved by means of external magnetic fields or ferromagnetic contacts, which may change the intentional spin state and may present additional challenges when downsizing devices. First, we predict that coupling magnetic molecules together opens up possibilities for all electrical control of both the molecular spin states as well as the current flow through the system. Tuning between the regimes of ferromagnetic and anti-ferromagnetic exchange interaction, the current can be, at least, an order of magnitude enhanced or reduced. The effect is susceptible to the tunnel coupling and molecular level alignment which can be used to achieve current rectification. Second, we address the electronically induced anisotropy field acting on a spin moment comprised in a vibrating magnetic molecule located in the junction between ferromagnetic metals. Under weak coupling between the electrons and molecular vibrations, the nature of the anisotropy can be changed from favoring a high spin (easy axis) magnetic moment to a low spin (easy plane) by applying a temperature difference or a voltage bias across the junction. For unequal spin-polarizations in the ferromagnetic metals it is shown that the character of the anisotropy is essentially determined by the properties of the weaker ferromagnet. By increasing the temperature in this metal, or introducing a voltage bias, its influence can be suppressed such that the dominant contribution to the anisotropy is interchanged to the stronger ferromagnet. With increasing coupling strength between the molecular vibrations and the electrons, the nature of the anisotropy is locked into favoring easy plane magnetism.

The field of semiconductor spintronics is well-established. However, the realization of semiconductor devices utilizing spin remains problematic. A search for the solutions of the known problems is now focused on low-dimensional nanostructures, for which the spatial scale of the potential profile matches the characteristic length for the quantum tunneling. The combination of spin effects and resonant tunneling leads to a number of new phenomena discussed in the talk. Tunneling between 2D layers separated by a potential barrier is essentially resonant. The energy and in-plane momentum conservation allow the tunneling only at zero bias between the layers so that the differential conductance exhibits a sharp resonance. Such behavior has been experimentally observed for heterostructures with two quantum wells (QW). With account for spin-orbit interaction (SOI) in the 2D layers the tunneling becomes more intriguing. We have shown theoretically that SOI would manifest itself in the complicated pattern of I-V tunnel characteristic and appears to be very sensitive to the SOI parameters. The phenomenon opens a way to use the 2D-2D tunneling as a highly sensitive spin-orbit spectroscopy tool [1]. The essentially resonant character of the transport between 2D systems also leads to an effective spin injection and high tunnelling magnetoresistance. For the two 2D ferromagnetic layers with the spin split subbands the tunneling is allowed only when the same spin subbands of the two layers are matched, i.e. when the magnetizations of the layers are parallel. Analogously, for the tunneling from a 2D layer with the spin split band into a non-magnetic 2D channel the tunneling is allowed for one spin projection and blocked for the opposite, thus leading to an efficient spin injection. We believe this mechanism of the spin injection can be realized in GaMnAs heterostructures [2]. We also propose a mechanism for the dynamic spin injection in semiconductor heterostructures with a QW and a magnetic impurity layer spatially separated from the QW. The spin polarization of the carriers in a QW originates from spin-dependent tunneling recombination at impurity states in the magnetic layer. The developed theory allowed us to explain the kinetics of photoexcited electrons in experiments with time-resolved photoluminescence in Mn-doped InGaAs heterostructures [3] and study the influence of the Coulomb correlations at the impurity site on the spin injection [4]. [1] I. V. Rozhansky, N. S. Averkiev, E. Lahderanta, Phys.Rev. B 93, 195405 (2016). [2] M. Buchner, T. Kuczmik, M. Oltscher et.al., Phys. Rev. B 95, 035304 (2017). [3] I.V. Rozhansky, K.S. Denisov, N.S. Averkiev et.al., Phys. Rev. B 92, 25428 (2015). [4] N.S. Maslova, I.V. Rozhansky, V.N. Mantsevich et.al., arXiv:1802.06352 (2018).

The recently discovered unidirectional spin-Hall magnetoresistance in nonmagnet-ferromagnet (NM-FM) bilayer structures is believed the only way to electrically sense the in-plane 180-degree magnetization rotation in such NM-FM systems without adding additional terminals or structures. On the other front, spintronic memory and logic devices involving topological insulators (TIs) are being studied intensively due to TI’s exciting potential of efficiently generate spins. In our work, we observed the unidirectional spin-Hall and Rashba−Edelstein magnetoresistance (USRMR) in topological insulator-ferromagnet (TI-FM) layer heterostructures for the first time. TI films (Bi2Se3 or (Bi1−xSbx)2Te3) were first grown by molecular beam epitaxy and then transferred to a magnetron sputtering chamber where the CoFeB and capping MgO were deposited. Then the stacks were patterned into Hall bars and tested with harmonic measurements in a Quantum Design PPMS. The measured longitudinal second harmonic resistance contains the USRMR signal plus contributions from other thermoelectric effects. Then series of transverse second harmonic measurements with various external magnetic field strengths were carried out to carefully determine the contributions of these effects. Finally, the data was analyzed, and results show non-zero USRMR. By varying the temperature, TI thickness and TI material and comparing the USRMR performances, we have also concluded that the topological surface states of TI played important roles in generating USRMR signal. The transport properties and conditions seem to govern heavily on the USRMR performance. And the largest USRMR was observed in 6 quintuple layer thick Bi2Se3 sample at 150K and being more than twice as large as the best reported USMR in Ta/Co samples.

It was recently demonstrated in bilayers of permalloy and platinum, that by combining spin torques arising from the spin Hall effect with Oersted field-like torques, magnetization dynamics can be induced with a directional preference.¹ This “unidirectional” magnetization dynamic effect is made possible by exploiting the different even and odd symmetry that damping-like and field-like torques respectively have when magnetization is reversed. The experimental method used to demonstrate this effect was the spin-torque ferromagnetic (ST-FMR) resonance technique; a popular tool used in the phenomenological quantification of a myriad of damping-like and field-like torques. In this report, we review the phenomenology which is used to describe and analyze the unidirectional magnetization dynamic effect in ST-FMR measurements. We will focus on how the asymmetry in the dynamics also is present in the phase angle of the magnetization precession. We conclude by demonstrating a utility of this directional effect; we will outline an improved experimental method that can be used to distinguish a phase-shifted field-like torque in a ST-FMR experiment from a combination of field-like and damping-like torques.

We theoretically investigate a new kind of nonlinear magnetoresistance on the surface of three-dimensional topological insulators (TIs). At variance with the unidirectional magnetoresistance (UMR) effect in magnetic bilayers, this nonlinear magnetoresistance does not rely on a conducting ferromagnetic layer and scales linearly with both the applied electric and magnetic fields; for this reason, we name it bilinear magneto-electric resistance (BMER). We show that the sign and the magnitude of the BMER depends sensitively on the orientation of the current with respect to the magnetic field as well as the crystallographic axes - a property that can be utilized to map out the spin texture of the topological surface states via simple transport measurement, alternative to the angle-resolved photoemission spectroscopy (ARPES).

Recent theoretical work on circular dichroic effects, for absorption processes in chiral materials, has reopened questions over the possibility that the interactions of vortex beams may display a sensitivity to material handedness. The interest in such a phenomenon arises from the fact that any engagement of optical phase gradients, in quadrupole-allowed electronic transitions, will represent a distinctive form of engagement with chiral matter. This is an issue that numerous careful experiments have so far failed to fully resolve, with some of them giving a clear null result, yet others giving positive indications. A definitive outcome from any such investigation would represent a touchstone for a broader, yet more challenging question: is there any mechanism by means of which twisted light, which conveys both orbital and spin angular momentum, can exhibit coupling between the two? It emerges that such a possibility can be identified, but the constraints upon its manifestation are severe. This presentation sets out the principles and the conclusions to which they lead, informing the pathway for ongoing experimentation.

We present inelastic light scattering experiments on low-energy intrasubband spin-density excitations (SDE) in 12-nm-wide (001)-oriented GaAs-AlGaAs single quantum well samples with balanced Rashba and Dresselhaus spin-orbit interaction strengths. This unique symmetry causes an effective spin-orbit field either parallel or antiparallel to specific in-plane crystal directions, which supports the persistent spin helix. This results in a highly anisotropic splitting of intrasubband SDEs in the conduction band. Measurements are performed in backscattering geometry, where the SDE is formed by spin-flip intrasubband transitions. A wave-vector transfer into the two-dimensional electron system is realized by tilting the sample. By rotating the sample with a rotary stage, we can precisely map the anisotropic spin splitting, which appears for various crystal directions as a double peak line shape of the spectra. In the presence of external magnetic fields, a superposition of both, the intrinsic spin-orbit field and the external magnetic field, occurs. We analyze our experimental spectra via a lineshape analysis, based on the Lindhard-Mermin lineshape, including the effects of the anisotropic spin splitting and the external magnetic field. This allows us to quantitatively deduce the spin-orbit parameters, the electron g factor, and the single-particle relaxation time from our observations.

In this paper, we demonstrate a time-reversal methodology to create diffraction-limited optical focal spot with arbitrarily oriented magnetic dipolar field component using a 4pi microscopic configuration. Through combining the magnetic dipole radiation pattern and the Richards–Wolf vectorial diffraction method, the required illuminations at the pupil plane of a 4pi focusing configuration for the reconstruction of magnetic dipole focal field are found analytically. In general, the calculated pupil field is a complex optical field with amplitude, phase and polarization variations within the cross section. Such required pupil fields can be experimentally generated with the recently develop Vectorial Optical Field Generator. Furthermore, the orientation of the magnetic field component within the doughnut shape focal field can be rotated arbitrarily by modulating the pupil field distribution carefully while maintaining the diffraction-limited focal spot size. These unique focal field distributions are expected to exhibit novel phenomena when interact with various type of structured-materials. These interactions may find important applications in super-resolution microscopy, particle trapping and manipulation, materials characterization, as well as three-dimensional high-density optical storage.

STT-MRAM cell design with dual magnetic tunnel junctions (D-MTJ) is a novel design that show a factor of ~2 in switching performance compared to conventional MRAM design. However, the disadvantage is D-MTJ tends to show lower TMR. In this presentation, we demonstrate it's possible to achieve high TMR D-MTJ cell design without compromising its performance gain. We accomplished this by thinning down the secondary MgO barrier. We observe that when thinning down the secondary barrier, the device level TMR reaches a level close to conventional MRAM design of the same free layer while still preserving its high switching performance

Josephson junctions containing ferromagnetic materials have attracted intense interest both because of their unusual physical properties and because they have potential application for cryogenic memory. There are two ways to store information in such a junction: either in the amplitude of the critical current or in the ground-state phase difference across the junction; the latter is the topic of this paper. We have recently demonstrated two different ways to achieve phase control in such junctions: the first uses junctions containing two magnetic layers in a pseudo spin valve configuration, while the second uses junctions containing three magnetic layers with non-collinear magnetizations. The demonstration devices, however, have not yet been optimized for use in a large-scale cryogenic memory array. In this paper we outline some of the issues that must be considered to perform such an optimization, and we provide a speculative phase-diagram for the nickel-permalloy spin-valve system showing which combinations of ferromagnetic layer thicknesses should produce useful devices.

Quantum dots are nanostructures made of semiconducting materials that are engineered to hold a small amount of electric charge (a few electrons) that is controlled by external gate and may hence be considered as tunable artificial atoms. A quantum dot may be contacted by conductive leads to become the active part of a single-electron transistor, a device that is highly conductive only at very specific gate voltages. In recent years a significant attention has been given to more complex hybrid devices, in particular superconductor-semiconductor heterostructures. Here I review the theoretical and experimental studies of small quantum-dot devices contacted by one or several superconducting leads. I focus on the research on the low-lying localized electronic excitations that exist inside the superconducting gap (Yu-Shiba-Rusinov states) and determine the transport properties of these devices. The sub-gap states can be accurately simulated using the numerical renormalization group technique, often providing full quantitative understanding of the observed phenomena.

Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the topological phase transition. A potent alternative explanation for many of the recent experimental Majorana reports is that a non-topological Andreev state localizes near the end of a nanowire. We compare Andreev and Majorana modes a investigate ways to clearly distinguish the two phenomena. We are also exploring how Andreev states can be chained together along the nanowire to realize the one-dimensional Kitaev model, a discrete way of generating Majorana modes.

The extremely large magnetoresistance demonstrated with graphene nanoribbons (GNRs) is highly attractive as a potential spintronic switch. To develop a computing system in which GNRs can be cascaded requires a mechanism for transforming the GNR resistance into a magnetic field that can activate the magnetoresistance of other GNRs. All-carbon spin logic provides this necessary direct cascading by routing the GNR current through carbon nanotubes (CNTs) positioned adjacent to other GNRs. Applying a constant voltage bias across each CNT-GNR-CNT path results in a magnetic field-dependent electrical current that can perform logical functions, providing a scaling path toward large-scale computing systems.

Magnonics represents a promising alternative to conventional electronics for the development of energy efficient computing platforms. In this context, the nanoscale engineering of spin textures is highly appealing for the development and realization of new nanomagnonic device concepts. Here, we show that reconfigurable nanopatterned spin textures can be used to manipulate spin waves. Magnetic domains and domain walls are written by thermally assisted magnetic scanning probe lithography (tam-SPL) in exchange bias systems. In such structures, we demonstrate through microfocused Brillouin Light Scattering and time resolved scanning transmission X-ray microscopy measurements, the channeling and propagation of confined spin waves. This work opens the way to the use of engineered spin-textures as building blocks of magnonics computing devices.

The surface states of a topological insulator (TI) can be induced with superconductivity through proximity with a conventional s-wave superconductor (S). To study the coupling between two superconducting TI surfaces, we report the growth and fabrication of vertical Josephson junctions with the topological insulators (Bi_0.5Sb_0.5)₂Te₃ sandwiched between Nb electrodes. We observed two Josephson critical currents on the I-V characteristic at 3.5 K, attributed to the bulk Nb and the proximity-induced superconducting (Bi_0.5Sb_0.5)₂Te₃ surfaces, respectively. The enhancement of conductance at these two critical currents leads to a bump and a plateau on the differential conductance spectroscopy. By further cooling down to 300 mK, a zero bias conductance peak (ZBCP) appears on the plateau, which is taken as a feature from the unconventional paring at the (Bi_0.5Sb_0.5)₂Te₃-Nb interface.

Since the discovery of the large spin Hall effect in certain heavy metals, there has been continuous interest in utilizing this spin-orbit torque (SOT) effect in constructing a non-volatile memory that can be switched by an electric current. The key to future application of this type of memory is achieving both a short write time and a low write current, which will lower the energy cost compared to existing and other emerging memory technologies. We demonstrate an efficient way of reducing the switching current in tungsten-based three terminal magnetic tunnel junctions (MTJs) with in-plane magnetization (IPM) using a sub-atomic layer of Hf dusting inserted between the free FeCoB layer and the MgO tunnel barrier. We show with a simple FeCoB-MgO-FeCoB MTJ structure that in addition to low write current, fast pulse switching can be achieved with pulses ≤ 1 ns. We also confirm that in an SAF balanced MTJ structure with a PtHf spin Hall channel that the nanosecond switching behavior is typical of the switching of IPM three terminal spin-orbit-torque devices. We report write error rate of these structures down to ~10^-6 at for 1 ns pulses, demonstrating feasibility for high performance cache memory.

Complementary metal oxide semiconductor (CMOS) technology is reaching towards an inevitable bottleneck due to sizing constraints, power consumption, and speed. Spintronic devices, namely Magnetic Tunnel Junction (MTJ), have emerged as a better replacement for CMOS technology. Several pieces of research have been carried out for MTJ based designs; both at device and circuit levels especially for memory and logic applications. Spin transfer torque (STT) and spin-hall effect (SHE) are two popularly used switching mechanisms in perpendicular magnetic anisotropy (PMA) MTJs. This paper presents the compact model of a differential spin hall effect (DSHE) device that uses both STT and SHE switching mechanisms for memory application. The structure of the model consists of a heavy hall metal sandwiched between two nanomagnets (PMA MTJs). The two nanomagnets, having two free layers (FL) for switching, produces complimentary or, differential magnetic states in each FL through STT+SHE mechanism. It provides efficient write and read operations because combined STT+SHE induced spin current, helps in switching the magnetization of nanomagnets efficiently and the two nanomagnets stores complementary data. The device demonstrates 10 times faster write and faster read operation as compared to conventional two-terminal MTJ. The DSHE device is capable of storing differential bit at the cost of single and very less write energy of 0.076 pJ. It is reported that the device can be used for complementary spin logic applications. The performance metrics such as operational delay, power consumption, and area of the device has been compared with a conventional STT-MRAM and SHE-MRAM.

The recent proposal of cascading spin logic in low-dimensional carbon materials by Friedman et al. [1] has provided a new paradigm to perform efficient high-performance computing. In this scenario, graphene nanoribbon (GNR) transistors, in concert with metallic carbon nanotubes (CNT) interconnect, provide compact logic, low current requirements, and fast switching times. The idea is based on the recent discovery of CNT unzipping mechanisms and exceptional negative magnetoresistance in GNRs for which partially unzipped CNTs permit the development of an all-carbon spintronic logic family. In this talk, we describe the physical model that predicts the onset of spin polarization on zig-zag GNR edges in the presence of magnetic field. The model is based on a tight-binding Hamiltonian in the presence of non-local magnetic field with site-to-site interaction up to the third nearest neighbor. Coupled to a non-equilibrium green function (NEGF) formalism, our model shows electrons of opposing spin result in two stable states: a global antiferromagnetic (AFM) ground state with zero net magnetization, and a ferromagnetic (FM) state at a slightly higher energy with a net magnetization. Both magnetic states have local AFM ordering and large magnetization at the edge sites. A magnetic field arising from a current passing through a CNT close to the edge alters the GNR's magnetic energy via the Zeeman interaction. When the field is strong enough, the change of the GNR ground state from AFM to FM switches its electrical behavior from insulating to conducting. 1. J.S. Friedman et al., Nature Communications 8, 15635 (2017).

Magnetism in two-dimensional (2D) materials has been traditionally limited to the study of extrinsic effects such as the introduction of local magnetic moments in non-magnetic 2D crystals via doping or defect engineering.1 The realization of long-range magnetic order in a 2D material has therefore been a tantalizing concept given the high stakes of the incorporation of magnetism in heterostructures for applications such as spintronics or topological superconductivity. Although the vast majority of van der Waals materials are intrinsically diamagnetic, the family of the transition metal trihalides is an exception. In this talk I will present our recent discovery of the first 2D magnet made out of a single layer of a ferromagnetic insulator: chromium triiodide (CrI3).2 Our experiments also showcase the dramatic layer dependence of the magnetic phase transitions in atomically-thin van der Waals crystals down to the monolayer and the intriguing metamagnetism emerging in bilayer CrI3. I will conclude by outlining the potential of this new class of 2D magnets for optoelectronic applications in view of the combination of magnetic hysteresis with helical luminescence in the monolayer limit.3 [1] O. V. Yazyev, Rep. Prog. Phys. 73 (5), 056501 (2010) [2] B. Huang et al. Nature 117, 610 (2017) [3] K. L. Seyler et al. Nat. Phys. (2017)

Pristine materials seldom appear as we want them. Instead, their appeal typically comes from suitable modifications. Proximity effects are a versatile method to transform a given material by acquiring the properties of its neighbors and becoming, superconducting, magnetic, valley-polarized, or topologically nontrivial [1-3]. This approach is particularly suitable for 2D materials in which the length of the proximity effects exceeds their thickness [1,2]. Advances in (quantum) spin and anomalous Hall effect, as well as (anomalous) valley Hall effect suggest the electronic degrees of freedom (spin, charge and valley) can be used as different information carriers [3]. The realization of multiple Hall effects in a single material provides a fascinating opportunity to manipulate the implementation of such information. Here we predict the realization and manipulation of multiple Hall effects in the proximitized 2D materials based on first-principles calculations and tight binding models. Harnessing such Hall effects associated with multiple degrees of freedom of electrons could enable novel applications in electronics, spintronics, and valleytronics. [1]. P. Lazić, K. D. Belashchenko, I. Žutić, Phys. Rev. B 2016, 93, 241401. [2]. B. Scharf, A. Matos-Abiague, J. E. Han, E. M. Hankiewicz, I. Žutić, Phys. Rev. Lett. 2016, 117, 166806. [3]. T. Zhou, J. Zhang, Y. Xue, B. Zhao, H. Zhang, H. Jiang, Z. Yang, Phys. Rev. B 2016, 94, 235449.

The discovery of new spin-to-charge conversion effects (spin Hall effect (SHE), Rashba-Edelstein effect, spin-momentum locking) is expanding the potential of applications such as the magnetization switching of ferromagnetic elements for memories [1] or the recent proposal of a spin-orbit logic [2] which can have a strong technological impact. Finding routes to maximize the SHE is not possible as long as it remains unclear which is the dominant mechanism in a material. I will present a systematic study in Pt, the prototypical SHE material, using the spin absorption method in lateral spin valve devices. We find a single intrinsic spin Hall conductivity in a wide range of conductivities, in good agreement with theory. By tuning the conductivity, we observe for the first time the crossover between the moderately dirty and the superclean scaling regimes of the SHE, equivalent to that obtained for the anomalous Hall effect. Our results explain the dispersion of values in the literature and find a route to maximize this important effect [3]. We also studied Ta, a material with a claimed giant SHE. We experimentally demonstrate the dominance of the intrinsic mechanism in Ta and the observation of a record value of the spin Hall angle (-35 %) [4]. Finally, I will show how to optimize the spin-to-charge current conversion at room temperature by combining Pt with a graphene channel [5], opening up exciting opportunities towards the implementation of spin-orbit-based logic circuits. [1] C. K. Safeer et al., Nat. Nanotech. 11, 143 (2016) [2] S. Manipatruni et al., arXiv:1512.05428v2 (2017) [3] E. Sagasta et al., Phys. Rev. B 94, 060412(R) (2016) [4] E. Sagasta et al., submitted. [5] W. Yan et al., Nature Comms. 8, 661 (2017)

The cumulative effects of the charge, spin, and heat carried by electrons give rise to a variety of phenomena, most notably the realization of spin-driven thermoelectrics. We model the anomalous Nernst (ANE) and the spin Nernst (SNE) effects and a Seebeck-induced spin Hall conductance (SHC) in high spin-orbit coupled (soc) compounds. The lead chalcogenide, PbTe, is chosen as the representative material. PbTe films, by virtue of a large soc and narrow bulk band gap possess a significant Rashba spin-orbit coupling (RSOC) leading to a Berry curvature that induces anomalous thermal behaviour. In presence of a temperature gradient (ΔT) and magnetic field, the ANE and SNE drive thermal and spin currents - characterized by their respective coefficients - which acquire higher values for a stronger RSOC. Additionally, an extrinsic soc generated by an in-plane electric field offers a gate-like mechanism to control the anomalous thermal and spin currents. As a measure of the thermodynamic efficiency of such processes, we set up a Carnot engine that optimizes for low effective masses – which is a direct outcome of a robust intrinsic soc. Furthermore, contrary to the RSOC-originated anomalous effects, a PbTe-like gradient doped film clamped between contacts maintained at a specific ΔT exhibits a thermally-controlled SHC. The ΔT generates a Seebeck current (I-Seebeck) carried by the RSOC- spin-polarized electrons. The spin current generated by the passage of I-Seebeck is gauged via the SHC as a function of the Seebeck coefficient (S) and ΔT. A larger S manifests in a more pronounced SHC.

Exploiting the spin degrees of freedom of electrons in solid state devices is considered as one of the alternative state variables for information storage and processing beyond the charge based technology. In this regard, two-dimensional (2D) atomic crystals and their heterostructures provide an ideal platform for spintronics. Our findings demonstrate all-electrical spintronic device at room temperature with the creation, transport and control of the spin in 2D materials heterostructures, which can be key building blocks in future device architectures. Graphene is an excellent medium for long-distance spin communication in future spintronic technologies. We demonstrated a long distance spin transport over 16 µm and spin lifetimes up to 1.2 ns in large area CVD graphene at room temperature [1]. Hexagonal boron nitride (h-BN) is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. We demonstrated the spin filtering effect in few layer h-BN junctions leading to a large negative spin polarization up to 65 % in graphene at room temperature [2, 3]. Furthermore, we combined graphene and MoS2 in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin-orbit interaction and spin lifetime at room temperature [4]. [1] Nature Commun. 6, 6766 (2015). [2] Scientific Reports, 4: 61446 (2014) [3] Scientific Reports, 6, 21168 (2016). [4] Nature Commun. 8, 16093 (2017).

Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nano-electronic, opto-electronic and spintronic applications. However, the demonstration of an electron spin transport through a semiconducting MoS2 channel remains challenging. Here we show the evidence of the electrical spin injection and detection in the conduction band of a multilayer MoS2 semiconducting channel using a two-terminal spin-valve configuration geometry. A magnetoresistance around 1% has been observed through a 450 nm long, 6 monolayer thick MoS2 channel with a Co/MgO tunnelling spin injector and detector [1]. It is found that keeping a good balance between the interface resistance and channel resistance is mandatory for the observation of the two-terminal magnetoresistance. Moreover, the electron spin-relaxation is found to be greatly suppressed in the multilayer MoS2 channel with an in-plane spin polarization. The long spin diffusion length (approximately 235 nm) could open a new avenue for spintronic applications using multilayer transition metal dichalcogenides. In this talk, we will present the main issues of the spin-injection problem at ferromagnet/Tunnel barrier/MoS2 interfaces as well as the spin-propagation in multilayered channel like played by MoS2 involving Schottky depletion layers giving thus an extension to earlier modeling of spin-injection in ferromagnet/ssource/ferromagnet systems [2-3]. [1] Shiheng Liang et al., Nat. Comm. 8, 14947 (2017) [2] A. Fert, H. Jaffrès, Phys Rev B64, 184420, 2001 [3] A. Fert, J.-M. George, H. Jaffrès, R. Mattana, IEEE, 54, 921-932 (2007)

The unique electronic band structure in monolayer transition metal dichalcogenides (TMDs) provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL) and a high degree of circular polarization has been theoretically predicted for resonant excitation of TMDs such as MoS2, MoSe2, WS2 and WSe2, yet rarely observed experimentally. In fact, a wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer TMDs, although the reasons for the disparity are unclear. Here we investigate the room-temperature Pcirc in TMD monolayers synthesized via chemical vapor deposition. In each material system, a wide range of Pcirc and PL intensity values are observed. However, there is a pronounced inverse correlation between Pcirc and PL intensity, which we attribute to sample-dependent variations in the exciton radiative and non-radiative lifetime components. Samples that demonstrate weak PL emission and short exciton relaxation time exhibit a high degree of valley polarization. The short exciton lifetime results in a higher measured polarization by limiting opportunity for depolarizing scattering events. These findings clarify the disparities among previously reported values and suggest a means to engineer valley polarization via controlled introduction of defects and non-radiative recombination sites.

Monolayer transiton-metal dichalcogenides (TMDs) have attracted wide attention over the last several years due to their interesting optical properties and promising applications in spintronics, valleytronics, optoelectronics, and energy harvesting [1,2]. In this talk, we present a new Coulomb potential for TMDs, which not only captures the nonhydrogenic Rydberg series of exciton binding energies, but also correctly describes the weak dependence of the trion binding energy on the dielectric materials below and on top of the monolayer [3]. In addition, we explain how various many-body interactions affect the spectrum of monolayer TMDs [4], with emphasis on the coupling between neutral excitons and shortwave plasmons that originate from the spin-split conduction-band valleys [5]. Finally, we explain how exciton-phonon interactions affect the optical emission spectrum in these materials. [1] J.R. Schaibley, et al., Nature Reviews Materials 1, 16055 (2016). [2] S. Manzeli, et al., Nature Reviews Materials 2, 17033 (2017). [3] D. V. Tuan, M. Yang, and H. Dery, arXiv:1801.00477. [4] B. Scharf, D. V. Tuan, I. Zutic, and H. Dery. arXiv: 1801.06217. [5] D. V. Tuan, B. Scharf, I. Zutic, and H. Dery. Phys. Rev. X 7 (4), 041040 (2017).

Magnetic insulators (MIs), especially iron-based garnets, possess remarkable properties such as ultralow damping and long magnon decay lengths, which can provide significant advantages for practical applications with respect to their metallic magnetic counterparts. Recently, robust perpendicular magnetic anisotropy (PMA) is obtained in ferrimagnetic films of thulium, europium, and terbium iron garnet (TmIG, EuIG, and TbIG) with high structural quality down to a thickness of 5.6 nm with saturation magnetization close to bulk [1,2]. By using the spin Hall effect in platinum we have demonstrated efficient spin current injection through the TmIG/Pt interface, which we quantified by the spin Hall magnetoresistance and harmonic Hall effect measurements [1-3]. We then achieved deterministic spin-orbit torque-driven magnetization switching of TmIG(~10 nm)/Pt bilayer both with quasi-dc (5 ms) as well as pulsed currents down to 2 ns width[1,4]. The switching current density is found to be of the order of ~10^7 (~10^8) A/cm2 using dc (pulsed) current, comparable to reported values for Pt/Co[5]. We reveal that the threshold switching current strongly depends on the absence or presence of an initially reversed domain in the structure implying a reversal mediated by efficient current-driven domain wall motion. Ultimately, we investigated the current-driven dynamics of domain walls in TmIG. We found that by solely using electrical currents, domain walls can be efficiently moved with very high mobility. Moreover, the flow regime threshold is found to be an order of magnitude lower with respect to conventional ferromagnets with interfacial PMA, owing to structural quality and bulk-like behavior of 7-nm-thick TmIG. These results suggest the utility of PMA rare earth garnets and pave the road towards ultralow dissipation spintronic devices based on MIs. [1] Avci et al., Nat. Mater. 16, 309 (2017); [2]Quindeau et al., Adv. Elec. Mater. 3, 1600376 (2017); [3]Avci et al., PRB 95, 115428 (2017); [4]Avci et al., APL 111, 072406 (2017); [5]Miron et al., Nature 476, 189 (2011).

Spin Hall nano-oscillator (SHNO) [1] devices hold great promise as extremely compact, broad-band, and versatile microwave oscillators and have unique opportunities for magnonic devices. SHNOs exhibit a strong nonlinearity, which increases their phase noise, but at the same time, strengthens their propensity for injection locking [2] to external sources, and ultimately the possibility of mutual synchronization. Here, we present the first experimental demonstration of the mutual synchronization of nano-constriction SHNOs [3] and the recent progress in the mutual synchronization of such devices. The mutual synchronization is observed both as a strong increase in the power and coherence of the electrically measured microwave signal. The mutual synchronization is also optically probed using scanning micro-focused Brillouin light scattering microscopy (µ-BLS), providing the first direct imaging of synchronized nano-magnetic oscillators. By tailoring the connection region between the nano-constrictions, we have been able to synchronize SHNOs separated by up to 4 micrometers, and we have demonstrated mutual synchronization of as many as nine SHNOs; we will show as well the ability to synchronize a much larger number of such devices 100 NC-SHNOs so far. We will discuss as will the perspectives of our results on mutual synchronization of SHNOs, and how it opens up a direct route for the design of very large SHNO based oscillator networks and pave the way for many research and application opportunities where coherent phase locking is needed, in particular, energy efficient spin wave computing on the nanoscale as spintronic “neuromorphic computing”. [1] V. E. Demidov, et al., Nature Mater. 11, 1028 (2012) V. E. Demidov, et al., Appl. Phys. Lett. 105, 172410 (2014). [2] V. E. Demidov, et al., Nat. Commun. 5, 3179 (2014). [3] A. A. Awad, et al., Nat. Phys. 13, 292–299, (2017).

Inertia is a fundamental property of a particle that can be understood from Newtons laws of motion. In a similar way, any magnetized body must possess magnetic inertia by the virtue of its magnetization. The influence of possible magnetic inertia effects has recently drawn attention in ultrafast magnetization dynamics and switching. Magnetization dynamics at the inertial regime has been investigated using thermodynamic theories that predicted the magnetic inertia can become impactful at shorter timescales. However, at the fundamental level, the origin of magnetic inertial dynamics is still unknown.

Here, we derive rigorously a description of magnetic inertia in the extended Landau-Lifshitz-Gilbert (LLG) equation starting from a fundamental and relativistic Dirac-Kohn-Sham framework. We use a unitary transformation, the so called called Foldy-Wouthuysen transformation, up to the order of 1/c⁴. In this way, the particle and anti-particle in fully relativistic description become decoupled and a Hamiltonian describing only the particles is derived. This Hamiltonian involves the nonrelativistic Schrodinger-Pauli Hamiltonian together with the relativistic corrections of the order 1/c² and 1/c⁴.

With the thus-derived Hamiltonian, we calculate the corresponding spin dynamics leading to the LLG equation of motion. Our result exemplify that the relativistic correction terms of 1/c² are responsible for the Gilbert damping, however, the relativistic correction terms of 1/c⁴ are responsible for magnetic inertial dynamics. Therefore, we predict that the intrinsic magnetic inertia is a higher-order relativistic spin-orbit coupling effect and is expected to be prominent only on ultrashort timescales (subpicoseconds). We also show that the corresponding Gilbert damping and magnetic inertia parameters are related to one another through the imaginary and real parts of the magnetic susceptibility tensor, respectively.

We concisely review the equivalence among the dynamics of spin, a spinning top, and an electron subjected to a magnetic monopole. The equivalence becomes clearer when the finite inertia of spin is taken into account. We describe how to calculate spin inertia, considering metallic ferromagnets as an example, where conduction electrons flow among localized magnetic moments. The presence of conduction electrons effectively changes the dynamics of the localized magnetic moments; in particular, the electrons add a finite inertial term to the magnetic moments. We also introduce the interesting history of monopole harmonics, which is a generalized concept of spherical harmonics describing an electron wave function subject to a monopole magnetic field. It is named by Wu and Yang in 1976, but the function itself appeared in much older literature, such as the Landau- Lifshitz's textbook on quantum mechanics. It appears in quantization of the motion of spinning tops or diatomic molecules, which was already considered in as early as 1926, soon after Schrodinger published his pioneering paper on quantum wave mechanics.

The same abstract as above

In the Edelstein effect, a current in the crystal induces a spin polarization. It occurs in crystals without inversion symmetry, as studied in Rashba systems and in the surface of topological insulators. The spin polarization vanishes in equilibrium, but in the presence of the current, the off-equilibrium electron distribution leads to nonzero spin polarization, due to the spin-split band structure. We propose an analogous effect for orbital angular momentum. For example, in a crystal with helical structure such as tellurium (Te), we propose that a current along the helical axis induces an orbital magnetization. This is analogous to solenoids in classical electrodynamics. Within this analogy to solenoids, we quantify this effect by introducing a dimensionless parameter, which represents a number of turns in the unit cell when regarded as a classical solenoid. We show that it can become much larger than unity, i.e. it is much larger than the value naïvely expected from the lattice structure. Moreover, we propose a similar effect appears for phonons. In crystals, each phonon eigenmode has angular momentum due to rotational motions of the nuclei, but their sum is zero in equilibrium. Meanwhile a heat current in the Te crystal induces a nonzero total angular momentum. We evaluate this effect for GaN and Te by ab initio calculation, and propose experiments to measure this effect.

Generation of spin current is critical to develop technologies based on the current induced control of magnetization via spin orbit torques. The spin Hall effect and the Rashba-Edelstien effect convert charge current to spin current: the size of spin current in some systems is sufficiently large to enable magnetization control of nearby magnetic layers. We have studied various spin conversion processes in heterostructures with large spin orbit coupling. To characterize the magnitude of spin current, transport and voltage measurements are used. We show results of the charge-spin and heat-spin conversion processes in Pt/Co bilayers and multilayers, in which an anomalously large spin Hall magnetoresistance is found. Light-spin conversion, which emerge in structures with spin-split bands, are studied in similar heterostructures.

Spin-related phenomena, such as the giant magnetoresistance and the spin-transfer torque, have led to a new era of nano-spintronics in last two decades. The discovery of these physical phenomena has contributed to a substantial increase in the data storage capacity of computers. Another revolutionary phenomenon, the interfacial Dzyaloshinskii-Moriya interaction (DMI), was recently discovered in ultra-thin ferromagnet/heavy-metal bilayers. The DMI stabilizes nanometre-sized chiral spin textures such as Néel domain walls and hedgehog skyrmions. These chiral objects find the potential for applications in ultra-high density, low-energy, and high-speed memory devices because they are stable due to topological protection and easy to move with high efficiency. As both stability and current-driven speed of chiral spin textures are proportional to the DMI strength [A. Thiaville et al., Europhys Lett. 100, 57002 (2012); A. Fert, V. Cros, and J. Sampiao, Nat. Nanotechnol. 8, 152 (2013)], tremendous efforts are being devoted to finding high-DMI materials. In this respect, understanding the microscopic origin of DMI is of critical importance. Here we discuss the microscopic origin of the interfacial DMI with experimental and theoretical studies as follows: First, we show the temperature dependence of the DMI for a Pt/Co/MgO trilayer; the DMI increases with decreasing temperature in a range from 300 to 100 K. To discuss this temperature dependence of the DMI, that of the spin and orbital magnetic moments of Co and Pt is studied by X-ray magnetic circular dichroism (XMCD) spectroscopy. We find that spin moment values of Co and Pt show temperature dependences due to change in Heisenberg exchange. Furthermore, the intra-atomic magnetic dipole moment, which is due to the asymmetric spin-density distribution, shows strong temperature dependence, suggesting a sizable modification of the charge distribution between the in-plane and the out-of-plane d-orbitals under temperature variation. We also find that the out-of-plane orbital moment shows large temperature dependence while in-plane orbital moment does not, revealing a close connection between the anisotropy of orbital moment and the DMI. The ab-initio and the tight-binding model calculations suggest that the ISB-dependent electron hopping, which gives rise to the asymmetric charge distribution at the interface of the FM/HM, is a possible microscopic origin of the correlation between the orbital anisotropy and the DMI.

We evaluate the non-equilibrium spin polarization induced by an applied electric field for a tight-binding model of electron states at oxides interfaces in LAO/STO heterostructures. By a combination of analytic and numerical approaches we investigate how the spin texture of the electron eigenstates due to the interplay of spin-orbit coupling and inversion asymmetry determines the sign of the induced spin polarization as a function of the chemical potential or band filling, both in the absence and presence of local disorder. With the latter, we find that the induced spin polarization evolves from a non monotonous behavior at zero temperature to a monotonous one at higher temperature. Our results may provide a sound framework for the interpretation of recent experiments.

Gate tunable inverse Edelstein effect is observed at room temperature in Rashba-split 2DEG at the complex oxides interface.

A two-dimensional electron gas (2DEG) formed at LaAlO3/SrTiO3 (LAO/STO) interfaces, which consists of itinerant d-orbital electrons, has been observed a lot of attractive physics, such as ferromagnetism, superconductivity and even their coexistence. The ability of d-electron 2DEG for spintronics has been also demonstrated as a high-efficient spin-charge conversion via the Rashba field and theoretical estimations of a long spin lifetime, so that a spintronics application of d-electron 2DEGs is one of the hottest issues in the oxide physics. Here, we demonstrate room-temperature spin transport at LAO/STO interfaces. We fabricated NiFe (Py) and Pt or Ta wires, separated by less than 1 micron, on LAO/STO substrate (5 unit cells in LAO-layer thickness). Spins are injected from the Py wire into the LAO/STO interface by excitation of the ferromagnetic resonance (FMR) of the Py (spin pumping). The spin current through the LAO/STO interface can be detected as an electromotive force (EMF) by the inverse spin Hall effect (ISHE) from the non-local nonmagnetic wire. We successfully observed the electromotive force from the Pt under the FMR. The EMF has symmetric and asymmetric line shape. The symmetric part is attributed to the EMF of the ISHE (VISHE). In particular, the sign of the VISHE was reversed by inverting the direction of the external magnetic field and by switching detector material from Pt to Ta, where the latter possess a negative spin Hall angle. These results strongly support the notion that over 100 nm spin propagation in the 2DEG indeed has been successfully achieved.

Understanding how spins move at pico- and femtosecond time scales is the focus of much of contemporary research in magnetism. I will go through some basic and more advanced concepts in the exciting emerging field of terahertz (THz) magnetism, where electromagnetic radiation in the 0.1-10 THz range, the so-called THz gap, is used to probe or to control spin dynamics at these time scales. I will give an overview of the current research in THz magnetism. As illustrating examples, I will briefly discuss how low-intensity THz radiation can be used to probe the fundamentals of spin- dependent transport in the linear regime [1]. I will then describe how intense THz fields can be used to drive coherent and incoherent ultrafast spin dynamics in nonlinear regimes, both with broadband [2] and narrowband radiation [3]. Finally, I will show some recent implementation of metamaterials [4] aimed at selectively enhancing the terahertz magnetic field in the near-field [5]. I will also illustrate the design of an anti-reflection coating that allows for table-top, femtosecond pump-probe experiments in generic nanostructures surrounded by highly reflective metamaterials [6]. [1] Z. Jin et al., Nature Physics 11, 761 (2015) [2] S. Bonetti et al, Physical Review Letters 117, 087205 (2016) [3] Z. Wang et al., Selective THz control of magnetic order: new opportunities from superradiant undulator sources, Journal of Physics D: Applied Physics, in press (2018) [4] Hou-Tong Chen et al., Terahertz Science and Technology 1, 42 (2008) [5] D. Polley, et al. Journal of Physics D: Applied Physics 51, 084001 (2018) [6] M. Pancaldi et al. Optics Letters 26, 2917 (2018)

Ultrafast control of magnetic order in materials requires a fundamental understanding of how energy and angular momentum flow between electronic, magnetic, and vibrational degrees-of-freedom. We investigate the ultrafast response of Au/TmIG and Au/YIG bilayers to ultrafast laser heating of the Au electrons. In the picoseconds after heating, large interfacial spin currents occur due to a temperature imbalance between electrons and phonons in the metal, and magnons and phonons in the magnetic insulator. We utilize four different optical probes to develop a complete picture of the heat and spin transport in Au/TmIG and Au/YIG. Magneto-optic Kerr effect measurements of Au at a wavelength of 800 nm detects the spin accumulation in the normal metal that results from interfacial spin-currents. Magneto-optic Kerr effect measurements at 400 nm monitor the ultrafast magnetization dynamics of the garnet insulator that occur due to increases in magnon population. Finally, thermoreflectance measurements at 690 and 950 nm monitor the temperature evolution of the Au electrons and phonons, respectively. Together, these measurements allow us to estimate the magnitude of the transport coefficients responsible for the longitudinal spin-Seebeck effect in these systems. These coefficients include the electron-magnon conductance of the Au/TmIG and Au/YIG interfaces, the electron-phonon coupling in the Au layer, and the magnon-phonon coupling in the TmIG and YIG layers.

Spin currents have been shown to play a key role in the ultrafast laser-driven demagnetization process in ferromagnetic thin films. Here, we show that an oscillating spin current can be generated in the film via the application of a femtosecond laser pulse in the visible range. In particular, we prove that the spin current is due to ballistic electrons traveling back and forth in the film. The dependence of the current intensity on the driving electric field is also investigated.

Generating and controlling spin currents at magnetic/nonmagnetic layer interfaces using ultrashort laser pulses has triggered the development of new high- operational frequency spintronic devices. Recent studies showed that laser-driven spin currents and opto-magnetic torques acting on spins are most effective when an interface is created between ferromagnetic Co and nonmagnetic Pt thin films. Our study focuses on the role of the Co-Pt interface on laser-induced optical torques in the strongly spin-orbit coupled Co/Pt model system. We varied the average roughness at the interface, in the range of 0.1-1.0 nm, by tuning the deposition pressure conditions during fabrication. With the aid of time-resolved THz-emission spectroscopy we detected both the laser-induced helicity-independent(HI) and helicity-dependent(HD) THz-emission due to spin-Hall and spin-orbit torque effects, respectively. We reveal a dramatic change in the detected THz-signals when the interface roughness is varied. For example, the HD-THz emission is observed only when the roughness is 0.3 nm or above. To study the role of intermixing a CoPt spacer layer, with varying compositions, is introduced at the interface. However, the detected THz-emission signals rule out the intermixing effects in determining the helicity-dependence. Moreover, static spin-hall conductivity measurements provide with new insights in understanding the role of spin-orbit coupling, at the Co/Pt interfaces, in laser-induced optical torques on net magnetization. This research is funded by DOE:DE-SC001823

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin texture around their interfaces. This talk will discuss our recently developed approach [1] to this problem where we combine density functional theory (DFT) with Green's function techniques to obtain the spectral function at any plane passing through atoms of TI (such as Bi¬2Se3) and FM (such as Co) or normal metal (such as Cu) layers comprising the interface. In contrast to widely assumed but thinly tested Dirac cone gapped by the proximity exchange field spectral function, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi¬2Se3 in contact with Co near the Fermi level, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions injected by metallic layers and pushed, due to charge transfer from Co or Cu layers, few tenths of eV below the Fermi level for both Bi¬2Se3/Co and Bi¬2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of the Fermi level. Crucially for experiments on spin-orbit torque in TI/FM heterostructures [3], few monolayers of Co adjacent to Bi¬2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures (despite Co magnetization being out of plane) due to proximity spin-orbit coupling in Co induced by Bi¬2Se3. I will also discuss spectral functions and spin texture at heavy-metal/FM (such as Ta/Co and Pt/Co) [2] and Weyl-semimetal/FM interfaces, as well as how DFT Hamiltonian of these heterostructures can be combined with Keldysh Green's functions to compute field-like and antidamping-like components of spin-orbit torque [3,5,6] or spin memory loss [2] at interfaces from first principles. References [1] J. M. Marmolejo-Tejada, K. Dolui, P. Lazić, P.-H. Chang, S. Smidstrup, D. Stradi, K. Stokbro, and B. K. Nikolić, Nano Lett. 17, 5626 (2017). [2] K. Dolui and B. K. Nikolić, . Phys. Rev. B 96, 220403(R) (2017). [3] F. Mahfouzi, B. K. Nikolić, and N. Kioussis, Phys. Rev. B 93, 115419 (2016). [4] P.-H. Chang, T. Markussen, S. Smidstrup, K. Stokbro, and B. K. Nikolić, Phys. Rev. B 92, 201406(R) (2015). [5] F. Mahfouzi and B. K. Nikolić, SPIN 3, 1330002 (2013). [6] F. Mahfouzi, N. Nagaosa, and B. K. Nikolić, Phys. Rev. Lett. 109, 166602 (2012).

Individual magnetic impurities or small collections of magnetic impurities in III-V semiconductors can be identified via scanning tunneling microscopy (STM) [1,2], their exchange interaction can be measured [3], and they can have remarkably long spin coherence times [4]. Spin-1/2 impurities are able to be addressed individually and the eigenstates tailored allowing the construction of engineered spin networks [5]. We describe an approach to explore the coherent spin dynamics of a spin-1/2 defect coupled to an additional spin-1/2 defect via exchange interaction with a spin-polarized STM contact through low-field magnetoresistance. The inherent anisotropy [2,3,5,6] in conjunction with the applied magnetic field should allow one to describe a single spin Hanle curve. In addition, measurements of the spin coherence time and the local hyperfine interaction should be feasible. This analysis is then used to guide the examination of coherent spin-dynamics involving coupled Mn-hole complexes in III-V semiconductors. [1] J. M. Tang and M. E. Flatté, Phys. Rev. Lett. 92, 047201 (2004). [2] A. M. Yakunin et al., Phys. Rev. Lett. 92, 216806 (2004). [3] D. Kitchen et al., Nature 442, 436 (2006). [4] R. C. Myers et al., Nature Materials 7, 203 (2008). [5] K. Yang et al., Phys. Rev. Lett. 119, 227206 (2017). [6] R. E. George et al., Phys. Rev. Lett. 110, 027601 (2013).

Working within the framework of the classical theory of electrodynamics, we derive an exact mathematical solution to the problem of self-field (or radiation reaction) of an accelerated point-charge traveling in free space. We obtain relativistic expressions for the self-field as well as the rates of radiated energy and linear momentum without the need to renormalize the particle’s mass – or to discard undesirable infinities.

I will present a platform for functional sub-50 nm room temperature magnetic skyrmions. Multilayers of Ir/Fe(x)/Co(y)/Pt enable us tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of 2 and 10 respectively. For a systematic investigation of the magnetization dynamics, we determined the damping parameter characterizing the magnetization response, and identified a gyrotropic skyrmion excitation that persists over a wide range of temperatures and across varying sample compositions. Importantly, the skyrmion excitation spectrum is demonstrably renormalized by the interplay of interlayer and interfacial magnetic interactions. To tailor the phenomenology of nanoscale skyrmions, including topological stability and malleability we also studied the formation and evolution of skyrmions at zero field through confinement effects. Notably, confinement-induced stabilization of room temperature skyrmions at zero field in nanodots is found to occur over a wide range of magnetic and geometric parameters. The zero-field skyrmion size can be as small as 50 nm, and varies by a factor of 4 with dot size and magnetic parameters. Crucially, skyrmions with varying thermodynamic stability exhibit markedly different confinement phenomenologies. Through these studies, I will discuss quantifiable insights towards understanding skyrmion stability and dynamics in multilayers, and immediate directions for exploiting their properties in nanoscale devices.

Electric field can tune interfacial magnetism, thus paving the way towards new low power devices using gate voltage. In particular, its effect on skyrmions, which are promising to code information bits, is a critical issue. Here, we first address the effect of electric field on interfacial anisotropy in Pt/Co/AlOx ultrathin trilayers and the possibility to control skyrmion nucleation and annihilation with gate voltage. Then we report the effect of electric field on the interfacial interaction responsible for skyrmions, namely Dzyaloshinskii-Moriya Interaction (DMI). We demonstrate an unprecedented large electric field effect on DMI (β_DMI = 600 fJV^-1m^-1) in Ta/FeCoB/TaOx ultrathin trilayers through Brillouin Light Scattering spectroscopy. Additional Kerr effect observations lead us to propose that electric field could ultimately reverse the sign of DMI, resulting in chirality switch.

Skyrmions are topologically protected spin textures, which can be used in spintronic devices for information storage and processing. Ferromagnetic skyrmions attracted a lot of attention because they are small in size, better than domain walls at avoiding pinning sites, and can be moved very fast by electric currents in ferromagnet/heavy-metal bilayers due to novel spin-orbit torques [1, 2]. Meanwhile, the ferromagnetic skyrmions also have certain disadvantages to employ them in spintronic devices, such as the presence of stray fields and transverse to current dynamics [2]. To avoid these unwanted effects, we propose to look at these topological objects in antiferromagnets. An antiferromagnetic skyrmion has no stray fields and its dynamics are faster compared to its ferromagnetic analogue. More importantly, due to unusual topology it experiences no skyrmion Hall effect [3], and thus is a better candidate for spintronic applications [4]. I will also discuss the lifetimes of both antiferromagnetic skyrmions at finite temperatures [5]. Lastly, I will talk about antiskyrmions – anisotropic topological objects, which were recently observed in systems with anisotropic Dzyaloshinskii-Moriya interaction [6]. I will explain their long lifetimes and current driven dynamics in antiferromagnets based on the transformation between skyrmion and antiskyrmion. [1] I. Ado, O. A. Tretiakov, and M. Titov, Phys. Rev. B 95 , 094401 (2017). [2] K. Litzius, I. Lemesh, B. Kruger, P. Bassirian, L. Caretta, K. Richter, F. Buttner, K. Sato, O. A. Tretiakov, J. Forster, R. M. Reeve, M. Weigand, I. Bykova, H. Stoll, G. Schutz, G. S. D. Beach, and M. Klaui, Nature Physics 13 , 170 (2017). [3] J. Barker and O. A. Tretiakov, Phys. Rev. Lett. 116 , 147203 (2016). [4] C. A. Akosa, O. A. Tretiakov, G. Tatara, and A. Manchon, submitted to Phys. Rev. Lett. (2017); arXiv:1709.02931. [5] P. F. Bessarab, D. Yudin, D. R. Gulevich, P. Wadley, M. Titov, and O. A. Tretiakov, submitted to Phys. Rev. Lett. (2017); arXiv:1709.04454. [6] A. K. Nayak, V. Kumar, T. Ma, P. Werner, E. Pippel, R. Sahoo, F. Damay, U. K. Rößler, C. Felser, and S. S. P. Parkin, Nature 548, 561 (2017).

Magnetic skyrmions are magnetic textures, topologically different from the uniform ferromagnetic state, holding a lot of promise for applications as well as of fundamental interest. They have been observed in magnetic multilayers at room temperature only a couple of years ago [1]. In magnetic multilayers, a key to stabilize magnetic skyrmions is the Dzyaloshinskii-Moriya interaction, obtained at the interfaces between ferromagnetic layers and heavy-metal/oxides spacers, which promotes a unique chirality of the skyrmionic spin textures. Combined with spin-orbit torques generated in heavy-metal layers, this unique chirality allows very efficient current-induced motion at speeds reaching 100m/s [2]. In this presentation, we report about our predictions and observations of hybrid chirality in skyrmionic systems, arising from a competition between the Dzyaloshinskii-Moriya interaction and the other magnetic interactions. After having demonstrated a direct evidence of such hybrid chirality [3] by probing the surface spin ordering of multilayers with circular dichroism in X-ray resonant magnetic scattering [4], we will discuss the impact of hybrid chirality in technologically relevant multilayers depending on different parameters such as the number of stacked layers, interfacial anisotropy or interlayer exchange coupling. In the perspective of technological applications of skyrmions, controlling their chirality to match the spin-orbit torques injection geometry of the multilayers is required to achieve efficient current-induced motion. [1] C. Moreau-Luchaire et al, Nat. Nano. 11, 444 (2016). [2] A. Hrabec et al, Nat. Comm. 8, 15765 (2017). [3] W. Legrand et al, arXiv:1712.05978v2 (2017). [4] J.-Y. Chaleau et al, Phys. Rev. Lett. 120, 037202 (2018).

Spin correlations at hopping are responsible for large magnetoresistance at trap-assisted resonance tunneling between normal metallic and ferromagnetic electrodes. The reason for the spin correlations at hopping is the spin-selective escape rate, which results in non-zero average spin at a trap. This causes a dependence of the trap occupation and, therefore, the current on the average spin. Surprisingly, strong spin dephasing enhances the amplitude of the magnetoresistance at trapassisted tunneling from a normal metal to a ferromagnet. Spin dephasing can also boost the tunneling magnetoresistance in magnetic tunnel junctions. Spin relaxation, however, reduces the spin correlations and associated effects, as expected.

Since the spin on the trap is a vector quantity, it produces unusual correlations in multi-terminal devices. Our analysis of a three-terminal device with normal metallic and ferromagnetic electrodes and trap-assisted hopping implies that the spin correlations result in current-voltage dependences characteristic to a single-electron transistor. Importantly, the transfer characteristics are determined by the spin correlations and the spin blockade alone as, because of the finite transition rate between the trap and the normal metallic electrodes, the current is not Coulomb blocked and it always flows through the trap-source, trap-drain, and trap-gate junctions. However, when both the gate and the source electrodes are ferromagnetic with high interface spin polarizations and anti-parallel, the current through all junctions is either suppressed or it flows only between source and drain depending on the voltages applied, in complete analogy to a single electron transistor.

We theoretically investigate the tunneling transport through a localized region of uniaxial strain in a graphene nanoribbon. When a tunneling current flows along the nanoribbon, the gauge field generated by local strain induces a finite tunneling valley Hall response. A top gate located in the strained region can be used for selective switching of Klein tunneling in dependence of the momentum of the tunneling carriers. This produces a large valley Hall response. Due to time reversal symmetry, the transverse valley signal does not carry net charge. However, the presence of proximity-induced magnetization in the graphene nanoribbon breaks the time reversal symmetry, resulting in the emergence of a charge Hall conductance, even when the magnetization lies in the plane of the nanoribbon. Unlike the recently proposed tunneling planar Hall effect in topological insulators, the strain-induced tunneling Hall effect in graphene nanoribbons does not rely on spin-momentum locking. We discuss the various functionalities of the magnetically proximitized, strained graphene nanoribbon and the possibility of using both the tunneling longitudinal and Hall response for applications in spin straintronic devices.

The anisotropic magnetoresistance (AMR) effect is a fundamental phenomenon in which the electrical resistivity depends on the relative angle between the magnetization direction and the electric current direction. The efficiency of the effect ``AMR ratio'' is defined by (r_parallel - r_perp)/r_perp, where r_parallel (r_perp) is a resistivity for the case of the electrical current parallel to the magnetization (a resistivity for the case of the current perpendicular to the magnetization). The AMR effect has been experimentally studied for various ferromagnets since about 150 years ago. The intuitive explanation about the AMR effect, however, has scarcely been reported. In this study, we first derive a general expression of the AMR ratio extending the conventional model [1] to a more general one [2 - 4]. Using the general expression, we next give the intuitive explanation about the AMR effect [3]. In addition, we show that the negative AMR ratio is a necessary condition for half-metallic ferromagnets [2,3,5,6,7]. [1] I. A. Campbell et al., J. Phys. C 3, S95 (1970). [2] S. Kokado, M. Tsunoda et al., J. Phys. Soc. Jpn. 81, 024705 (2012). [3] S. Kokado and M. Tsunoda, Adv. Mater. Res. 750-752, 978 (2013). [4] S. Kokado and M. Tsunoda, J. Phys. Soc. Jpn. 84, 094710 (2015). [5] F. Yang, Y. Sakuraba, S. Kokado et al., Phys. Rev. B 86, 020409 (2012). [6] Y. Sakuraba, S. Kokado et al., Appl. Phys. Lett. 104, 172407 (2014). [7] S. Kokado, Y. Sakuraba, and M. Tsunoda, Jpn. J. Appl. Phys. 55, 108004 (2016).

In this paper, we present the functionality and model the performance of a new spin-based logic device called the voltage-controlled topological-spin switch (vTOPSS). This device stores information in the magnetization of a thin magnetic insulator (MI) layer, which has ultra-fast dynamics and low-energy dissipation due to its small damping factor. To control the magnetization of the MI, a voltage signal is applied to a proximal topological insulator (TI) layer, which has a high charge-to-spin conversion efficiency at room temperature. The information in the MI layer is read using a magnetic tunnel junction (MTJ) voltage divider with sub-100 mV read voltages. Since its input/output state variables are in the voltage domain, the vTOPSS device does not require any transduction circuitry to be integrated with the CMOS technology. Device optimization shows that the vTOPSS device can operate with sub-25 aJ energy dissipation and < 30 nW power in on-state, these values are much lower than those reported in contemporary spin-based devices. Results confirm that the dominant component of energy dissipation is due to the TI leakage, which can be reduced by suppressing the surface and bulk charge conduction in the TI. Unlike CMOS devices, energy dissipation of the VTOPSS device is proportional to its switching delay. To simultaneously achieve low latency and energy dissipation in vTOPSS, a TI material with a large spin Hall conductivity and negligible charge conductivity is preferred. Interconnect burden on the performance of the vTOPSS device is minimal, which opens up the possibility of using highly resistive nanowires as potential interconnects for this technology.

Topological insulators (TIs) have attracted much attention due to the gapless metallic surface states (SSs) that are protected by the time-reversal symmetry (TRS). The SSs are promising particularly because of the giant spin-charge current conversion efficiencies. However, because the SSs of TIs are protected by the TRS, they are easily broken when a ferromagnetic material is deposited on the surface of TIs due to a magnetic perturbation. Meanwhile, the SSs in topological crystalline insulators (TCIs) are protected by the mirror reflection symmetry of the crystal. Thus, the influence of the breaking of the TRS by the magnetic perturbation is different in TCIs from that in other TIs. SnTe is a typical and promising TCI. The strong spin-orbit coupling in bulk SnTe is also attractive for spintronic applications. However, there have been no reports of successful spin injection into either the SSs or the bulk state of SnTe. In this study, using a high-quality epitaxial (001)-oriented Fe/ SnTe/ CdTe/ ZnTe heterostructure grown on GaAs, we have successfully observed the inverse spin Hall effect in SnTe induced by spin pumping. A relatively large spin Hall angle of ~0.01 was obtained for bulk SnTe at room temperature. This large value may be partially caused by the surface states. Our result suggests that SnTe can be used for efficient spin-charge current conversion. [S. Ohya et al., Phys. Rev. B 96, 094424 (2017).] This work was supported by Grants-in-Aid for Scientific Research and Spintronics Research Network of Japan (Spin-RNJ).

Band gap opening and TI properties can be induced at room temperature in α-Sn (001) thin films by strain and quantum finite-size effects. Indeed, angle-resolved photoemission spectroscopy (ARPES) measurements by Ohtsubo et al. performed on thin α-Sn (001) films grown in-situ by molecular beam epitaxy revealed a Dirac cone (DC) linear dispersion with helical spin polarization around the Γ point of the surface Brillouin zone. We recently reported that a very efficient spin-to-charge conversion (SCC) can be achieved at room temperature by spin pumping into this α-Sn thin films, in clear relation with the inverse Edelstein effect (IEE) induced by the counterclockwise helical spin configuration of the DC identified by ARPES . We will present our work focused on the research of efficient interface for charge current to spin current. We will be discuss in details our study by ARPES of the thickness dependence, as well as the impact of a metallic or insulating capping layer on the α-Sn surface states. For all the thicknesses (20 to 50 atomic layers) a linear energy dispersion of the surface states has been observed, allowing the Fermi velocity, the Fermi level and the density of states to be estimated. Following ARPES experiments we performed magneto-transport experiments using the same samples. It is then possible by tracking the Shubnikov-de Haas oscillations to evidence the signature of the surface states, supporting that transport measurements can indeed reveal surface states.

An electric field modulation of tunneling anisotropic magnetoresistance (TAMR) is reported at the Schottky interface of Ni and Nb-doped SrTiO₃ at room temperature. TAMR response as high as 0.11% is observed in the bias dependence. A bias is applied across the Schottky junction whose variation also modulates the built-in electric field across the interface. This is simulated from the electrostatic modelling across the Schottky interface. Strength of the TAMR response and its modulation with electric field is strongly dependent on the large dielectric permittivity of SrTiO₃ at room temperature and on the modulation of the Rashba spin-orbit field across the Schottky interface respectively. This experiment, shows an unique method to store and manipulate spin states by an electric field across the Schottky interface.

Recently, there has been impressive progress in the field of artificial intelligence. A striking example is Alphago, an algorithm developed by Google, that defeated the world champion Lee Sedol at the game of Go. However, in terms of power consumption, the brain remains the absolute winner, by four orders of magnitudes. Indeed, today, brain inspired algorithms are running on our current sequential computers, which have a very different architecture than the brain. If we want to build smart chips capable of cognitive tasks with a low power consumption, we need to fabricate on silicon huge parallel networks of artificial synapses and neurons, bringing memory close to processing. The aim of the presented work is to deliver a new breed of bio-inspired magnetic devices for pattern recognition. Their functionality is based on the magnetic reversal properties of an artificial spin ice in a Kagome geometry for which the magnetic switching occurs by avalanches.

Topologically protected magnetic features including Skyrmions have drawn a great deal of attention recently for future computing applications due to the unprecedented efficiency with which they can be manipulated by spin current. These features are stabilized by the Dzyaloshinskii-Moriya Interaction (DMI), which favors a chiral winding of neighboring electron spins. In this talk, we examine the impact of an interface-induced DMI on the structure of magnetic domain walls (DWs) in asymmetric [Pt/Co/Ni/Ir]xN based superlattices where structural inversion symmetry is broken. The interfacial DMI vector is measured by examining the asymmetric growth of magnetic bubble domains via Kerr microscopy where both in-plane and perpendicular magnetic fields are applied. Experimental growth velocity measurements are fit to the dispersive stiffness model[1] based on highly anisotropic energy of Dzyaloshinskii DWs coupled with an attempt frequency that depends on the DW’s internal magnetization akin to chiral damping.[2] DWs were directly imaged using high resolution Lorentz mode transmission electron microscopy on thicker asymmetric superlattices which display a remnant labyrinth domain pattern characteristic of bubble materials. Striking features of reconstructed Lorentz induction maps associated with Dzyaloshinskii DWs are presented and discussed theoretically. We confirm a preferred chirality of Néel domain walls in our system and demonstrate the ability to tune the DMI constant through variations in thickness and composition. [1] J. P. Pellegren, D. Lau, and V. M. Sokalski, "Dispersive Stiffness of Dzyaloshinskii Domain Walls," Physical Review Letters vol. 119, p. 027203, 2017. [2] E. Jue, C. K. Safeer, M. Drouard, A. Lopez, P. Balint, L. Buda-Prejbeanu, et al., "Chiral damping of magnetic domain walls," Nat Mater, vol. 15, pp. 272-277. 2016.

In ultrathin ferromagnetic films with perpendicular anisotropy a spin-reorientation transition from out-of-plane to in-plane orientation of the magnetization vector may occur. The competition of exchange and anisotropy energy on the one hand and the dipole interaction on the other hand leads to the formation of stripe domain patterns in the vicinity of the spin reorientation transition. Recently a strong Dzyaloshinskii-Moriya interaction (DMI) was found in various perpendicularly magnetized multilayer systems due to symmetry breaking at interfaces. The dependence of the stripe domain width on the DMI is theoretically and experimentally investigated. A domain spacing model applicable in the normal stripe domain phase is developed describing the dependence of the stripe domain width on the magnetic properties of the sample. We present a new approach to determine the magnitude of the DMI-constant by fitting the stripe domain width as a function of the effective perpendicular anisotropy on wedge-shaped samples with the model. By applying this method to the domain pattern of several ultrathin multilayer samples based on Ni/Fe/Cu(001) imaged by TP-MCD-PEEM the magnitude of the DMI of the FeNi- and NiFe-interfaces is determined. Furthermore we show that the DMI in Ni/Fe/Cu(001) can be considerably enhanced by adding an overlayer of platinum to the sample stack.

Magnetic skyrmions are topologically-protected spin textures with attractive properties suitable for future spintronic device applications. In this talk, our recent experimental observations of skyrmions by utilizing the state-of-the-art X-ray transmission microscopy will be presented. The presentation will first cover the observation of tunable nanosecond dynamics of skyrmions. The finding demonstrated that distinct dynamic states of magnetic skyrmions, triggered by current-induced spin-orbit torques, could be reliably tuned by changing the magnitude of spin orbit torques. [1] Second, I will demonstrate the experimental observation of antiferromagnetically-coupled skyrmions and their current-driven dynamics in a new material system: ferrimagnetic GdFeCo multilayer films. In the work, we confirmed that ferrimagnetic skyrmions can also move at reasonably high-velocity, ~50 m/s, with significantly reduced skyrmion Hall angle, θSkHE ~ 20 degrees. This observation highlights the possibility to build more reliable skyrmionic devices using ferrimagnetic and antiferromagnetic materials. [2] Lastly, the deterministic writing and deleting of a single magnetic skyrmions will be presented. In this work, the stroboscopic pump-probe X-ray measurement serves as a key technique to reveal the deterministic and completely reproducible nature of the observation. We experimentally present that an engineered current pulses can efficiently create and annihilate a single skyrmion in ferrimagnetic materials, GdFeCo, in nanosecond time scale. Micromagnetic simulations reveal the microscopic origin behind the observed topological fluctuation with great qualitative and quantitative agreement. [3] [1] S. Woo et al., Nat. Commun. 8, 15573 (2017) [2] S. Woo et al., Nat. Commun. in press (2018) [3] S. Woo et al., arXiv:1706.06726

Magnetic skyrmions, vortex-like topological spin textures, have attracted much attention in terms of both fundamental physics and spintronics applications. Thus far, skyrmions have been observed in thin-film multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI) and structurally-chiral magnets with bulk DMI. Recently, bulk-DMI-induced skyrmions have been observed above room temperature in Co-Zn-Mn alloys with a β-Mn-type chiral structure [1]. In most chiral magnets, however, skyrmions exist as a thermodynamically equilibrium state only in a narrow temperature and magnetic field region just below the magnetic transition temperature Tc. The limited region of the stable skyrmions is unfavorable for applications. Here, we focused on β-Mn-type Co9Zn9Mn2 (Tc ~ 400 K) and performed small-angle neutron scattering, magnetic susceptibility and Lorentz transmission electron microscope measurements. We demonstrated that skyrmions persist over almost the whole temperature region below 400 K as a long-lived metastable state by performing a conventional field-cooling process. Once created, metastable skyrmions survive above room temperature after removal of magnetic field [2]. These findings exemplify the topological robustness of the once-created skyrmions and provide a significant step toward applications of skyrmions in bulk chiral magnets. In the presentation, we will discuss the details of the above observations, and also show novel skyrmion states in Co-Zn-Mn alloys with other chemical compositions. [1] Y. Tokunaga et al., Nat. Commun. 6, 7638 (2015). [2] K. Karube et al., Phys. Rev. Mater. 1, 074405 (2017).

One major challenge in the next generation THz (1012) technology is to develop highly efficient, ultra-broadband and low-cost terahertz emitters with a gapless spectrum. Up-to-date, most broadband and table-top THz emitters are based on the femtosecond laser excitations, taking advantage exclusively of the charge property of the electron. Here, we introduce two novel types of broadband spin-based THz emitters composed of the ferromagnetic metallic heterostructures [1-4], e.g. (Co, Fe)/Pt and Fe/Ag/Bi. We have carried out detailed thickness-dependent experiments in these samples. Such investigations not only enable us to clarify the intrinsic mechanisms behind the THz radiation - the inverse spin Hall effect (ISHE) and the inverse Rashba-Edelstein effect (IREE), but also help to determine the key parameters to optimize the THz emission. The emitted THz wave, with its phase and polarization easily manipulated by changing the film stacking order and the magnetization direction, has an ultra-broadband width (~0.1-20 THz) and strong amplitude (comparable to the conventional nonlinear crystals). We also demonstrate that the THz radiation arising from both the ISHE and IREE can be selectively superimposed with each other. [1] Seifert, T. et al. Efficient metallic spintronic emitters of ultrabroadband terahertz radiation. Nat. Photon. 10, 483 (2016). [2] Yang, D. et al. Powerful and Tunable THz Emitters Based on the Fe/Pt Magnetic Heterostructure. Adv. Opt. Mater. 4, 1944 (2016). [3] Wu, Y. et al. High-performance THz emitters based on ferromagnetic/nonmagnetic heterostructures. Adv. Mater. 29, 1603031 (2017). [4] Zhou. C et al. Broadband terahertz generation via the interface inverse Rashba-Edelstein effect (submitted).

All-optical switching (AOS) of magnetization by femtosecond laser pulses has received enormous interest recently. Triggered by the notion that AOS might be realized in ultrathin magnetic systems fulfilling very similar requirements as needed for spintronic control of magnetization in magnetic nanowire racetrack architectures, we envision the development of integrated magneto-photonic memories. In such devices data should be coupled between photonic and magnetic degree of freedom without any intermediate electronic steps, while data should be moved along the magnetic racetrack by means of spin-orbit torques. In this presentation, we will report on highly efficient AOS and efficient current-induced domain wall motion in the very same system: Pt/Co/Gd trilayers with perpendicular magnetic anisotropy, a reduced magnetic moment by the anti-parallel coupling of Co and Gd sub-layers and displaying strong spin-orbit torques and Dzyaloshinskii-Moriya interaction. It will be shown how the magnetization of such synthetic ferromagnetic thin films systems can be reversed fully deterministically using single fs pulses. Exposing the system to an even number of pulses results in the original magnetization, while an odd number of pulses leads to the reversed magnetic state in a reducible way and even after thousands of laser pulses. Threshold fluences are determined as a function of Co thickness and record low efficiencies corresponding to below 50 fJ needed to switch a 50x50 nm2 are found. Moreover we quantitatively determined spin orbit torques and analyzed the coherent current-driven motion of opposite (up-down and down-up) domain walls.

It was recently reported that THz emission can be realized in ferromagnetic (FM) /non-FM bilayers via dynamical spin-to charge conversion originating from interfacial spin-orbit coupling or inverted spin-Hall effect (ISHE) [1-3]. We will present our results of THz emission provided by optimized grownth bilayers composed of a high-spin orbit material (Pt, Au :W) in contact with a thin ferromagnetic layer Co/Pt, NiFe/Au:W, NiFe/Au:Ta. Those bilayers are used in experiments combining RF-spin pumping and spin-to-charge conversion by ISHE [4-5]. In comparaison with Co/Pt systems, in NiFe/Au:Ta and NiFe/Au:W bilayers provide a smaller signal even though the spin Hall angles for Au:Ta and Au:W are larger than for Pt [6]. This is mainly due to their smaller spin-mixing conductance in comparaison with Co/Pt. We will discuss the role of the generalized spin-mixing conductance on the spin-transport properties and spin-orbit torques involved in the time-dependent diffusion and relaxation phenomena. We demonstrate that the THz signals strongly depend on the spin Hall angle of non-FM metal, spin diffusion length, and spin-mixing conductance. In the structures with large spin-mixing conductance and spin length diffusion, e.g., Co/Pt, the THz signal is comparable to ZnTe signal. It should be a rapid method to know the characteristic of spintronics samples. We have also studied in the static regime the unconventional Anomalous Hall effect (AHE) of Pt/[CoNi]_N multilayers showing up a characteristic AHE spin-inversion from Pt to Au:W samples by proximity effect. We analyze our results in the series of samples: the exact conductivity profile across the multilayers via the 'extended' Camley-Barnas approach [7] and the spin current profile generated by SHE. The values of spin Hall angles in layers are found: -0.2 for Pt (enhanced compared to CPP geometry)In [Co,Ni]N/Pt sample, we show that the transverse current changes from negative (N<20) to positive (N>20) values. [1] T. J. Huisman et al., Nat.Nano 11, 455–458 (2016) [2] T. Seifert et al., Nat.Photo 10, 483-488 (2016) [3] D. Yang et al., Adv.Opt.Mat. 4, 1944-1949 (2016) [4] J.C Rojas-Sanchez et al., PRL. 112, 106602 (2014) [5] P. Laczowski et al, APL 104, 142403 (2014) [6]P. Laczowski et al, Phys. Rev. B 96, 140405(R) (2017) [7] R. E. Camley et al., Phys. Rev. Lett. 63, 664 (1989); J. Barnas et al., Phys. Rev. B 42, 8110 (1990)