This PDF file contains the front matter associated with SPIE Proceedings Volume 10357, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The spin Hall magnetoresistance is the result of the combined action of the direct and inverse spin Hall effects, and also of the Hanle effect - depolarization of spins by a transverse magnetic field. Swapping spin currents consists in the interchange of the directions of spin polarization and spin flow. Theoretical ideas and experimental results are reviewed.

The angular momentum currents have three distinct forms known as the electron spin current, the incoherent magnon current and the coherent macroscopic magnon current. We present a unified theory to correlate these different angular momentum currents in magnetic multilayered structures by using semi-classical approaches for electrons and magnons. Within each layer, which could be magnetic or non-magnetic and metallic or insulating, the non-equilibrium electron or magnon distributions satisfy the Boltzmann equations for fermions and bosons. The momentum and angular momentum transfer among electrons, incoherent magnons, and order parameters are explicitly taken into account via collision terms. By establishing boundary conditions for spin chemical potentials and the angular momentum current at the interfaces, we determine the distributions and subsequently find the spatial dependence of angular momentum currents. The formalism is applied to various multilayer systems and a few predictions are outlined.

Since the first observation of tunnelling effect in La0.7Sr0.3MnO3 based magnetic structures, the search for high TMR values has been the main goal of research in this field, aiming at developing high sensitive magnetic sensors. Nevertheless, oxides TMR often have high level of noise when using SrTiO3 as insulating barrier drastically reducing the interest of such devices [1]. Recently we introduced the use of heavily doped n-type semiconductor SrTi0.8Nb0.2O3 as fully depleted layer to form the insulating barrier [2-3]. Magneto-transport properties of the MTJs were studied as a function of applied bias, temperature and barrier thickness. It is found that using semiconducting barrier at the place of the standard insulator, leads to a significantly improved reproducibility of results and in the spectral noise density reduced by three orders of magnitude at low temperature. We ascribe that fact to a strongly reduced amount of defects, such as oxygen vacancies, in doped SrTi0.8Nb0.2O3. This results brings novel opportunities to develop high sensitive magnetic devices working at low temperature. [1] A. Solignac, G. Kurij, R. Guerrero, G. Agnus, T. Maroutian, C. Fermon, M. PArnnetier-Lecoeur, Ph. Lecoeur, SPIE Proceedings Series, 2015, 9551, pp.95512F (2016) [2] G. Kurij, A. Solignac, T. Maroutian, G. Agnus, R. Guerrero, L. E. Calvet, M. Pannetier-Lecoeur, and Ph. Lecoeur, Appl. Phys. Lett. 110, 082405 (2017) [3] G. Kurij, L. E. Calvet, R. Guerrero, T. Maroutian, G. Agnus, A. Solignac, and Ph. Lecoeur, Thin Solid Films vol. 716, part B, 82-85 (2016)

We present a general drift-diffusion theory beyond linear response to explain the unidirectional magnetoresistance (UMR) observed in recent experiments in various magnetic heterostructures. In general, such nonlinear magnetoresistance may originate from the concerted action of current-induced spin accumulation and spin asymmetry in electron mobility. As a case study, we calculate the UMR in a bilayer system consisting of a heavy-metal (HM) and a ferromagnetic metal (FM), where the spin accumulation is induced via the spin Hall effect in the bulk of the HM layer. Our previous formulation [cf. PRB 94, 140411(R) (2016)] is generalized to include the interface resistance and spin memory loss, which allows us to analyze in details their effects on the UMR. We found that the UMR turns out to be independent of the spin asymmetry of the interfacial resistance, at variance with the linear giant-magnetoresistance (GMR) effect. A linear relation between the UMR and the conductivity-spin asymmetry is revealed, which provides an alternative way to control the sign and magnitude of the UMR and hence may serve as an experimental signature of our proposed mechanism.

We investigate the compatibility of the concept of "charge to spin current conversion" with the second law of thermodynamics in the context of the spin-Hall effect (SHE). This investigation is performed in the framework of the two spin channel model of the SHE. It is first shown that the spin-accumulation due to spin-flip scattering at the interface is independent of the We investigate the compatibility of the electric charge to spin current conversion with the second law of the thermodynamics in the spin-Hall effect (SHE). This investigation is performed in the framework of the two spin channel model of the SHE. It is first shown that the spin-accumulation due to spin-flip scattering at the interface is independent of the spin-accumulation due to SHE, if the spin-flip scattering length is much larger than the electrostatic screening length [1]. A variational technique based on the least dissipation principle is then applied. We show that, for a bulk paramagnet with spin-orbit interaction, in the case of the Hall bar geometry the principle of minimum dissipated power prevents the generation of transverse spin and charge currents while in the case of the Corbino disk geometry, transverse currents can be produced. More generally, we show that electric charge accumulation prevents the stationary spin to charge current conversion to occur inside the device [2]. [1] J.-E. Wegrowe, "Stationary state and screening equations in spin-Hall effect", arXiv:1701.0601 (2017) [2] J.-E. Wegrowe, R. V. Benda, and J. M. Rubi, Conditions for the generation of sin current in spin-Hall devices, arXiv :1609.03916v1 [cond-mat.mes-hall] 2016.

The electrical injection, transport and detection of spins in silicon is studied using non-local spin-transport devices with an n-type Si channel and Fe/MgO magnetic tunnel contacts. Clear spin-valve and Hanle spin signals with consistent magnitude are observed, unambiguously proving the existence of a spin accumulation in the Si channel. Importantly, the spin accumulation is very large, and increased by one to two orders of magnitude when compared to previous reports. We attribute this to the large tunnel spin polarization of the Fe/MgO contacts. Using devices with different growth procedures it is shown that the quality of the tunnel contacts and the magnitude of the non-local spin signals depend significantly on the details of the contact fabrication. The results demonstrate that a large spin accumulation can indeed be induced in Si, as is required for the development of Si spintronic devices with a large magnetic response.

We theoretically investigate the interplay between SOFs and proximity-induced magnetism in hybrid SM/F heterostructures (SM and F stand for semiconductor ferromagnet, respectively) as well as its effect on spectral and transport properties. The anisotropic spin-dependent transport leads to the emergence of novel magnetoresistive phenomena in planar SM/F multi-terminal devices, where both the longitudinal and transverse Hall-like responses become anisotropic with respect to the magnetization orientation. We also investigate the superconducting proximity effect in SM/S heterostructures (S stands for superconductor), where the interfacial SOFs can lead to the emergence of triplet pairing even for s-type S materials.

Semiconducting transition metal dichalcogenides (TMDs) have set a new paradigm for exploring atomic scale phenomena and future spin and valleytronic device applications. In this talk, I will present our recent investigations on TMDs, such as monolayers of WSe2, WS2, and MoTe2 using high-field magneto-optical spectroscopy. We use photoluminescence, photoluminescence excitation and reflectivity measurements to study the valley Zeeman splitting and valley polarization of neutral and charged excitons in these materials under magnetic fields up to 30 T at cryogenic temperatures [1-5]. While in MoTe2, the neutral A and B excitons as well as the charged A excitons show similar valley Zeeman splittings (g-factor ~ -4) [1], a more involved behavior is observed for singlet and triplet charged excitons in WS2 [3,4]. I will also present high-field Zeeman spectroscopy of single-photon emission in WSe2 [5]. Our results shine light on the salient aspects of the spin- and valley-resolved band structure of TMDs. [1] A. Arora et al., Nano Letters 16, 3624 (2016) [2] R. Schmidt, A. Arora et al., Phys. Rev. Lett 117, 077402 (2016) [3] G. Plechinger et al., Nat. Commun. 7, 12715 (2016) [4] G. Plechinger et al., Nano Lett. 16, 7899 (2016) [5] M. Koperski et al., Nat. Nanotech. 10, 503, (2015)

Though numerous spintronic switching devices have been proposed or demonstrated, there has been significant difficulty in translating these advances into practical computing systems. The challenge of cascading has impeded the integration of multiple devices into a logic family, and several proposed solutions potentially overcome these challenges. Here, the cascading techniques by which the output of each spintronic device can drive the input of another device are described for several logic families, including spin-diode logic (in particular, all-carbon spin logic), complementary magnetic tunnel junction logic (CMAT), and emitter-coupled spin-transistor logic (ECSTL).

Many key technologies of our society, including artificial intelligence (AI) and big data, have been enabled by the invention of transistor and its ever-decreasing size and ever-increasing integration at a large scale. There is a clear scaling limit to the conventional transistor technology, however. Many recently proposed advanced transistors are also having an uphill fight in lab because of necessary performance tradeoffs and limited scaling potential. In this talk, we argue for a new pathway that could enable exponential scaling for multiple generations. This pathway involves layering multiple technologies that are beyond the available functions of conventional and newly proposed transistors. We believe that this potential pathway is becoming clear through recent worldwide effort. In this talk, I will brief you my group’s recent progress on two selected topics along this line, one on the STT-RAM and one on spin logic. Meanwhile I will also introduce a team effort of C-SPIN Center of STARnet program, where systems designers, devices builders, materials scientists and physicist all work under one roof to tackle the scaling issue and overcome key technology barriers. Several successful examples such as the logic in memory, cognitive computing, probabilistic computing and reconfigurable information processing will be discussed.

In this paper, we provide a short overview of efforts to process information with spin as a state variable. We highlight initial efforts in spintronics where devices concepts such as spinwaves, field coupled nanomagnets, etc. were are considered as vehicles for processing information. We also highlight more recent work where spintronic logic and memory devices are considered in the context of information processing hardware for the internet of things (IoT), and where the ability to constantly "checkpoint" processor state can support computing in environments with unreliable power supplies.

The hexagonal close-packed (hcp) Co(0001)/MoS₂ and face-centered cubic (fcc) Ni(111)/WSe₂ interface atomic, magnetic and electronic structures are investigated using first-principles methods based on the density functional theory. We show that the MoS₂ and WSe₂ single layers are covalently bond to the Co(0001) and Ni(111) metal surfaces. We describe the consequences of this bonding on the spin magnetic moments and on the electron states at the vicinity of these interfaces, where MoS₂ and WSe₂ become metallic due to hybridization between Co (or Ni) and S (or Se) atomic orbitals. A finite spin-polarization at the Fermi level is calculated in the MoS₂ and WSe₂ layers at these two interfaces. We also give and estimation of the Schottky barrier height that may appear at the border between the metallic and semiconducting phases of MoS₂ (or WSe₂) near the edge of a Co/MoS₂ or Ni/WSe₂ metallic contact.

We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS₂, a prototypical incommensurate Charge Density Wave (CDW) material. A comparative study of the low-energy electronic structures of 2H-TaS₂ and two other related compounds, 2H-TaSe₂ and 2H-NbSe₂, identifies several generic features of their CDW orders. Firstly, Fermi surface (FS) nesting alone doesn’t seem to give rise to the CDW instability in these compounds. Secondly, partial gapping of the underlying FS surface in the CDW state is common to each of these materials. Finally, the CDW energy gap, unlike the energy gap in a superconductor, is not symmetric with respect to the chemical potential.

The Lorentz force law of classical electrodynamics states that the force 𝑭𝑭 exerted by the magnetic induction 𝑩𝑩 on a particle of charge 𝑞𝑞 moving with velocity 𝑽𝑽 is given by 𝑭𝑭 = 𝑞𝑞𝑽𝑽 × 𝑩𝑩. Since this force is orthogonal to the direction of motion, the magnetic field is said to be incapable of performing mechanical work. Yet there is no denying that a permanent magnet can readily perform mechanical work by pushing/pulling on another permanent magnet or by attracting pieces of magnetizable material such as scrap iron or iron filings. We explain this apparent contradiction by examining the magnetic Lorentz force acting on an Amperian current loop, which is the model for a magnetic dipole. We then extend the discussion by analyzing the Einstein-Laub model of magnetic dipoles in the presence of external magnetic fields.

Atomically sharp oxide heterostructures exhibit a range of novel physical phenomena that do not occur in the parent bulk compounds including conducting, superconducting, and magnetic states. We present a new emergent phenomenon at the LaMnO3/SrTiO3 interface in which an antiferromagnetic insulator abruptly transforms into a superparamagnetic state. Above a critical thickness of LaMnO3 of five unit cells, our scanning nanoSQUID-on-tip microscopy [1] shows spontaneous formation of isolated magnetic islands with in-plane moment of 10^4 to 10^5 μ_B with characteristic diameter of 10 to 50 nm [2]. The nanoscale islands display superparamagnetic dynamics of random moment reversals by thermal activation or in response to an in-plane magnetic field. We propose a charge reconstruction model of the polar LaMnO3/SrTiO3 heterostructure which describes a sharp emergence of thermodynamic phase separation leading to nucleation of metallic ferromagnetic islands in an insulating antiferromagnetic matrix. The model suggests that a gate tunable superparamagnetic-ferromagnetic transition can be induced, holding potential for applications in magnetic storage and spintronics. [1] D. Vasyukov et al., Nature Nanotechnology 8, 639 (2013). [2] Y. Anahory, L. Embon, C. J. Li, S. Banerjee, A. Meltzer, H. R. Naren, A. Yakovenko, J. Cuppens, Y. Myasoedov, M. L. Rappaport, M. E. Huber, K. Michaeli, T. Venkatesan, Ariando, and E. Zeldov, Nat. Commun. 7, 12566 (2016).

We discuss the results of recent theoretical studies of the spin structure of free carriers in semiconductor structures. In addition to spin-orbit splitting of the spectrum of bulk materials, in 2D systems, the spin degeneracy of the levels is lifted in presence of lateral wave vector. The linear spin splitting of the energy levels is important for spin relaxation in quantum well structures because the dominant mechanism of spin relaxation in 2D structures relies on the connection between spin and electron momentum. Also, it is important for description of states in semiconductor-based 2D topological insulators because the structure of the levels strongly depends on the spin-orbit interaction. Combining the envelope function theory and atomistic tight-binding approach, we calculate spin-orbit splitting constants for realistic quantum wells, study the relative importance of the interface and the bulk contributions to the spin splitting; show that the strain due to lattice mismatch is important in both conventional GaAs/AlGaAs and InGaAs/GaAs structures; and describe the fine structure of Dirac states in the HgTe/CdTe quantum wells of critical and close-to-critical thicknesses.

Topological insulators are characterized by the quantum anomalous Hall effect on the topological surface states under time-reversal symmetry breaking. While this effect has been recently observed in a magneto-optical setup upon illumination of weak linearly polarized light, the influence of intense optical field remains largely unexplored. Using the Keldysh-Floquet Green's function formalism, we develop a theory for the dynamical Hall conductivity for arbitrary incident optical frequency in the intense optical field regime. We apply our general theory to the adiabatic, low-frequency regime, and study the breakdown of the one-half Hall quantization under intense optical field. Our results reveal a strong nonlinear dependence of the dynamical Hall conductivity on the incident optical field, which is triggered by the formation of Floquet subbands and the transitions between them.

The topological properties of magnets, encoded in the reciprocal space distribution of the Berry phase, have caused a revolution in our understanding of their transport properties. The discovery that the non-trivial geometry of states in a solid is ultimately related to the orbital properties of electrons allows us to predict from theoretical arguments a pronounced orbital magnetism in various situations ranging from Rashba systems to Chern insulators. Moreover, we demonstrate that a combination of complex geometry in real and reciprocal spaces leads to an emergence of topological orbital magnetism in non-collinear magnets, which overall opens new vistas in large current-induced orbital magnetization response and magnetization manipulation in antiferromagnets. Finally, we predict that in insulating systems with non-trivial topologies the strength of the magneto-electric response as manifested in the magnitude of the current-induced spin-orbit torques and Dzyaloshinskii-Moriya interaction can exceed significantly that of conventional metallic magnets, which opens new perspectives in dissipationless control of magnetization in magnetic materials.

Interfacial spin-flip scattering plays an important role in magnetoelectronic devices. Spin loss at metallic interfaces has usually been quantified by matching the magnetoresistance data for multilayers to the Valet-Fert model, while treating each interface as a fictitious bulk layer whose thickness is $\delta$ times the spin-diffusion length. However, the relation between the parameter $\delta$ and the scattering properties of the interface has been missing. We establish this relation using the properly generalized magnetoelectronic circuit theory, for both normal and ferromagnetic interfaces. It is found that the parameter $\delta$ extracted from the measurements on multilayers scales with the square root of the probability of spin-flip scattering. The spin-flip scattering probabilities are calculated for several specific interfaces using the Landauer-Büttiker method based on the first-principles electronic structure, and the results are compared with experimental data.

Current pumping by an external potential is studied on the basis of the Keldysh Green’s function method, and a pumping formula written in terms of retarded and advanced Green’s functions is obtained. The formula is used to study the spin pumping effect in the case of strong s-d exchange interaction, and the driving field is identified to be the spin gauge field. At the lowest order in the precession frequency of magnetization, the spin gauge field works as a constant potential, and the system is shown to reduce to a static problem of spin current generation by a time-independent potential with off-diagonal spin components. and We theoretically explore the optical properties of a bulk Rashba conductor by calculating the transport coefficients at finite frequencies. It is demonstrated that the combination of direct and inverse Edelstein effects leads to a softening of the plasma frequency for the electric field perpendicular to the Rashba field, resulting in a hyperbolic electromagnetic metamaterial. In the presence of magnetization, a significant enhancement of anisotropic propagation (directional dichroism) is predicted because of the interband transition edge singularity. On the basis of an effective Hamiltonian analysis, the dichroism is demonstrated to be driven by toroidal and quadratic moments of the magnetic Rashba system.

In recent years, Spin Orbit interaction as a source of spin current has been widely used through the physics of Spin Hall Effect (SHE). The peculiar symmetry of SHE allows creating a spin accumulation at the interface between a spin-orbit metal and a magnetic insulator that could lead to a net pure spin current flowing from the metal into the insulator. This spin current will induce a torque on the magnetization and eventually could drive it into steady motion. As a ferromagnetic insulator with a very low Gilbert damping, Yttrium Iron Garnet (YIG) is a very promising candidate to investigate pure spin current phenomena. Only very recently, with the developments in preparation of high-quality nanometer-thick YIG films, the implementation of insulator-based spin-torque devices became practically feasible. Here, we report on the excitation of auto-oscillations in microstructures of YIG(20nm)\Pt(8nm) driven by Spin Orbit torque (SOT). By injection of a dc current in the adjacent Pt layer, we have been able to prove that the SOT due to SHE is sufficiently strong and efficient to drive the YIG magnetization dynamics at frequencies that closely follow the ferromagnetic resonance mode. These auto-oscillations have been detected either inductively using a spectrum analyzer or directly observed using micro-focus Brillouin Light Scattering. Furthermore, we achieved an efficient control of spin waves attenuation length in a YIG waveguide using the SHE in the sub-critical regime i.e. below the auto-oscillations threshold. We believe that our finding pave the path to active magnonics devices made out of YIG films.

When electrons in a magnetic metal are driven far from equilibrium via ultrafast heating of the electrons, the magnetic order undergoes radical changes within tens of femtoseconds due to massive flows of energy and angular momentum between electrons, spins, and phonons. In ferrimagnetic metals such as GdFeCo, ultrafast optical heating can deterministically reverse the magnetization in less than a picosecond. In this talk, I describe our experimental work to gain a better understanding of how energy is exchanged between electrons, phonon, and spins in a magnetic metal following ultrafast heating. We use time-resolved measurements of the magneto-optic Kerr effect to record the response of ferro- and ferri-magnetic metals to heating via ultrafast optical or electrical pulses. Picosecond electrical pulses are generated with photoconductive Auston switches. By comparing the magnetic dynamics that result from electrical vs. optical heating, we identify differences in the rate of energy transfer to phonons from thermal vs. nonthermal electrons. We also find that both optical and electrical heating are effective for ultrafast switching of ferrimagnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub-10 ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than other electrically controlled magnetic switching mechanisms.

Representing the future of spintronics, femtosecond spin current (SC) pulses constitute a versatile tool to transfer spin and control magnetization on the ultrafast timescale. It is therefore of paramount importance to understand the kinetics of these pulses and the fundamentals of their interaction with magnetized media. In our work, we demonstrate the key role of interfaces for the SC dynamics in Fe/Au/Fe multilayers. In particular, we argue that both (i) demagnetization caused by a pulse of hot electrons and (ii) spin transfer torque exerted by the orthogonal to the Fe magnetization projection of magnetic moment delivered by SC pulse are localized in the vicinity of the Fe/Au interface. We analyze both processes in details, showing that the SC-driven excitation of the sub-THz spin wave dynamics in Fe film is enabled by the spatial confinement of the exerted spin transfer torque. Moreover, a pulse of hot electrons leads to the efficient demagnetization of the Fe film. By disentangling the magneto-optical Kerr effect (MOKE) transients we demonstrate the strong spatial non-uniformity of this demagnetization. We argue that simultaneous recording of transient MOKE rotation and ellipticity is crucial for drawing such conclusions. Our findings have a twofold impact: firstly, they illustrate rich opportunities of utilizing SC pulses for manipulation of magnetization in ferromagnets and, secondly, they highlight the importance of spatial localization for understanding the ultrafast spin dynamics in multilayers.

The manipulation of magnetization by an electric current can be obtained thanks to the spin-transfer torque (STT) phenomenon. The two main mechanisms that give rise to this phenomenon are the spin filtering torque (SFT) and the spin-orbit torque (SOT). The former is obtained in multilayer systems by passing a spin-polarized current through the multilayers, whereas the latter is achieved through a direct transfer of angular momentum from the crystal lattice through the spin–orbit interaction. In this work, we study the influence of these two mechanisms on the magnetization dynamics of a three-terminal spin-torque oscillator. The device is composed of either a spin-valve (SV) or a magnetic tunnel junction (MTJ) on top of a Pt wire. The system can be excited either by SFT or by SOT depending on whether the current is applied through the magnetic structure (SV or MTJ) or through the Pt wire. Finally, each device is compared to a reference sample where the Pt wire is replaced by a Cu wire. Therefore, no SOT is expected in this second set of devices. In this presentation, we compare the different types of devices in order to understand the transport in the three terminal devices and study the role of the different mechanisms (SFT and STT) on the magnetization dynamics. Finally, we propose a method to compare the spin transfer efficiency of the two mechanisms on a single device.

Pulsed laser deposition (PLD) of high quality nanometers thick (YIG) films have recently allowed to open the field of spintronics and magnonics nanostructured magnetic insulators[1,2]. YIG is a versatile material in term of anisotropy or magnetization as doping and growth induced strain can significantly change those properties. We present here the effect of Bi substitution on Bi:YIG PLD grown films with thicknesses ranging from 10 nm to 40 nm. By using lattice matched substituted GGG substrates (sGGG) it is possible to stabilize for specific growth conditions an out-of plane easy magnetization axis. We present comprehensive structural charcterisation using X-ray diffraction and squid magnetometry that shows the transition from an easy in-plane magnetization direction to an out-of plane magnetization. The effect of Bi doping is to significantly increase the Faraday rotation of the films. Using magneto-optical Kerr microscopy, it is therefore possible to observ the shape and the sizes of the magnetic domains for films thicknesses down to 10 nm. Using the Kooy and Enz model it is possible to extract a domain wall energy of 0.49 erg/cm2. [1] O. D. Kelly et al., Applied Physics Letters 103, 4, 082408 (2013). [2] M. Collet et al., Nature communications 7, 10377 (2016). [3] C. Kooy and U. Enz, Philips Res. Rep., vol. 15, pp. 7-29(1960)

A lateral-type spin-photodiode having a refracting facet on a side edge of the device is proposed and fabricated experimentally. The light impinged on the side of the device is refracted and shed directly on the backside of a spin-detecting Fe contact where spin-polarized carriers are generated in a thin InGaAs active layer and injected in the Fe contact through a crystalline AlO_x tunnel barrier. Experiments are carried out at room temperature with photocurrent set up with circular polarization spectrometry, through which light-helicity-dependent photocurrent component, ΔI, is obtained with the spin detection efficiency F ≈ 0.4 %, where F is the ratio between ΔI and total photocurrent. This value is the highest reported so far for lateral-type spin-photodiodes. It is discussed that improving the quality of the p-InGaAs/x-AlOx/Fe interfaces will give rise to higher F values.

In this paper, we demonstrate a very efficient electrical spin injection into an ensemble of InAs/InGaAs quantum dots at zero magnetic field. The circular polarization of the electroluminescence coming from the dots, which are embedded into a GaAs-based Spin Light Emitting diode reaches a value as large as 20% at low temperature. In this device, no external magnetic field is required in order to inject or read spin polarization thanks to the use of an ultrathin CoFeB electrode (1.1 nm), as well as p-doped quantum dots (with one hole per dot in average) as an optical probe. The electroluminescence circular polarization of the dots follows the hysteresis loop of the magnetic layer and decreases as a function of bias for large voltages. In a reverse way, we have also investigated the possibility to use such a device as a photodetector presenting a photon helicity-dependent photocurrent. We observe a weak asymmetry of photocurrent under right and left polarized light that follows the hysteresis cycle of the magnetic layer, and the effect decreases for increasing temperatures and can be controlled by the bias.

The origin of the ultrafast demagnetisation has been a mystery for a long time. Recently we have proposed an approach based on spin dependent electron superdiffusion. [1-3] A number of experimental works have confirmed the importance and the amplitude of the superdiffusive spin transport for ultrafast magnetisation dynamics [4-7]. In particular the spin superdiffusion model predicted the transfer of magnetisation in the non-magnetic substrate and the possibility of increasing the magnetisation: both phenomena were experimentally confirmed. [4-5] We predict the possibility of injecting ultrashort (sub-picosecond) spin current pulses from a ferromagnetic metallic layer undergoing ultrafast demagnetisation into a semiconducting substrate. [8] After laser excitation, energetic carriers can overcome the semiconductor bandgap. We address the complex interplay of spin diffusion, the formation of high electric fields at the metal/semiconductor interface, and the concomitant thermalisation of the laser excited carriers by ad hoc numerical techniques. We show that spin currents pulses hundreds of femtoseconds long are injected in the semiconductor and present a record spin polarisation. Such spin current pulses have the possibility of becoming the carriers of information in future spintronics running at unprecedented frequencies above the THz regime. [1] M. Battiato, K. Carva, P.M. Oppeneer, Phys Rev. Lett. 105, 027203 (2010). [2] M. Battiato, K. Carva, P.M. Oppeneer, Phys Rev. B 86, 024404 (2012). [3] M. Battiato, P. Maldonado, P.M. Oppeneer, J. Appl. Phys. 115, 172611 (2012). [4] A. Melnikov et al., Phys. Rev. Lett. 107, 076601 (2011). [5] D. Rudolf, C. La-O-Vorakiat, M. Battiato et al., Nature Comm. 3, 1037 (2012). [6] A. Eschenlohr,* M. Battiato,* et al., Nature Mater. 12, 332 (2013). [7] T. Kampfrath, M. Battiato, et al, Nature Nanotechnol. 8, 256 (2013). [8] M. Battiato and K. Held, Phys Rev. Lett. 116, 196601 (2016).

Interfacial spin-orbit torques (SOTs) enable manipulation of the magnetization through an in-plane charge current, which has drawn increasing attention for spintronic applications. In the search for material systems that can provide efficient SOTs, much work has been focused on polycrystalline ferromagnetic metal/non-magnetic metal bilayers [1-3]. Here the current flows in the non-magnetic layers and induces a torque at the interfaces via the spin Hall effect of the non-magnetic layer [1], the Rashba effect at the interface [2] or spin-momentum locking when a topological insulator is involved [3]. In this presentation, we report the observation of robust SOT occuring in a well characterized single crystalline Fe/insulating GaAs (001) interface at room temperature where the SOT is caused by the lack of space inversion symmetry at the interface. We find that the magnitude of the interfacial SOT per unit charge current density is comparable in strength with that in ferromagnetic metal/non-magnetic metal systems. This large magnitude also allows for the observation of spin-to-charge current conversion at the interface, which is known as spin-galvanic effect [4]. The results suggest that single crystalline Fe/GaAs interfaces may enable efficient magnetization manipulation through purely electric means [5]. References [1] L. Liu, et al., Science 336, 555 (2012). [2] I. M. Miron, et al., Nature 476, 189 (2011). [3] A. R. Mellnik, et al., Nature 511, 449 (2014). [4] S. D. Ganichev, et al., Nature 417, 153 (2002). [5] L. Chen, et al., Nature Commun. 7, 13802 (2016).

We have studied an infinite Hubbard chain with a spin-orbit coupling term, submitted to a uniform magnetic field as well as local phonons. By means of a Lang-Firsov transformation, we show that an effective interacting fermion model emerges. Moreover, a self-consistent mean-field theory of this model, formulated in terms of thermal Green's functions, shows that a BCS term emerges, thus leading to a superconducting phase transition at zero temperature. We find analytical expressions for the phase boundary, that agree well with exact numerical diagonalization of the Hamiltonian.

The spin coupling between spins of conduction electron in heavy metal (HM) and localized electrons in ferromagnetic (FM) material influences the magnetization of the FM layer via the spin-orbit torque (SOT). The two main phenomena contributing to SOT are bulk spin Hall effect arising from spin scattering in the heavy metal layer and Rashba effect, which is an interfacial spin orbit coupling at the FM/HM interface. The intensity of the SOT is generally characterized by two effective fields: field-like term and damping-like term. Various techniques have been used to quantify the two effective fields, such as current-induced domain wall motion, ferromagnetic resonance techniques, and SOT-assisted magnetization switching. However, the reported amplitude of the fields is highly dependent on the experimental technique. We present our recent results on the effective fields measurements in multilayered ferromagnetic films and introduce a self-validating method, which simultaneously quantify both the field-like and damping-like terms in structures with in-plane magnetic anisotropy. An analytical expression is derived and experimentally verified using the harmonic Hall resistance measurement. The second harmonic Hall resistance is fitted with our derived equation, to extract both the field-like and damping-like effective fields. We show that in structures with in-plane magnetic anisotropy, the field-like term comprises of a constant component and an azimuth-angular-dependent term. For structures with perpendicular magnetic anisotropy, our analytical expression and experimental results reveals an angular dependence of the damping-like term. Finally, a novel approach to achieving field free bipolar SOT induced magnetization is presented.

Wide-ranging developments in optical angular momentum have recently led to refocused attention on issues of material chirality. The connection between optical spin and circular polarization, linking to well-known and utilized probes of chirality such as circular dichroism, has prompted studies aiming to achieve enhanced means of differentiating enantiomers – molecules or particles of opposite handedness. A number of newly devised schemes for physically separating mirror-image components by optical methods have also been gaining traction, together with a developing appreciation of how the scale of physical dimensions ultimately determines any capacity to differentially select for material chirality. The scope of such enquiries has substantially widened on recognition that suitably structured, topologically charged beams of light – often known as ‘twisted light’ or ‘optical vortices’ can additionally convey orbital angular momentum. A case can be made that understanding the full scope and constraints upon chiroptical interactions in the nanoscale regime involves the resolution of CPT symmetry conditions governing the fundamental interactions between matter and photons. The principles provide a sound theoretical test-bed for new methodologies.

Two-dimensional (2D) atomic crystals have extraordinary electronic and photonic properties and hold great promise in the applications of photonic and optoelectronics. Here, we review some of our works about the spin-orbit interaction of light on the surface of 2D atomic crystals. First, we propose a general model to describe the spin-orbit interaction of light of the 2D free standing atomic crystal, and find that it is not necessary to involve the effective refractive index to describe the spin-orbit interaction. By developing the quantum weak measurements, we detect the spin-orbit interaction of light in 2D atomic crystals, which can act as a simple method for defining the layer numbers of graphene. Moreover, we find the transverse spin-dependent splitting in the photonic spin Hall effect exhibits a quantized behavior. Furthermore, the spin-orbit interaction of light for the case of air-topological insulator interface can be routed by adjusting the strength of the axion coupling. These basic finding may enhance the comprehension of the spin-orbit interaction, and find the important application in optoelectronic.

Here we present our proposal and initial results on the magnetic field control of plasmon resonances in the mid IR region by the use of the Magneto-Refractive (MR) effect, i.e., a change in the optical properties of the system by magnetic field controlled electrical resistivity. For this we select a Giant Magneto Resistance model system (a Au/Permalloy multilayer), for which changes in resisitivity of the order of 10% by the application of small (of the order of 20 Oe) magnetic fields have been reported. The experiments are carried out in a dedicated magnetic field FTIR (M-FTIR) spectrometer.

We theoretically explore the optical properties of a bulk Rashba conductor by calculating the transport coefficients at finite frequencies. It is demonstrated that the combination of direct and inverse Edelstein effects leads to a softening of the plasma frequency for the electric field perpendicular to the Rashba field, resulting in a hyperbolic electromagnetic metamaterial. In the presence of magnetization, a significant enhancement of anisotropic propagation (directional dichroism) is predicted because of the interband transition edge singularity. On the basis of an effective Hamiltonian analysis, the dichroism is demonstrated to be driven by toroidal and quadratic moments of the magnetic Rashba system. The effective theory of the cross-correlation effects has the same mathematical structure as that of insulating multiferroics.

Magnonics aims to utilize magnons (quanta of spin-waves) to process and transfer digital and analog information, promising high throughput, low power computing for the post-CMOS era. Any such future magnonic circuits will require spin wave signal sources and amplifiers. We propose that acoustic pumping of spin waves provides a mechanism to implement these functions in efficient and highly localized devices. In support of this, we have developed a general theoretical model of linear and parametric magneto-acoustic interactions, covering all possible polarizations of acoustic waves and spin wave modes. The model combines the predictive power of analytical techniques with numerical micromagnetic simulations and is thus well-suited for the design of complex physical devices. Based on this, we determine the configurations most amenable to spin wave generation and amplification. As an experimental prototype we demonstrate an acoustically-pumped amplifier for spin-waves. Our device consists of an yttrium-iron-garnet (YIG) film grown on a gallium gadolinium garnet (GGG) substrate, with a bulk acoustic waves (BAW) transducer fabricated on the top of the GGG substrate. We show experimentally that the amplitude of the propagating spin-waves increases with the application of the BAW. Moreover, this scheme can be used as a signal correlator, where the modulated spin-waves and acoustic waves serve as signal inputs and the resulting modulation of the amplified spin wave serves as the output.

The interaction of a strongly nonlinear spin system with a crystalline lattice through magnetoelastic coupling results in significant modifications of the acoustic properties of magnetic materials, especially in the vicinity of magnetic instabilities associated with the spin-reorientation transition (SRT). The magnetoelastic coupling transfers the critical properties of the magnetic subsystem to the elastic one, which leads to a strong decrease of the sound velocity in the vicinity of the SRT, and allows a large control over acoustic nonlinearities. The general principles of the non-linear magneto-acoustics (NMA) will be introduced and illustrated in ‘bulk’ applications such as acoustic wave phase conjugation, multi-phonon coupling, explosive instability of magneto-elastic vibrations, etc. The concept of the SRT coupled to magnetoelastic interaction has been transferred into nanostructured magnetoelastic multilayers with uni-axial anisotropy. The high sensitivity and the non-linear properties have been demonstrated in cantilever type actuators, and phenomena such as magneto-mechanical RF demodulation have been observed. The combination of the magnetic layers with piezoelectric materials also led to stress-mediated magnetoelectric (ME) composites with high ME coefficients, thanks to the SRT. The magnetoacoustic effects of the SRT have also been studied for surface acoustic waves propagating in the magnetoelastic layers and found to be promising for highly sensitive magnetic field sensors working at room temperature. On the other hand, mechanical stress is a very efficient way to control the magnetic subsystem. The principle of a very energy efficient stress-mediated magnetoelectric writing and reading in a magnetic memory is described.

Surface acoustic wave devices (SAW) have a major interest in sensor applications due to their ease of manufacturing, their sensitivity, small size, and wireless structures. Indeed, especially in SAW resonator geometry, the sensor can be wireless addressed and measured with any embedded power. Surface Acoustic Wave sensors have been used to measure a large variety of stimuli like temperature, pressure, constrain [ref]. It is also known that the velocity or resonant frequency of SAW devices including a magnetostrictive material can be changed by applying a magnetic field. By using magnetostrictive single materials or composites with enhanced magnetoelectric coefficients, various magnetic SAW sensors have been proposed during these last ten years. However, their use to sense magnetic fields is restricted to low magnetic fields and the capability to wireless measure bipolar fields and/or high fields is lacking. Furthermore, the occurrence of hysteresis in the SAW response has not been addressed. In this paper, we report magnetic Surface Acoustic Wave (SAW) sensors that consist of interdigital transducers made of a magnetostrictive material (Ni and TbFe2) or based on exchange-biased multilayers (Co/IrMn and CoFe/IrMn). In the SAW resonator geometry, the wireless measure could be performed in a field range depending on the system studied. The intensity and sign of the applied field could be extracted. Finally, the control of the electrode magnetic properties insured reversible behavior in the SAW response.

In this presentation we will review the spintronic terahertz (THz) phenomena such as ultrafast transient demagnetization [1], generation of spin waves via THz Zeeman torque [2,3], Mott scattering [4], and ultrafast inverse spin-Hall effect [5]. [1] E. Beaurepaire, G. M. Turner, S. M. Harrel, M. C. Beard, J.-Y. Bigot, and C. A. Schmuttenmaer, "Coherent terahertz emission from ferromagnetic films excited by femtosecond laser pulses," Appl. Phys. Lett. 84, 3465 (2004) [2] T. Kampfrath, A. Sell, G. Klatt, A. Pashkin, S. Mahrlein, T. Dekorsy, M. Wolf, M. Fiebig, A. Leitenstorfer, and R. Huber, "Coherent terahertz control of antiferromagnetic spin waves," Nature Photonics 5, 31 (2011) [3] Z. Jin, Z. Mics, G. Ma, Z. Cheng, M. Bonn, and D. Turchinovich, "Single-pulse terahertz coherent control of spin resonance in the canted antiferromagnet YFeO3, mediated by dielectric anisotropy", Phys. Rev. B 87, 094422 (2013) [4] Z. Jin, A. Tkach, F. Casper, V. Spetter, H. Grimm, A. Thomas, T. Kampfrath, M. Bonn, M. Kläui, and D. Turchinovich, "Accessing the fundamentals of magnetotransport in metals with terahertz probes", Nature Physics 11, 761 (2015) [5] T. Seifert, S. Jaiswal, U. Martens, J. Hannegan, L. Braun, P. Maldonado, F. Freimuth, A. Kronenberg, J. Henrizi, I. Radu, F. Freimuth, E. Beaurepaire, Y. Mokrousov, P.M. Oppeneer, M. Jourdan, G. Jakob, D. Turchinovich, L. M. Hayden, M. Wolf, M. Münzenberg, M. Kläui, and T. Kampfrath, "Efficient metallic spintronic emitters of ultrabroadband terahertz radiation", Nature Photonics 10, 483 (2016)

Magnetic skyrmions are intensively explored for potential applications in ultralow-energy data storage and computing. To create practical skyrmionic memory devices, it is necessary to electrically create and manipulate these topologically-protected information carriers in thin films, thus realizing both writing and addressing functions. Although room-temperature skyrmions have been previously observed, fully electrically controllable skyrmionic memory devices, integrating both of these functions, have not been developed to date. In this talk, I will talk about our recent demonstration of a room-temperature skyrmion shift memory device, where individual skyrmions are controllably generated and shifted using current-induced spin-orbit torques. Particularly, it is shown that one can select the device operation mode in between: (i) writing new single skyrmions, or (ii) shifting existing skyrmions, by controlling the magnitude and duration of current pulses. Thus, we electrically realize both writing and addressing of a stream of skyrmions in the device. This prototype demonstration brings skyrmions closer to real-world computing applications.

In ultrathin magnetic films deposited on heavy elements substrates, the interface-induced Dzyaloshinkii-Moriya interaction allows to stabilize non-collinear spin structures like magnetic skyrmions. These topologically protected objects are interesting for future data storage technologies like racetrack memories. We present spin-polarized scanning tunneling microscopy investigations on a three-atomic-layer thick Fe film deposited on Ir(111). In this system, the large epitaxial strain is relieved by the formation of a dense dislocation lines network. This particular structure of the film induces a symmetry breaking with dramatic consequences on the magnetic state. In zero field, spin spirals propagate along the dislocations lines and their period depends on the spacing between these lines, i.e. on the strain relief, which is locally varying [1]. We attribute this effect on the spirals to modifications of the exchange coupling. Single skyrmions appear in external magnetic field. We demonstrate that they can reliably be written and deleted by an STM tip [2]. The strong-bias polarity dependence and the linear behavior of the threshold voltage for switching with the tip-sample distance shows that electric field plays the dominant role in the switching mechanism. This switching between the topologically distinct magnetic states by electric fields may be beneficial in future spintronic devices employing skyrmions as information carriers. We acknowledge financial support by the European Union via the Horizon 2020 research and innovation program under Grant No.665095. [1] Finco et al, PRB 94, 214402 (2016) [2] Hsu et al, Nat. Nanotech. (2016)

Sub-100-nm skyrmions are stabilized in magnetic metallic multilayers and observed using transmission electron microscopy, magnetic force microscopy, scanning transmission X-ray microscopy and X-ray resonant magnetic scattering. All these advanced imaging techniques demonstrate the presence of 'pure' Neel skyrmion textures with a determined chirality. Combining these observations with electrical measurements allows us to demonstrate reproducible skyrmion nucleation using current pulses, and measure their contribution to the transverse resistivity to detect them electrically. Once nucleated, skyrmions can be moved using charge currents. We find predominantly a creep-like regime, characterized by disordered skyrmion motion, as observed by atomic force microscopy and scanning transmission X-ray microscopy. These observations are explained qualitatively and to some extent quantitatively by the presence of crystalline grains of about 20nm lateral size with a distribution of magnetic properties.

In the last five years, Artificial Intelligence has made striking progress, and now defeating humans at subtle strategy games, such as Go, and even Poker. However, these algorithms are running on traditional processors which have a radically different architecture than the biological neural networks they are inspired from. This considerably slows them down and requires massive amounts of electrical power, more than ten thousand times what the brain typically need to function. This energy dissipation is not only becoming an environmental issue, but it also sets a limit to the size of neural networks that can be simulated. We are at a point where we need to rethink the way we compute, and build hardware chips directly inspired from the architecture of the brain. This is a challenge. Indeed, contrarily to current electronic systems, the brain is a huge parallel network closely entangling memory and processing. In this talk, I will show that, for building the neuromorphic chips of the future, we will need to emulate functionalities of synapses and neurons at the nanoscale. I will review the recent developments of memristive nano-synapses and oscillating nano-neurons, the physical mechanisms at stake, and the challenges in terms of materials. Finally, I will present the first achievements of neuromorphic computing with novel nanodevices and the fascinating perspectives of this emerging field.

The brain is a complex network of interconnected circuits that exchange electrical signals with each other. These electrical signals provide insight on how neural circuits code information, and give rise to sensations, thoughts, emotions and actions. Currents methods to detect and modulate these electrical signals use implanted electrodes or optical fields with light sensitive dyes in the brain. These techniques require complex surgeries or suffer low resolution. In this talk we explore a new method to both image and stimulate single neurons using spintronics. We propose using a Spin Transfer Nano-Oscillators (STNOs) as a nanoscale sensor that converts neuronal action potentials to microwave field oscillations that can be detected wirelessly by magnetic induction. We will describe our recent proof-of-concept demonstration of both detection and wireless modulation of neuronal activity using STNOs. For detection we use electrodes to connect a STNO to a lateral giant crayfish neuron. When we stimulate the neuron, the STNO responds to the neuronal activity with a corresponding microwave signal. For modulation, we stimulate the STNOs wirelessly using an inductively coupled solenoid. The STNO rectifies the induced microwave signal to produce a direct voltage. This direct voltage from the STNO, when applied in the vicinity of a mammalian neuron, changes the frequency of electrical signals produced by the neuron.

Topological Hall effect (THE) is a recently discovered transport phenomenon occurring in various magnetic systems due to free carriers interaction with chiral magnetization textures, such as magnetic skyrmions. THE mechanism is based on exchange interaction, thus it is fundamentally different from normal Hall effect and anomalous Hall effect. THE is considered as a very perspective tool to probe topologically nontrivial spin structures as well as for potential device applications based on topology-related properties of nanostructures, one of the most popular concepts in this field is racetrack memory based on magnetic skyrmions. Despite its great importance for fundamental and applied research, the topological Hall effect has still lacked a proper theoretical description. The existing theories of THE consider two limiting cases of either infinitely strong exchange interaction when an adiabatic Berry phase approach is applicable or the case of a weak exchange interaction allowing for perturbation theory analysis. These two theoretical approaches are known to give qualitatively different results regarding THE. The adiabatic Berry phase approximation has revealed that THE is accompanied with a pronounced transverse spin current, so the appearance of transverse charge Hall current requires spin polarization of the carriers in the sample, similarly to the anomalous Hall effect [1]. In the opposite case of a weak exchange the transverse charge current can occur in the absence of a spin current, in this regime THE is expected even for non-polarized carriers [2]. We have developed a theory of THE based on exact solution of a problem of electron scattering on a chiral spin field. The suggested theory fills the gap between the two limiting regimes of THE. We discovered a nontrivial crossover between the two regimes, the transverse pure charge current in the weak coupling case transforms into a pronounced transverse spin current in the adiabatic regime. Thus, the apparent contradiction between the results of adiabatic and perturbative theoretical approaches to THE have been eliminated for the first time [3]. [1] P.Bruno, V.K.Dugaev, M.Taillefumier, Phys.Rev.Lett. 63, 096806 (2004). [2] K.S. Denisov, I. V. Rozhansky, N. S. Averkiev, E. Lähderanta, Phys.Rev.Lett, 117, 027202 (2016) [3] K.S.Denisov, I.V.Rozhansky, N.S.Averkiev, E.Lähderanta, arXiv:1702.04985 (2017).

The orbital magnetic moment is usually associated with the relativistic spin-orbit interaction, but recently it has been shown that noncollinear magnetic structures can also be its driving force. This is important not only for magnetic skyrmions, but also for other noncollinear structures, either bulk-like or at the nanoscale, with consequences regarding their experimental detection. In this work we present a minimal model that contains the effects of both the relativistic spin-orbit interaction and of magnetic noncollinearity on the orbital magnetism. A hierarchy of models is discussed in a step-by-step fashion, highlighting the role of time-reversal symmetry breaking for translational and spin and orbital angular motions. Couplings of spin-orbit and orbit-orbit type are identified as arising from the magnetic noncollinearity. We recover the atomic contribution to the orbital magnetic moment, and a nonlocal one due to the presence of circulating bound currents, exploring different balances between the kinetic energy, the spin exchange interaction, and the relativistic spin-orbit interaction. The connection to the scalar spin chirality is examined. The orbital magnetism driven by magnetic noncollinearity is mostly unexplored, and the presented model contributes to laying its groundwork.

Spin torque magnetic memory (ST-MRAM) is currently under intense academic and industrial development, as it features non-volatility, high write and read speed and high endurance. However, one of its great challenge is the probabilistic nature of programming magnetic tunnel junctions, which imposes significant circuit or energy overhead for conventional ST-MRAM applications. In this work, we show that in unconventional computing applications, this drawback can actually be turned into an advantage. First, we show that conventional magnetic tunnel junctions can be reinterpreted as stochastic “synapses” that can be the basic element of low-energy learning systems. System-level simulations on a task of vehicle counting highlight the potential of the technology for learning systems. We investigate in detail the impact of magnetic tunnel junctions’ imperfections. Second, we introduce how intentionally superparamagnetic tunnel junctions can be the basis for low-energy fundamentally stochastic computing schemes, which harness part of their energy in thermal noise. We give two examples built around the concepts of synchronization and Bayesian inference. These results suggest that the stochastic effects of spintronic devices, traditionally interpreted by electrical engineers as a drawback, can be reinvented as an opportunity for low energy circuit design.

Determining a ranked list or degree of match is frequent computational task, and is one that does not require high numerical precision. A variety of methods for performing such calculations are being explored, such as hardware methods based on lower precision Boolean logic and threshold logic, among others. We have been exploring nonBoolean hardware methods of performing these calculations using phase locking of spin torque oscillator arrays. In this scheme, phase locking of the oscillators in the array maps on to a distance metric based on an L2 norm. We will discuss the progress and challenges of implementing larger (>4) arrays of independently controllable spin torque oscillators into microwave systems that exhibit phase locking, and require stable, high speed, phase sensitive detection. Finally, we will discuss how these results may impact the prospects for applying such oscillator arrays to computational schemes that are more bio-inspired or neuromorphic in nature.

While the high-frequency performance of conventional lasers is limited by the coupled carrier-photon dynamics, spin-polarized lasers have a high potential to overcome this limitation and to push the direct modulation bandwidth beyond 100 GHz. The key is to utilize the ultrafast polarization dynamics in spin-polarized vertical cavity surface-emitting lasers (spin-VCSELs) which is decoupled from the intensity dynamics and its fundamental limitations. The polarization dynamics in such devices, characterized by the polarization oscillation resonance frequency, is mainly determined by the amount of birefringence in the cavity. Using an approach for manipulating the birefringence via mechanical strain we were able to increase the polarization dynamics to resonance frequencies of more than 40 GHz. Up to now these values are only limited by the setup to induce birefringence and do not reflect any fundamental limitations. Taking our record results for the birefringence-induced mode splitting of more than 250 GHz into account, the concept has the potential to provide polarization modulation in spin-VCSELs with modulation frequencies far beyond 100 GHz. This makes them ideal devices for next-generation fast optical interconnects. In this paper we present experimental results for ultrafast polarization dynamics up to 50 GHz and compare them to numerical simulations.

Spin-polarized vertical (external) cavity surface-emitting lasers [Spin-V(E)CSELs] using quantum dot (QD) material for the active region, can display polarization switching between the right- and left-circularly polarized fields via control of the pump polarization. In particular, our previous experimental results have shown that the output polarization ellipticity of the spin-V(E)CSEL emission can exhibit either the same handedness as that of the pump polarization or the opposite, depending on the experimental operating conditions. In this contribution, we use a modified version of the spin-flip model in conjunction with combined time-independent stability analysis and direct time integration. With two representative sets of parameters our simulation results show good agreement with experimental observations. In addition periodic oscillations provide further insight into the dynamic properties of spin-V(E)CSELs.

The injection of spin polarized carriers in semiconductor lasers greatly modifies the device operation. Although the vast majority of spin lasers are based on semiconductors with zinc-blende structure[1], there is a recent exception using nitride-based compounds with wurtzite structure[2], which still lacks a reliable theoretical description. In order to address such deficiency, we investigated (In,Ga)N-based wurtzite quantum wells following typical device geometries[3]. The small spin-orbit coupling in such nitride materials allows the simultaneous spin polarization of electrons and holes, providing an unexplored path to control spin lasers. For instance, based on microscopic gain calculations[3,4] we found a robust gain asymmetry, one of the key signatures of spin laser operation. In addition, we combine these microscopic gain calculations with phenomenological rate equations[5] to investigate threshold reduction features. We show that the lasing threshold has a nonmonotonic dependence on electron spin polarization, even for a nonvanishing hole spin polarization. The complementary information of these theoretical frameworks provides a powerful predictive materials-specific tool to understand and guide the operation of semiconductor spin lasers. [1] Holub et al., PRL 98, 146603 (2007); Lindemann et al., APL 108, 042404 (2016); Rudolph et al., APL 82, 4516 (2003); Frougier et al., APL 103, 252402 (2013). [2] Cheng et al., Nat. Nanotech. 9, 845 (2014). [3] Faria Junior et al., arXiv:1701.07793 (2017). [4] Faria Junior et al., PRB 92, 075311 (2015). [5] Lee et al., APL 105, 042411 (2014).

We present a general method for modeling spin-lasers such as spin-polarized vertical cavity surface emitting laser (spin-VCSELs) with multiple quantum wells including anisotropic effects such as i) the emission of elliptically-polarized photons and originating from unbalanced spin-up and spin-down pumps, ii) the linear gain dichroism originating from the reduction from Td to C2v symmetry group at the III-V ternary interfaces and iii) the locally linear birefringence due to the anisotropic strain field at surface of ½ VECSELs an optical birefringence of quantum wells from the Henry’s factor. New recurrence calculations, together with analytically gain tensor derived from Maxwell-Bloch equations, enable to model emission from multiple quantum well active zones to find the laser resonance conditions and properties of eigenmodes. The method is demonstrated on real semiconductor laser structures. It is used for the extraction of optical permittivity tensors of surface strain and of quantum wells (QWs). The laser structures are also experimentally studied via ellipsometry methods by measurement of the rotation spectra of complete Mueller matrix in the reflection geometry. The anisotropic optical permittivity constants in the spectral range from 0.73 to 6.4 eV are modeled in order to disantangle surface and QWs contributions to the linear optical birefringence of the structures.

Deep Machine Learning is the emerging brain-inspired computing approach that employs artificial neural networks to solve such important problems as image and voice recognition, market behavior prediction, etc. It however still relies on digital CMOS technologies that approach their fundamental limits. As a consequence, there is now significant research activity aimed at finding hardware platforms that would allow for the native implementation of the artificial neural networks. There are already models available that describe human brain operation via synchronization phenomena in complex networks of nonlinear oscillators. This research topic remains mostly theoretical, or numerical, since large-scale oscillator networks are needed, but not easily implemented. However, it was recently demonstrated that so-called spin torque and spin Hall nano-oscillators can act as artificial neurons [1], and their propensity for mutual synchronization on the nano-scale can open up for very large non-linear oscillator networks with different degrees of mutual interactions. To this end, we here present the first experimental demonstration of mutual synchronization of nano-constriction spin Hall nano-oscillators (SHNOs) [2]. 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. In addition, we have demonstrated mutual synchronization of as many as nine SHNOs. Our results opens up a direct route for the design of very large SHNO based oscillator networks and pave the way for the development of a spintronic brain-inspired computing technology. [1] J. Grollier, D. Querlioz, M.D. Stiles, PIEEE 104, 2024 (2016) [2] A. A. Awad, P. Dürrenfeld, A. Houshang, M. Dvornik, E. Iacocca, R. K. Dumas and J. Åkerman, Nature Physics 13, 292–299 (2017).

Conventional transistor and magnet-based memory devices make use of deterministic bits that are either a "0" or a "1". In a series of recent papers, we proposed a probabilistic framework that makes use of unstable devices with low barriers to represent probabilistic bits (p-bit). In this paper, we review some of this earlier work and suggest possible future directions.

Progress of silicon based technology is nearing its physical limit, as minimum feature size of components is reaching a mere 5 nm. The resistive switching behavior of transition metal oxides and the associated memristor device is emerging as a competitive technology for next generation electronics. Significant progress has already been made in the past decade and devices are beginning to hit the market; however, it has been mainly the result of empirical trial and error. Hence, gaining theoretical insight is of essence. In the present work we report a new connection between the resistive switching and shock wave formation, a classic topic of non-linear dynamics. We argue that the profile of oxygen ions that migrate during the commutation in insulating binary oxides may form a shock wave, which propagates through a poorly conductive region of the device. We validate the scenario by means of model simulations.

Majorana bound states (MBSs) are a promising candidate for a realization of topological quantum computer and memory, which uses non-Abelian anyons to encode and manipulate quantum information. Since Kitaev's toy model for creating MBSs using the unpaired sites at the ends of a spinless p-wave superconducting wire, it has been shown that a conventional s-wave superconductor with spin-orbit coupling (SOC) subject to Zeeman or proximity-induced exchange field has effective p-wave pairing and thus can also support these nonlocal quasiparticles. In systems lacking an extrinsic SOC, an effective SOC can be provided through a nonuniform magnetic texture or field. Here, we explore stabilization of MBSs through a proximity effect with a noncollinear ferromagnet supporting skyrmions. Skyrmions can be easily manipulated via currents or temperature gradients thus providing means for braiding and manipulation of MBSs. In particular, we demonstrate how a nonabelian statistics of MBSs can be revealed by moving proximity coupled skyrmions in a vicinity of a tri-junction with different superconducting phases.

Topological insulators and graphene, the two representative 2D Dirac electron systems, have both been widely studied for spintronics applications. On the one hand, strong spin-orbit coupling in topological insulators makes them obvious spin source candidates. On the other hand, minute spin-orbit coupling in graphene makes it a promising spin transport channel. In this talk, I will first present our work on the charge-spin conversion in topological insulators. Our data demonstrate orders of magnitude improvement over conventional spin-Hall metals [1][2]. Furthermore, they indicate that the high spin generation efficiency originates from the spin-momentum locking of the topological surface states. In the second part of the talk, I will discuss the charge-spin conversion in graphene, when proximity coupled to a model magnetic insulator EuS. The interfacial exchange coupling produces a substantial Zeeman field (>= 14 T) in graphene, which yields orders-of-magnitude enhancement in spin generation by the Zeeman spin-Hall effect. Furthermore, the strong exchange field lifts the spin degeneracy in the graphene quantum Hall regime, which leads to novel spin-polarized edge transport features, potentially interesting for classical and quantum information processing [3]. [1] Luqiao Liu, A. Richardella, Ion Garate, Yu Zhu, N. Samarth, and Ching-Tzu Chen, Physical Review B 91, 235437 (2015). [2] Luqiao Liu, Ching-Tzu Chen, and J. Z. Sun, Nature Physics 10, 561--566 (2014). [3] Peng Wei, Sunwoo Lee, Florian Lemaitre, Lucas Pinel, Davide Cutaia , Wujoon Cha , Ferhat Katmis, Yu Zhu, Donald Heiman, James Hone, Jagadeesh S. Moodera, and Ching-Tzu Chen, Nature Materials 15, 711 (2016).

Manipulation of spin accumulation has shown to be a versatile tool for both fundamental research and functional application since it enables to study spin injection and transport in non-magnetic materials, as well as spincaloritronics, spin-orbitronics effects and spin transfer torque [1]. In this presentation, we shows how spin accumulation can be engineered, and what functional behaviors can be obtain through spin accumulation control: By manipulating spin accumulation in a lateral device through precession or absorption [2, 3], it is possible to extract independently transport parameters of a ferromagnetic material or of a heavy metal with strong spin orbit interaction. Anisotropic absorption in a ferromagnetic element notably enables to obtain both its spin diffusion length and its spin precession length. Lateral devices are an adapted tool to manipulate spin accumulation for functional application thanks to the high flexibility of their geometry. Nevertheless, lateral devices had always been confined to fundamental research due to the smallness of the signal they emit. We briefly show that lateral devices can emit signal comparable to what is obtain from CPP geometry, as giant magnetoresistance exceeding 10% in an downscaled CoFe-based lateral spin valve [4,5]. We then highlight that the use of lateral devices enables many applicable functions as enhancing spin signal amplitudes, or engineering simultaneously several spin accumulation degrees of freedom. Finally, using a confined spin accumulation located between two tunnel barriers, we show that it is possible to observe a novel magnetoresistive effect that only requires one ferromagnetic element. [1] Y. Otani et al., Phil. Trans. of the Royal Society of London A 369, 3136 (2011) [2] F. J. Jedema, et al., Appl. Phys. Lett. 81, 5162 (2002) [3] M. Isasa et al., Physical Review B 91, 2 (2015) [4] Y. K. Takahashi, et al. Applied Physics Letters 100, 5 (2012) [5] G. Zahnd, et al., IOP. Nanotechnology 27, 3 (2015).

Despite extensive applications of Pt for spin-charge conversion in spintronics, its spin-dependent transport properties are still debated. We study spin transport in Pt by utilizing current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) in nanoscale Permalloy (Py)-based spin valves with Pt inserted in the nonmagnetic spacer. We will discuss our results for the temperature-dependent spin diffusion length (SDL) of Pt, extracted from the dependence of GMR on the Pt thickness and calculations based on the Valet-Fert model. By comparing samples with Pt sandwiched between Cu spacers and samples where Pt is in direct contact with Py, we determine that the spin relaxation rate at the Pt/Py interface is significantly smaller than at the Pt/Cu interface. We interpret our results in terms of two relevant spin scattering mechanisms: spin flipping due to the orbital scattering (Elliot-Yafet mechanism, EY), and spin precession around the effective spin-orbit field (Dyakonov-Perel mechanism, DP). We argue that DP relaxation is suppressed at Pt/Py interfaces due to the dominance of the proximity-induced effective exchange field. By comparing the SDL as a function of temperature to the mean free path, we show that EY contribution to scattering in the bulk is dominant at temperatures above 150K. We also analyze samples with ultrathin Pt spacer, where scattering at interfaces should be dominant. Temperature dependence of GMR of these samples is consistent with the dominance of DP relaxation. Our results provide a pathway for the optimization of spin scattering and spin/charge current conversion in Pt-based spintronic devices.

We study time evolution of the opposite spin electron occupation numbers for the single Anderson impurity localized between two macroscopic leads in the presence of applied bias voltage. It was shown that non-stationary spin-polarized currents are present in the both leads and their polarization and direction in each lead can be controlled by the the applied bias voltage changing.

The finite-temperature electrical transport properties depending on the spin are essential for spintronics research focused on developing devices that should operate not only in the conditions of low temperatures. In this study we present a theoretical approach incorporating both chemical and temperature-induced disorder within the coherent potential approximation and the tight-binding linear muffin-tin orbital method, and the linear response theory is used to obtain spin-resolved electrical conductivity. Both nonmagnetic and magnetic materials are studied from the first principles in a wide temperature range. It was found, with neglected magnetic disorder, that vertex corrections to the total conductivity and spin-flip contributions to the conductivity are small; therefore, the spin-resolved coherent conductivities can be used to describe spin-dependent electrical transport. The developed formalism is applied to pure nonmagnetic platinum and to ferromagnetic random Cu-Ni alloys. For the latter system, the spin polarization of the current is nearly constant in the examined temperature range.

Manipulation of a spin current at nanoscale is desired in many proposed spintronics devices. Magnetic multilayers consisting of ferromagnetic, ferrimagnetic and nonmagetic materials show rich phenomena when a spin current propagates through the multilayers. An interface of ferromagnetic and nonmagnetic metals has been demonstrated to play an important role in the generation and dissipation of a spin current. Using first-principles scattering calculation, we study the transport and relaxation of spin currents in typical transition metals and alloys and their interfaces. In particular, we focus on identifying the correlation of spin transport and relaxation with the specific order parameters of magnetic materials. By examining the spin-Hall conductivity and spin-flip diffusion length as a function of conductivity (resistivity), we are able to distinguish different dominant physical mechanisms of the generation and dissipation of spin currents.

The spin Seebeck effect (SSE) has been widely studied as a potential mechanism for energy harvesting. However, the efficiency of such devices, utilizing the spin thermoelectric effect in thin film form, has not yet reached a sufficient value to make them economically viable. It is therefore imperative that advances are made to investigate means by which the thermoelectric signal can be enhanced. Multilayers of Co₂MnSi and Pt are fabricated and characterized in an attempt to observe enhanced voltages. We report that bilayers of ferromagnetic conductor/normal metal (FM/NM) exhibit a Longitudinal SSE response and that repetitive stacking of such bilayers results in an increased thermoelectric voltage that is highly dependent upon the quality of CMS/Pt and Pt/CMS interfaces.

Magnetization manipulation is an indispensable tool for both basic and applied research [1]. The dynamics of the response depends on the energy transfer from the laser excited electrons to the spins within the first femtoseconds. This determines the speed of the ultrafast magnetization. A special material of interest for magnetic storage development is FePt. In a seminal experiment all optical writing had been demonstrated for FePt nanoparticle of a magnetic hard disc media, completely surprisingly, by Lambert et al. in Science 2014. But, the mechanism remained unclear and it opened many questions about the extension of possibilities of all optical writing as a general mechanism. Meanwhile writing experiments by single laser spots point to an asymmetric writing per each shot. This is consistently observed by different groups. These effects can be described within different rate models. I will review the current understanding of the interaction of ultrafast excitation and heating, influence of magnetic dichroism and the presence of the inverse Faraday effect and attempts of understanding of these processes so far. From the experimental side in especial single shot writing experiments, that show a kind of accumulation effects of the writing, allow to pinpoint the underlying mechanism of writing in these media. Together, ab-initio calculations of the optical effects (inverse Faraday effect and magnetic dichroism induced heating) and the thermal modeling, allow to calculate the switching rates of the individual FePt nanoparticles. The latter then provides a rate of switching of the ensemble. A careful experimental determination of the absorbed fluence in the spherical geometry of the nanoparticles gives us a complete picture of the competing effects, of heating and writing asymmetries, and we can trace the different processes from the beginning of the laser pulse impact. In addition, this theoretical description allows us to optimize the number of shots needed to write the magnetization of the FePt nanoparticles and to pinpoint how to optimize the all optical writing. In my talk, I will review these recent developments that may lead to address an individual nanosized magnetic element in the far future all optically, for writing magnetic memory and memory storage. [1] J. Walowski and M. Münzenberg, Perspective: Ultrafast magnetism and THz spintronics, J. Appl. Phys. 120, 140901 (2016).

Experiments on THz magnetization dynamics provide insights into a yet unexplored time scale of transient magnetics and spintronics [1-3]. Coherent responses of the magnetization to THz pulses have been demonstrated [1,2]. Further, THz spin currents can be generated by non-equilibrium hot electrons [3]. These THz transients provide new ways to ultrafast spin control and its technical applications. Here, an analytical model is presented that describes transient magnetization dynamics up to the THz regime [4]. The model is used to determine the magnetization response to ultrafast multi and single-cycle THz pulses for a variety of parameters like carrier frequency, width, phase, frequency chirp, and polarization. The THz pulse shape and polarization provide a vectorial control of the magnetization on the sub-picosecond time scale. It is shown that an optimum timing for coherent magnetization control can be achieved. Dynamics of the magnetization induce a spin current in an adjacent non-magnetic material. This effect s known as spin pumping [5]. Here, calculations of THz transient spin current generation by spin pumping are presented. An effective spin current generation is found far above the ferromagnetic resonance up to THz frequencies although dynamic magnetization amplitudes are very small at THz frequencies. At THz frequencies, the coherent reaction of the magnetization also causes a coherent spin current. In contrast to the dc spin current which scales with the susceptibility of the magnetization, the ac spin current does not vanish above the ferromagnetic resonance. Instead the THz ac spin current reaches a value that is comparable to the dc spin current at resonance. The behavior far above resonance can be used to efficiently generate ultrafast spin currents without the need for magnetic systems with very high resonance frequencies. Spin currents on picosecond time scales can be achieved by THz magnetization dynamics. References: [1] C. Vicario et al., Nat. Photonics 7, 720 (2013), [2] T. Kampfrath et al., Nat. Photonics 5, 31 (2011) [3] T. Kampfrath et al., Nat. Nanotechnl. 8, 256 (2013) [4] L. Bocklage, Sci. Rep. 6, 22767 (2016) & ‎J. Magn. Magn. Mater 429, 324 (2017) [5] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Phys Rev. Lett. 88, 117601 (2002) [6] D. Wei et al., Nat. Comm. 5, 3768 (2014) [7] L. Bocklage, (submitted)

Self-assembled InAs Quantum Dots (QDs) are often called “artificial atoms" and have long been of interest as components of quantum photonic and spintronic devices. Although there has been substantial progress in demonstrating optical control of both single spins confined to a single QD and entanglement between two separated QDs, the path toward scalable quantum photonic devices based on spins remains challenging. Quantum Dot Molecules, which consist of two closely-spaced InAs QDs, have unique properties that can be engineered with the solid state analog of molecular engineering in which the composition, size, and location of both the QDs and the intervening barrier are controlled during growth. Moreover, applied electric, magnetic, and optical fields can be used to modulate, in situ, both the spin and optical properties of the molecular states. We describe how the unique photonic properties of engineered Quantum Dot Molecules can be leveraged to overcome long-standing challenges to the creation of scalable quantum devices that manipulate single spins via photonics.

Topological insulators (TIs), with their helically spin-momentum-locked topological surface states (TSS), are considered promising for spintronics applications. Several recent experiments in TIs have demonstrated a current induced electronic spin polarization that may be used for all-electrical spin generation and injection. Here, we report spin potentiometric measurements in TIs that have revealed a long-lived persistent electron spin polarization even at zero current. Unaffected by a small bias current and persisting for several days at low temperature, the spin polarization can be induced and reversed by a large “writing” current applied for an extended time. While the exact mechanism responsible for the observed long-lived persistent spin polarization remains to be better understood, we speculate on possible roles played by nuclear spins hyperfine coupled to TSS electrons and dynamically polarized by the spin-helical “writing current”. Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information.

Complex architectures of nanostructures are currently routinely elaborated using bottom-up or nanofabrication processes. This technological capability allows scientists to engineer materials with properties that do not exist in nature, but also to manufacture model systems to explore fundamental issues in condensed matter physics. Two-dimensional frustrated arrays of magnetic nanostructures are one class of systems for which theoretical predictions can now be tested experimentally. In particular, magnetic imaging techniques offer the appealing opportunity to observe a wide range of phenomena within the concept of lab-on-a-chip. For example, several exotic magnetic phases have been discovered in artificial frustrated spin systems. Besides, these systems allow the study of classical analogues of magnetic monopoles. These recent results have stimulated new research activities motivated by the quest for magnetic monopoles in condensed matter physics. In this contribution, we'll show that the micromagnetic properties of the elements constituting artificial frustrated arrays of nanomagnets introduce the concept of chiral monopoles. Injecting and manipulating experimentally the chirality of a magnetic monopole provide a new degree of freedom in the system. This offers the opportunity to control their motion under an external magnetic field, thus allowing to envision applications in magnetronics.

In physics, frustration appears in a system when it is impossible to minimise all pairwise interactions simultaneously. Frustration exists in some particular rare-earth based compounds, such as spin ices [1]. Their internal frustration gives rise to unusual properties, like a residual entropy at low temperature or the presence of monopole-like excitations [2]. However, experimental techniques are unable to probe each spin individually in these compounds. In 2006, Wang and coworkers opened a new way for studying magnetically frustrated spin systems [3]. Using e-beam lithography, one can make arrays of nanomagnets with the desired design. The state of each nanomagnet can then be probed individually in real space at room temperature using magnetic imaging (eg. Magnetic Force Microscopy). In this context, the square geometry received a considerable interest, since it is closely related to condensed matter spin ice compounds. But for geometrical reasons, this system orders instead of showing a disordered low energy manifold In this contribution, we explain how to bring back the massive ground state degeneracy in the square array of nanomagnets. We present the first experimental evidence of a Coulomb phase in this system [4]. We also report the presence of magnetic monopoles defects within the Coulomb phase. This study makes a new step toward a direct study of the dynamic of monopoles excitations (e.g. creation, annihilation or diffusion processes). [1] M.J. Harris et al., Phys. Rev. Lett. 79, 2554 (1997). [2] C. Castelnovo et al., Nature 451, 7174 (2008). [3] R.F. Wang et al., Nature 439, 303 (2006). [4] Y. Perrin et al., Nature 540, 410 (2016).

Graphene is a candidate material for next-generation high performance transparent conducting film (TCF) to replace indium tin oxide (ITO) materials. However, the sheet resistance of large area graphene obtained by the chemical vapor deposition (CVD) method is higher than other kinds of TCFs. The main strategies for improving the electrical conductivity of graphene films have been based on various doping treatments. AuCl₃ is one of the most effective dopants. In this paper, we investigate the influence of AuCl₃ doping on the conductive stability of CVD-grown graphene. Large area graphene film synthesized by CVD and transferred to glass substrates is taken as experimental sample. AuCl₃ in nitromethane is used to dope the graphene films to improve the electrical conductivity. Another sample without doping is prepared for comparison. The resistances of graphene under periodic visible light irradiation with and without AuCl₃ doping are measured. Results show that the resistances for all samples increase exponentially under lighting, while decrease slowly in an exponential form as well after the light is switched off. The relative resistance changes for undoped and doped samples are compared under 445nm light irradiation with 40mW/cm², 60mW/cm², 80mW/cm², 100mW/cm² in atmosphere and vacuum. The change rate and degree for doped graphene are greater than that of undoped graphene. It is evident from the experimental data that AuCl₃ doping may induce conductive unstability for CVD-grown graphene on glass substrate.

Damping in ferromagnetic alloys such, as Co₄₀Fe₄₀B₂₀ are very sensitive to thermal effects both during deposition and post deposition. Depositions using and alloyed Co₄₀Fe₄₀B₂₀ target were done on si (100) and amorphous glass substrates at room temperature up to 500°C. The samples deposited onto si (100) at 500°C crystalized into face centered cubic (110) Co₇Fe₃ whereas the samples on glass remained amorphous. The gilbert damping parameter α was reduced from .0141 to .0095 when deposited on glass however increased from .0097 to .0131 on si (100) at temperatures from 20°C- 400°C. At low deposition temperatures on glass large metallic grains of 50-75nm were found. The crystallization of Co₄₀Fe₄₀B₂₀ to Co₇Fe₃ at 500°C resulted in broader ferromagnetic resonance peak-to-peak line widths of 1050 Oe and increased in plane coercivity values of 200 Oe compared to 35 Oe and 5 Oe for amorphous Co40Fe40B20. Post deposition annealing of Co₄₀Fe₄₀B₂₀ on si (100) from 20-350°C showed reductions in damping from .0131 to .0076 and effective magnetization increased from 11.8 kOe to 13.3 kOe.