Spin Current Induced by a Surface Plasmon Polariton

We propose a novel type of photonic-crystal (PC)-based nanostructures for efficient and tunable optically-induced spin current generation via the spin Seebeck and inverse spin Hall effects. It has been experimentally demonstrated that optical surface modes localized at the PC surface covered by ferromagnetic layer and materials with giant spin-orbit coupling (SOC) notably increase the efficiency of the optically-induced spin current generation, and provides its tunability by modifying the light wavelength or angle of incidence. Up to 100% of the incident light power can be transferred to heat within the SOC layer and, therefore, to the spin current. Importantly, the high efficiency becomes accessible even for ultra-thin SOC layers. Moreover, the surface patterning of the PC-based spintronic nanostructure allows for the local generation of spin currents at the pattern scales rather than the diameter of the laser beam.

Spin wave resonance in Ni81Fe19/Pt thin wire arrays has been investigated using the inverse spin-Hall effect (ISHE). The spin wave in the Ni81Fe19 layer drives spin pumping, generation of spin currents from magnetization precession, and the pumped spin current is converted into a charge current by ISHE in the Pt layer. We found an electromotive force transverse to the spatial and the spin-polarization directions of the spin current. The experimental results indicate that the amplitude of the electromotive force is proportional to the spin wave resonance absorption intensity, enabling the electric measurement of spin wave resonance in nanostructured magnetic systems.

Spin Hall effect and its reciprocal, the inverse spin Hall effect (ISHE), provide the means for conversion between spin and charge currents [1, 2]. In particular, the ISHE transforms a pure spin current into a transverse charge current due to spin-orbit coupling. Recently, the decisive role of the ISHE effect on extending the field of spintronics into the terahertz (THz) regime was revealed [3, 4]. Here, we demonstrate the generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses of Fe (2 nm) / Pt (3 nm) were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer [5]. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals. Furthermore, we correlate the interface structural properties with the THz-E-field amplitude and the bandwidth of the THz radiation. By allowing the Pt layers to grow along different crystallographic directions on top of Fe we modify the THz emission characteristics of the optimized emitters. We present a theoretical model which correlates the loss of energy of the hot electrons and the electron-phonon/defect scattering lifetime at the Fe/Pt interface with the ISHE current that causes the THz emission. By taking into account the response function of the THz detector we describe the influence of the crystal structure of Pt onto the THz signal shape and spectrum. The demonstration of the role of the individual layer thicknesses and of the interface quality in the THz emission spectra paves the way for more efficient spintronic THz emitters.

One way to compensate for the surface plasmon polariton (SPP) propagation losses is to use a gain medium. However, simply ensuring high enough gain is not sufficient because it may violate the bounded character of the wave. Therefore, a detailed theoretical analysis is needed for the determination of the conditions for lossless or amplified SPP propagation. Here presented is an exact theoretical analysis of the SPP propagation in the case of an infinite metal/gain medium boundary. It is shown that the conditions for lossless/amplified SPP propagation can be conveniently examined and presented as a simply connected region in the complex plane of the gain medium dielectric function. Effective and minimum gain parameters are introduced, which facilitates the simultaneous analyses of different gain media/metals combinations. The practical application of these results is illustrated for several gain media/metal (silver, gold and aluminium) systems.

This chapter describes an experiment on the inverse spin Hall effect (ISHE) induced by spin pumping. Spin pumping is the generation of spin currents as a result of magnetization M(t) precession; in a ferromagnetic/paramagnetic bilayer system, a conduction-electron spin current is pumped out of the ferromagnetic layer into the paramagnetic conduction layer in a ferromagnetic resonance condition. The sample used in the experiment is a Ni81Fe19/Pt bilayer film comprising a 10-nm-thick ferromagnetic Ni81Fe19layer and a 10-nm-thick paramagnetic Pt layer. For the measurement, the sample system is placed near the centre of a TE011 microwave cavity at which the magnetic-field component of the microwave mode is maximized while the electric-field component is minimized.

The interaction of light with solid state quasiparticles, such as excitons and plasmons, on the nanometer-femtosecond spatio-temporal scale illuminates ultrafast physical and chemical processes on surfaces. In this thesis, I report on the generation, control, and spatio-temporal evolution of 2D evanescent electromagnetic waves confined at the silver (Ag)/vacuum interface; such fields, known as surface plasmon polaritons (SPPs), have joint particle-wave nature. SPPs are generated by the interaction of light with the collective response of conduction band free-electrons of metals. I image and study the ultrafast dynamics of SPP fields by interferometric time-resolved multi-photon photoemission electron microscopy (ITR-mP-PEEM). First, I report on the generation and propagation of SPPs excited on epitaxially grown Ag nanocrystals. The PEEM images record an interference pattern between SPPs and vacuum light, defined by a mismatch in their propagation wave vectors. Next, I explore the light polarization as a control parameter for the SPP generation, where the in-plane and out-of-plane components of optical electric fields couple differently. For equilateral triangle Ag island samples, the SPP interference patterns strongly depend on both the linear and circular polarizations. For circularly polarized light, the SPP coupling depends on the matching between spin angular momenta (SAM) of light and SPPs. The SAM of evanescent waves like SPPs is transverse and points oppositely when the propagation wave vector is reversed; this is known as the photonic quantum spin Hall effect (QSHE). I demonstrate that QSHE affects the function of an SPP lens coupling structure through a vectorial superposition of longitudinally and transversely coupled SPP waves that are launched by TE waves (s-polarized) and TM waves (p-polarized), respectively. Finally, I combine my understanding of SPP generation and imaging in a normal-incidence PEEM measurement to explore SPP dynamics when formed by an Archimedean spiral coupling structure. The geometrically defined phase structure of such SPP fields generates plasmonic vortices, whose singularities and time evolution are imaged by PEEM. Based on simulations, I conclude that the SPPs SAM distribution at the vortex core has a stable topological texture of a Neel type Skyrmion, and experimentally locate it by imaging the SPP field singularities.

Features of the current spin-orbit induced magnetic dynamics in multilayer nanostructures with nonmagnetic heavy metal layers possessing by a strong spin-orbit interaction are studied. These structures include ferromagnetic (F) (antiferromagnetic AF)/normal metal (N) nanostructures based on both conductive and insulating magneticsand heavy normal metals (e. g., FeCoB/Ta, YIG/Pt, Nio/Pt). The spin Hall effect of the conversion of an incoming charge current into a transverse (with respect to the charge current) spin current induces a spin-transfer torque and magnetic dynamics including a magnetic precession and switching. The magneto-dynamic effect of a spin current pumping generation together with the inverse spin Hall effect of conversion of the spin current into the incoming charge current provide the influence of the magnetic dynamics on the incoming charge current. These feedforward and feedback between the incoming charge current and the magnetic dynamics can be the basis for the spin-orbit driven self-sustained auto-oscillations of a magnetic order in the nanostructures. It is shown that the considered magnetic nanostructures possess by properties of controlled microwave radiation attaining tens THz in the antiferromagnetic case. Magnetic-induced changes of the electric resistance in the mentioned nanostructure are considered.

In many interacting-electron systems, the microscopic spin angular momentum of electrons is conserved along with the macroscopic angular momentum in electrical current flows, i.e., vorticity. Such spin-vorticity coupling expands the choice of materials for spintronics devices. In this paper, we evaluate the magnitude of an alternating spin current generated by the spin-vorticity coupling with a gigahertz-order surface acoustic wave in a Cu thin film. We measure the gigahertz alternating spin current by an electrical method based on the inverse spin-Hall effect. From the amplitude of the spin current, we can determine the conversion efficiency of the angular momentum between local lattice rotation and electron spin in the Cu film. The conversion efficiency is four orders of magnitude larger than the case of spin current generation via kilohertz-order vorticity in turbulent flow of liquid mercury [M. Matsuo et al., Phys. Rev. B 96, 020401(R) (2017)]. Such a huge conversion efficiency is attributable to a smaller inconsistency of energy scale between lattice rotation and electron spin than the case of liquid vorticity.

The discovery of magnetic order in atomically-thin van der Waals materials has strengthened the alliance between spintronics and two-dimensional materials. An important use of magnetic two-dimensional materials in spintronic devices, which has not yet been demonstrated, would be for coherent spin injection via the spin-pumping effect. Here, we report spin pumping from Cr2Ge2Te6 into Pt or W and detection of the spin current by inverse spin Hall effect. The magnetization dynamics of the hybrid Cr2Ge2Te6/Pt system are measured, and a magnetic damping constant of ~ 4–10 × 10−4 is obtained for thick Cr2Ge2Te6 flakes, a record low for ferromagnetic van der Waals materials. Moreover, a high interface spin transmission efficiency (a spin mixing conductance of 2.4 × 1019/m2) is directly extracted, which is instrumental in delivering spin-related quantities such as spin angular momentum and spin-orbit torque across an interface of the van der Waals system. The low magnetic damping that promotes efficient spin current generation together with high interfacial spin transmission efficiency suggests promising applications for integrating Cr2Ge2Te6 into low-temperature two-dimensional spintronic devices as the source of coherent spin or magnon current.

The magnetoresistance of Ni 81 Fe 19 and Co 83 Gd 17 ferromagnetic thin films is measured in Corbino disk geometry, and compared to magnetoresistance of same films measured in Hall-bar geometry. The symmetry of magnetoresistance profiles is drastically modified by changing geometry of sample, i.e., by changing boundary conditions. These properties are explained in a simple model, showing that Corbino magnetoresistance is defined by potentiostatic boundary conditions while Hall-bar magnetoresistance is defined by galvanostatic boundary conditions. The Hall was first measured in 1879 by Hall [1] by applying a magnetic field to a conducting slab contacted to an electric generator at extremities. Later on, Corbino [2] found a similar by applying a magnetic field on a disk geometry with two concentric electrodes. Quickly question arose on whether measured by Corbino (the so-called Corbino effect) in a disk and by Hall in a bar have same origin. In 1914, Adams and Chapman measured Corbino in many different metals [3] by using an oscillating current flowing from center of disk to its outer. Adams concluded in 1915 that the Corbino is, essentially, same as Hall effect [4]. However, question remains about exact meaning of adverb essentially. In 1950's, Hall in Corbino geometry was studied for its practical applications. The magnetoresistance of InSb slabs was shown to depend strongly on shape of samples [5]. The reason is that near current injection edge, Hall electric field is shorted and a transverse electric current appears which causes an increase of resistance as in Corbino geometry [6-9]. Accordingly, one can see Corbino geometry as extreme scenario where Hall electric field is zero everywhere and a Hall current is flowing, or, in other terms, one can view Corbino disk as a Hall bar in which electrostatic charge accumulation is reduced to zero everywhere. The system cannot generate a voltage between edges so that a Hall current is flowing and Joule heating is higher than in Hall bar for an equivalent volume [10-12]. The mechanism responsible for both Hall and Corbino is indeed same, but Corbino disk is a device that is more constrained than Hall bar, due to change of boundary conditions. At turn of last century, emergence of spintronics has shown possibility of exploiting spin-polarized currents * jean-eric.wegrowe@polytechnique.edu and spin-dependent potentials, and has paved way to realization of new electronic devices. Recently, various developments about spin-Hall effects (anomalous Hall effect, spin-Hall effect, spin-pumping effect, spin-Seebeck effects, etc. [13,14]) tend to show that usual Hall-bar conditions with spin relaxation could be turned into Corbino-like boundary conditions, in sense that electric charge accumulation drops to zero at edges and a pure spin current can be generated instead of a Hall voltage [11]. In this context, goal of this Rapid Communication is to study NiFe and GdCo ferromagnetic Corbino disks and Hall bars, in order to understand behavior of magnetoresistance [13,15-17] when boundary conditions are switched (by changing geometry) from spin current to spin-dependent voltage. The alloys Ni 81 Fe 19 and Co 83 Gd 17 are chosen for their maximum contribution to anisotropic magnetoresistance and anomalous Hall magnetoresistance (that defines anomalous Hall angle), respectively. First, we will present our measures of Corbino magne-toresistance performed on NiFe and CoGd rings. The results are then analyzed in framework of generalized Ohm's law by defining Corbino magnetotransport coefficients C as a function of usual Hall-bar coefficients [see Eq. (12) below]. The consistency of proposed explanation is checked independently, by measuring magnetotransport coefficients of Hall bar. The samples studied are 20-nm-thick layers of Ni 81 Fe 19 and Co 83 Gd 17 sputtered on glass substrates. The magnetic layers are sandwiched between 5-nm-thick Ta buffers and 3-nm-thick Pt caps. The magnetic properties of thin layers have been previously studied [18] (see Supplemental Material [19]). The sample magnetization is uniform for quasistatic states, although nonuniform states could take place at low magnetic fields (this regime is, however, not considered in present study). The NiFe is textured with small uniaxial anisotropy lying in sample plane. The coercivity field in in-plane geometry is of order of 1 mT. The out-2469-9950/2018/98(22)/220405(5) 220405-1

Theory of spin-charge conversion effects in spintronics are presented in terms of correlation functions of physical observables, spin and electric current. Direct and inverse spin Hall effects and spin pumping effect are studied considering metallic systems with random spin-orbit interaction and spatially nonuniform Rashba interaction. The theory is free from ambiguity associated with spin current, and provides clear physical picture of the spin-charge conversion effects. In the present approach, so-called the spin current transmission efficiency is essentially the nonuniform component of magnetic susceptibility.

Spin pumping in ferromagnets provides a source of pure spin currents. Via the\ninverse spin Hall effect a spin current is converted into a charge current and\na corresponding detectable DC-voltage. The ratio of injected spin current to\nresulting charge current is given by the spin Hall angle. However, the number\nof experiments more or less equals the number of different values for spin Hall\nangles, even for the most studied normal metal platinum. This publication\nprovides a full study of inverse spin Hall effect and anisotropic\nmagnetoresistance for different NiFe(Py) / normal metal bilayers using a\ncoplanar waveguide structure. Angle and frequency dependent measurements\nstrongly suggest that spin pumping and inverse spin Hall effect can be used to\nquantify spin Hall angles only if certain conditions are met. Ruling out the\nanisotropic magnetoresistance as a parasitic voltage generating effect\nmeasurements of the inverse spin Hall effect in Py/Pt and Py/Au yield spin Hall\nangles of 0.09 and 0.008 respectively. Furthermore, DC-voltages at\nferromagnetic resonance for Py/Pt are studied as a function of temperature and\nthe results are compared to theoretical models.\n

Noble metal nanowires are excellent candidates as subwavelength optical components in miniaturized devices due to their ability to support the propagation of surface plasmon polaritons (SPPs). Nanoscale data transfer based on SPP propagation at optical frequencies has the advantage of larger bandwidths but also suffers from larger losses due to strong mode confinement. To overcome losses, SPP gain has been realized, but so far only for weakly confined SPPs in metal films and stripes. Here we report the demonstration of gain for subwavelength SPPs that were strongly confined in chemically prepared silver nanowires (mode area = λ(2)/40) using a dye-doped polymer film as the optical gain medium. Under continuous wave excitation at 514 nm, we measured a gain coefficient of 270 cm(-1) for SPPs at 633 nm, resulting in partial SPP loss compensation of 14%. This achievement for strongly confined SPPs represents a major step forward toward the realization of nanoscale plasmonic amplifiers and lasers.

We study the background (equilibrium), linear and nonlinear spin currents in\n2D Rashba spin-orbit coupled systems with Zeeman splitting and in 3D\nnoncentrosymmetric metals using modified spin current operator by inclusion of\nthe anomalous velocity. The linear spin Hall current arises due to the\nanomalous velocity of charge carriers induced by the Berry curvature. The\nnonlinear spin current occurs due to the band velocity and/or the anomalous\nvelocity. For 2D Rashba systems, the background spin current saturates at high\nFermi energy (independent of the Zeeman coupling), linear spin current exhibits\na plateau at the Zeeman gap and nonlinear spin currents are peaked at the gap\nedges. The magnitude of the nonlinear spin current peaks enhances with the\nstrength of Zeeman interaction. The linear spin current is polarized out of\nplane, while the nonlinear ones are polarized in-plane. We witness pure\nanomalous nonlinear spin current with spin polarization along the direction of\npropagation. In 3D noncentrosymmetric metals, background and linear spin\ncurrents are monotonically increasing functions of Fermi energy, while\nnonlinear spin currents vary non-monotonically as a function of Fermi energy\nand are independent of the Berry curvature. These findings may provide useful\ninformation to manipulate spin currents in Rashba spin-orbit coupled systems.\n

We consider propagation of surface plasmon polaritons (SPPs) in linear periodic chains (LPCs) of prolate and oblate metallic spheroids. We show that the SPP group velocity can be efficiently controlled by varying the aspect ratio of the spheroids. For sufficiently small aspect ratios, a gap appears in the first Brillouin zone of the chain lattice in which propagating modes do not exist. Depending on the SPP polarization, the gap extends to certain intervals of the Bloch wave number $q$. Thus, for transverse polarization, no propagating SPPs exist with wave numbers $q$ such that ${q}_{c}^{\ensuremath{\perp}}<|q|<\ensuremath{\pi}/h$, $h$ being the chain period. For longitudinally polarized SPPs, the gap spans the interval $|q|<{q}_{c}^{\ensuremath{\parallel}}$. Here ${q}_{c}^{\ensuremath{\perp}}$ and ${q}_{c}^{\ensuremath{\parallel}}$ are different constants, which depend on the chain parameters, spheroid aspect ratio, and its orientation with respect to the chain axis. The dependence of the dispersion curves on the spheroid aspect ratio leads to a number of interesting effects. In particular, bandwidth of SPPs that can propagate in an LPC can be substantially increased by utilizing prolate or oblate spheroids. When $q$ is close to a critical value, so that $|q\ensuremath{-}{q}_{c}^{\ensuremath{\perp}}|⪡\ensuremath{\pi}/h$ or $|q\ensuremath{-}{q}_{c}^{\ensuremath{\parallel}}|⪡\ensuremath{\pi}/h$, the decay length of the SPPs is dramatically increased. In addition, the dispersion curves acquire a very large positive or negative slope. This can be used to achieve superluminal group velocity for realistic chain parameters. We demonstrate superluminal propagation of Gaussian wave packets in numerical simulations. Both theory and simulations are based on Maxwell equations with account of retardation and, therefore, are fully relativistic.