Generation Of Pure Spin Currents Research Articles

All-optical detection of interfacial spin transparency from spin pumping in β-Ta/CoFeB thin films.

Generation and utilization of pure spin current have revolutionized energy-efficient spintronic devices. Spin pumping effect generates pure spin current, and for its increased efficiency, spin-mixing conductance and interfacial spin transparency are imperative. The plethora of reports available on generation of spin current with giant magnitude overlook the interfacial spin transparency. Here, we investigate spin pumping in β-Ta/CoFeB thin films by an all-optical time-resolved magneto-optical Kerr effect technique. From variation of Gilbert damping with Ta and CoFeB thicknesses, we extract the spin diffusion length of β-Ta and spin-mixing conductances. Consequently, interfacial spin transparency is derived as 0.50 ± 0.03 from the spin Hall magnetoresistance model for the β-Ta/CoFeB interface. Furthermore, invariance of Gilbert damping with Cu spacer layer thickness inserted between β-Ta and CoFeB layers confirms the absence of other interface effects including spin memory loss. This demonstrates a reliable and noninvasive way to determine interfacial spin transparency and signifies its role in generation of pure spin current by spin pumping effect.

Magnetothermoelectric Properties of Ferrocene-Based Compounds Sandwiched by Transition Metals in the Presence of Gold Electrodes

Using transition metals (TM) sandwiched by ferrocene (Fc) molecules coupled to the gold electrodes through the cobalt atoms, we investigate the effect of different TMs such as V, Cr, Mn, Fe, Co, and Ni on magnetothermoelectric and thermoelectric properties of the junction in a linear response regime. Our calculations are performed based on density functional theory coupled to nonequilibrium Green function formalism. All considered junctions have acceptable spin filter efficiency (SFE) because of a good spin splitting at Fermi energy. The distinct difference between the spin-up and spin-down population of the Cr sandwiched molecule (Co(Fc)2CrCo) causes outstanding results such as nearly perfect spin polarization (SFE = 98.2%) and high spin and charge thermopower. The higher thermoelectric and magnetothermoelectric efficiencies of Co(Fc)2CrCo junction at the Fermi level prove that it is a remarkable candidate for charge and pure spin-current generation.

Unveiling the mechanisms of the spin Hall effect in Ta

Spin-to-charge current interconversions are widely exploited for the generation and detection of pure spin currents and are key ingredients for future spintronic devices including spin-orbit torques and spin-orbit logic circuits. In the case of the spin Hall effect, different mechanisms contribute to the phenomenon and determining the leading contribution is peremptory for achieving the largest conversion efficiencies. Here, we experimentally demonstrate the dominance of the intrinsic mechanism of the spin Hall effect in highly resistive Ta. We obtain an intrinsic spin Hall conductivity for \ensuremath{\beta}-Ta of $\ensuremath{-}820\ifmmode\pm\else\textpm\fi{}120(\ensuremath{\hbar}/e)\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ from spin absorption experiments in a large set of lateral spin valve devices. The predominance of the intrinsic mechanism in Ta allows us to linearly enhance the spin Hall angle by tuning the resistivity of Ta, reaching up to \ensuremath{-}35 \ifmmode\pm\else\textpm\fi{} 3%, the largest reported value for a pure metal.

Pure spin current transport in a SiGe alloy

Using four-terminal nonlocal magnetoresistance measurements in lateral spin-valve devices with Si0.1Ge0.9, we study pure spin current transport in a degenerate SiGe alloy (n ∼ 5.0 × 1018 cm−3). Clear nonlocal spin-valve signals and Hanle effect curves, indicating generation, transport, and detection of pure spin currents, are observed. The spin diffusion length and spin lifetime of the Si0.1Ge0.9 layer at low temperatures are reliably estimated to be ∼0.5 µm and ∼0.2 ns, respectively. This study demonstrates the possibility of exploring physics and developing spintronic applications using SiGe alloys.

Studies of spin related processes in fullerene C60 devices

We have investigated spin related processes in fullerene C60 devices using a several experimental techniques, which include magnetic field effect of photocurrent and electroluminescence in C60-based diodes; spin polarized carrier injection in C60-based spin-valves; and pure spin current generation in NiFe/C60/Pt trilayer devices.

Investigating magnetic proximity effects at ferrite/Pt interfaces

Spintronic devices based on pure spin currents have drawn a lot of attention during the last few years for low energy device design. One approach to generate pure spin currents is to combine a metallic or insulating ferromagnetic layer with a non-magnetic metallic layer with a large spin-orbit coupling. A recent controversy has arisen in the possible role of magnetic proximity effects at ferromagnetic/non-magnetic interfaces, which can hamper the understanding of pure spin current generation mechanisms. While magnetic proximity effects have been frequently observed at ferromagnetic metal/non-magnetic interfaces, there are only a few studies on ferromagnetic insulator/non-magnetic interfaces. Regarding the use of ferromagnetic insulators, the focus has been mainly on yttrium iron garnet (YIG). However, investigation of induced magnetic moments at YIG/Pt interfaces has engendered contradictory results. Here, we propose to study insulating ferrites for which electronic and magnetic properties can be modulated. Magnetic proximity effects have been investigated at MnFe2O4/Pt, CoFe2O4/Pt, and NiFe2O4/Pt interfaces by X-ray circular magnetic dichroism (XMCD) measurements at the Pt L3 edge. Although hybridization with Pt seems to be different among the ferrites, we do not detect any XMCD signal as the signature of an induced magnetism in Pt. We have then studied the Fe3O4 ferrite below and above the Verwey transition temperature. No XMCD signal has been measured in the insulating or conducting phase of Fe3O4. This suggests that the absence of magnetic proximity effects at ferrite/Pt interfaces is not linked to the insulating character or not of the ferrites.

Large room temperature spin-to-charge conversion signals in a few-layer graphene/Pt lateral heterostructure

Electrical generation and detection of pure spin currents without the need of magnetic materials are key elements for the realization of full electrically controlled spintronic devices. In this framework, achieving a large spin-to-charge conversion signal is crucial, as considerable outputs are needed for plausible applications. Unfortunately, the values obtained so far have been rather low. Here we exploit the spin Hall effect by using Pt, a non-magnetic metal with strong spin-orbit coupling, to generate and detect pure spin currents in a few-layer graphene channel. Furthermore, the outstanding properties of graphene, with long-distance spin transport and higher electrical resistivity than metals, allow us to achieve in our graphene/Pt lateral heterostructures the largest spin-to-charge output voltage at room temperature reported so far in the literature. Our approach opens up exciting opportunities towards the implementation of spin-orbit-based logic circuits and all electrical control of spin information without magnetic field.

All-optical detection of the spin Hall angle inW/CoFeB/SiO2heterostructures with varying thickness of the tungsten layer

The development of advanced spintronics devices hinges on the efficient generation and utilization of pure spin current. In materials with large spin-orbit coupling, the spin Hall effect may convert charge current to pure spin current and a large conversion efficiency, which is quantified by spin Hall angle (SHA), is desirable for the realization of miniaturized and energy efficient spintronic devices. Here, we report a giant SHA in beta-tungsten (\b{eta}-W) thin films in Sub/W(t)/Co20Fe60B20(3 nm)/SiO2(2 nm) heterostructures with variable W thickness. We employed an all-optical time-resolved magneto-optical Kerr effect microscope for an unambiguous determination of SHA using the principle of modulation of Gilbert damping of the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A non-monotonic variation of SHA with W layer thickness (t) is observed with a maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a very low value at t = 6 nm. This variation of SHA with W-thickness correlates well with the thickness dependent structural phase transition and resistivity variation of W above the spin diffusion length of W, while below this length the interfacial electronic effect at W/CoFeB influences the estimation of SHA.

Optical generation of pure spin currents at the indirect gap of bulk Si

We report on the optical generation of a pure spin current at the indirect gap of bulk Si at room temperature in the photon energy range comprised between 1.2 and 1.8 eV. Spin-polarized electrons are promoted to the Δ-valleys of the Si Brillouin zone by circularly polarized light. The photo-generated spin current is then detected by exploiting a Schottky Pt/Si(001) junction: spin-polarized electrons diffuse toward the Pt/Si interface and enter the Pt layer where the spin current is converted into a transverse electromotive field through the inverse spin-Hall effect (ISHE). The photon energy dependence of the ISHE signal is interpreted in the frame of a one-dimensional spin drift-diffusion model, which allows estimating the electron spin lifetime to be τs=15±5 ns.

Robust spin-current injection in lateral spin valves with two-terminal Co2FeSi spin injectors

We demonstrate generation and detection of pure spin currents by combining a two-terminal spin-injection technique and Co2FeSi (CFS) spin injectors in lateral spin valves (LSVs). We find that the two-terminal spin injection with CFS has the robust dependence of the nonlocal spin signals on the applied bias currents, markedly superior to the four-terminal spin injection with permalloy reported previously. In our LSVs, since the spin transfer torque from one CFS injector to another CFS one is large, the nonlocal magnetoresistance with respect to applied magnetic fields shows large asymmetry in high bias-current conditions. For utilizing multi-terminal spin injection with CFS as a method for magnetization reversals, the terminal arrangement of CFS spin injectors should be taken into account.

Modulation of the Spectral Characteristics of a Nano-Contact Spin-Torque Oscillator via Spin Waves in an Adjacent Yttrium-Iron Garnet Film

NiFe-Cu-Co trilayer nano-contact spin-torque oscillators (NC-STOs) fabricated on an yttrium-iron garnet (YIG) film were studied in two different modes. In passive mode, i.e. without any NC-STO auto-oscillations, a microwave current through the nano-contact can excite spin waves (SW) in the YIG film, and, vice versa, antenna generated SWs in the YIG film can be detected by the nano-contact. In active mode, i.e., in the presence of auto-oscillations, significant changes appear in the NC-STO spectrum when its frequency approaches that of the SWs excited in the YIG. These results demonstrate strong coupling between NC-STOs and SWs in YIG and open new possibilities of 1) pure spin-current generation in YIG by NC-STOs; 2) mutual locking of a number of NC-STOs through SWs in YIG; and 3) improvement of NC-STO spectra through SW feedback in YIG.

Generation of the pure spin current via Auger recombination in Rashba quantum wells

We propose new non-optical mechanism of the pure spin current generation via Auger recombination in the quantum wells with Rashba spin-orbit coupling. It is shown that such process is allowed due to interference between the Coulomb matrix elements corresponding to two different transitions during Auger recombination, leading to non-diagonal transversal components of the spin current tensor, JR = Jxy = −Jyx. In the limit of low temperatures the total spin current is proportional to the Rashba constant γR, spin relaxation time τs and the third power of both the concentration n, inverse quantum well width a-1 and Eg-2. Estimations show that typical magnitude of the generated spin current by this way is much greater than the ones obtained using intraband optical excitation mechanisms and comparable with spin currents driven by interband optical excitation, provided that the quantum well bandgap is narrow enough (Eg < 0.5 eV).

Generation of pure spin currents via Auger recombination in quantum wells with Rashba splitting

We propose a nonoptical mechanism for generating spin current via Auger recombination in semiconductor quantum wells (QWs) with spin–orbit splitting associated with structural QW asymmetry. It is shown that Auger recombination in narrow-bandgap semiconductors makes it possible to produce spin currents that exceed those that are obtained in the case of intraband as well as interband optical excitation. Analysis shows that the interference term in the expression for the Auger-recombination rate is responsible for the generation of spin currents.

Generation and Detection of Spin Currents in Semiconductor Nanostructures with Strong Spin-Orbit Interaction.

Storing, transmitting, and manipulating information using the electron spin resides at the heart of spintronics. Fundamental for future spintronics applications is the ability to control spin currents in solid state systems. Among the different platforms proposed so far, semiconductors with strong spin-orbit interaction are especially attractive as they promise fast and scalable spin control with all-electrical protocols. Here we demonstrate both the generation and measurement of pure spin currents in semiconductor nanostructures. Generation is purely electrical and mediated by the spin dynamics in materials with a strong spin-orbit field. Measurement is accomplished using a spin-to-charge conversion technique, based on the magnetic field symmetry of easily measurable electrical quantities. Calibrating the spin-to-charge conversion via the conductance of a quantum point contact, we quantitatively measure the mesoscopic spin Hall effect in a multiterminal GaAs dot. We report spin currents of 174pA, corresponding to a spin Hall angle of 34%.

Generation of pure spin currents via spin Seebeck effect in self-biased hexagonal ferrite thin films

Light-induced generation of pure spin currents in a Pt(2.5 nm)/BaFe12O19(1.2 μm)/sapphire(0.5 mm) structure is reported. The BaFe12O19 film had strong in-plane uniaxial anisotropy and was therefore self-biased. Upon exposure to light, a temperature difference (ΔT) was established across the BaFe12O19 thickness that gave rise to a pure spin current in the Pt via the spin Seebeck effect. Via the inverse spin Hall effect, the spin current produced an electric voltage across one of the Pt lateral dimensions. The voltage varied with time in the same manner as ΔT and flipped its sign when the magnetization in BaFe12O19 was reversed.

The inelastic spin Seebeck effects in a controllable Aharonov–Bohm interferometer based on a molecular quantum dot

Using non-equilibrium Green's functions' technology combined with small polaron transformation, we theoretically study the inelastic spin Seebeck effects in a controllable Aharonov–Bohm interferometer based on a degenerate molecular quantum dot level. The numerical results show that when the direct coupling between electrodes is switched on, the spin-dependent thermoelectric efficiency will be suppressed, but the magnetic flux can modulate the thermoelectric line-shapes regularly and the pure spin current generation can be achieved by the temperature gradient. When the direct coupling between electrodes is closed, the spin-dependent thermoelectric efficiency will be raised greatly and the heating resulting from the electron–phonon coupling can improve it effectively. • We develop a new numerical method to implement the calculation for inelastic Fano-resonance thermoelectric transport. • When the electrodes' direct coupling is open, the magnetic flux can modulate the line-shape of thermoelectric functions regularly. • The pure spin current generation can be achieved by the temperature gradient and the electrodes' direct coupling. • When the electrodes' coupling is closed, the spin-dependent thermoelectric efficiency will be raised greatly and the heating to the phonon bath can improve it effectively.

Generation of adjustable pure spin currents in negative-U systems

Single-particle sequential tunneling is studied through a negative-U center hybridized with a superconducting, a ferromagnetic, and a normal metal electrodes. In stark contrast to the case of positive U, the single-particle tunneling in attractive charging energy is usually prohibited by ground states with electrons in pairs. We find a microscopic mechanism to induce single-particle sates from pair states. As a consequence, in the nonpolarized metal terminal a remarkable pure spin current with no charge currents survives over a wide range of gate- and bias-voltages, which is rather crucial for experimental observation and design of spintronic devices. In addition, a significant spin-filter effect is presented in certain bias regime.

Photoinduced pure spin-current injection in graphene with Rashba spin-orbit interaction

We propose a photoexcitation scheme for pure spin current generation in graphene subject to a Rashba spinorbit coupling. Although excitation using circularly-polarized light does not result in optical orientation of spins in graphene unless an additional magnetic field is present, we show that excitation with linearly-polarized light at normal incidence yields spin current injection without magnetic field. Spins are polarized within the graphene plane and are displaced in opposite directions, with no net charge displacement. The direction of the spin current is determined by the linear polarization axis of the light, and the injection rate is proportional to the intensity. The technique is tunable via an applied bias voltage and is accessible over a wide frequency range. We predict a spin current polarization as high as 75% for photon frequencies comparable to the Rashba frequency. Spin current injection via optical methods removes the need for ferromagnetic contacts, which have been identified as a possible source of spin scattering in electrical spin injection in graphene.

Generation and coherent control of pure spin currents via terahertz pulses

We inspect the time and spin-dependent, inelastic tunneling in engineered semiconductor-based double quantum well driven by time-structured terahertz pulses. An essential ingredient is an embedded spin-active structure with vibrational modes that scatter the pulse driven carriers. Due to the different time scales of the charge and spin dynamics, the spin-dependent electron-vibron coupling may result in pure net spin current (with negligible charge current). Heating the vibrational site may affect the resulting spin current. Furthermore, by controlling the charge dynamics, the spin dynamics and the generated spin current can be manipulated and switched on and off coherently.

Greatly enhanced generation efficiency of pure spin currents in Ge using Heusler compound Co2FeSi electrodes

We show nonlocal spin transport in n-Ge-based lateral spin-valve devices with highly ordered Co2FeSi/n+-Ge Schottky tunnel contacts. Clear spin-valve signals and Hanle effect curves are demonstrated at low temperatures, indicating the generation, manipulation, and detection of pure spin currents in n-Ge. The obtained spin generation efficiency of ∼0.12 is about two orders of magnitude larger than that for a previously reported device with Fe/MgO tunnel barrier contacts. Considering the spin-related behavior with temperature evolution, we infer that it is necessary to simultaneously demonstrate a high spin generation efficiency and improve the quality of the transport channel to realize Ge-based spintronic devices.