Pumped Spin Current Research Articles

Amplification of electron-mediated spin currents by stimulated spin pumping

Stimulated emission is a process during which an atomic system gives away energy to create a coherent photon. It is fundamental to the operation of the optical amplifier. Here we propose two mechanisms for amplifying AC spin currents in a solid-state magnetic medium by a stimulated spin pumping process. The first is synchronous and consists of phase-locked pulses that perturb a precessing magnetic moment. The second is asynchronous and is driven by DC spin currents. The amplification relies on a non-adiabatic interaction taking place in a ferromagnetic medium in which the magnetic moment emits spin angular momentum in the form of spin current before equilibrating with the environment. The pumped spin current amplifies or absorbs the injected AC spin current mimicking the operation of the optical gain medium as readily seen from the gain saturation profiles. The mechanisms we propose are a first step towards a realistic spin current amplifier.

Elastodynamically Induced Spin and Charge Pumping in Bulk Heavy Metals.

Analogous to the spin-Hall effect (SHE), abinitio electronic structure calculations reveal that acoustic phonons can induce charge (spin) current flowing along (normal to) its propagation direction. Using the Floquet approach we have calculated the elastodynamically induced charge and spin pumping in bulk Pt and demonstrate that (i)the longitudinal charge pumping originates from the Berry curvature, while the transverse pumped spin current is an odd function of the electronic relaxation time and diverges in the clean limit. (ii)The longitudinal charge current is of nonrelativstic origin, while the transverse spin current is a relativistic effect that to lowest order scales linearly with the spin-orbit coupling strength. (iii)Both charge and spin pumped currents have parabolic dependence on the amplitude of the elastic wave.

Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: a first-principles Floquet-nonequilibrium Green function study

Motivated by the recent experiment (Vaidya et al 2020 Science 368 160) on spin pumping from sub-THz radiation-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt hosting strong spin-orbit (SO) coupling, we compute and compare pumped spin currents in Cu/MnF2/Cu and Pt/MnF2/Cu heterostructures. Recent theories of spin pumping by AFI have relied on simplistic Hamiltonians (such as tight-binding) and the scattering approach to quantum transport yielding the so-called interfacial spin mixing conductance (SMC), but the concept of SMC ceases to be applicable when SO coupling is present directly at the interface. In contrast, we use a more general first-principles quantum transport approach which combines noncollinear density functional theory with Floquet-nonequilibrium Green’s functions in order to take into account: SO-proximitized AFI as a new type of quantum material, different from isolated AFI and brought about by AFI hybridization with adjacent HM layer; strong SO coupling at the interface; and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting rather than insulating as in the isolated AFI. The DC component of pumped spin current vs. precession cone angle of the Néel vector l of AFI does not follow putative , except for very small angles for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. In addition, the angular dependence differs for opposite directions of precession of the Néel vector, leading to twice as large SMC for the right-handed than for the left-handed chirality of the precession mode.

Study of interfacial spin transport with an antiferromagnetic Cr2O3 interlayer

Interfacial spin transport has been studied in a $\mathrm{YIG}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}/\mathrm{Pt}$ trilayer device via the spin-pumping technique. It is found that the linewidths and the magnitudes of the spin-pumping voltage signals suddenly decrease at just below the N\'eel temperature of the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interlayer. A detailed analysis shows that the inner decay in the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interlayer dominates the interfacial spin transport in a wide temperature range, while the interfacial suppression of pumped spin currents occurs at a temperature slightly lower than the N\'eel temperature, both of which contribute to the sudden changes of spin-pumping voltage signals around the N\'eel temperature. This work carries forward the understanding of the spin transport in an antiferromagnetic system, which would serve to inspire, as well as facilitate, further breakthroughs of the limits of spin-based devices.

Spin injection and pumping generated by a direct current flowing through a magnetic tunnel junction

A charge flow through a magnetic tunnel junction (MTJ) leads to the generation of a spin-polarized current which exerts a spin-transfer torque (STT) on the magnetization. When the density of applied direct current exceeds some critical value, the STT excites high-frequency magnetization precession in the "free" electrode of MTJ. Such precession gives rise to microwave output voltage and, furthermore, can be employed for spin pumping into adjacent normal metal or semiconductor. Here we describe theoretically the spin dynamics and charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostructure connected to a constant-current source. The magnetization dynamics in the free CoFeB layer with weak perpendicular anisotropy is calculated by numerical integration of the Landau-Lifshitz-Gilbert-Slonczewski equation accounting for both STT and voltage controlled magnetic anisotropy associated with the CoFeB|MgO interface. It is shown that a large-angle magnetization precession, resulting from electrically induced dynamic spin reorientation transition, can be generated in a certain range of relatively low current densities. An oscillating spin current, which is pumped into the Au overlayer owing to such precession, is then evaluated together with the injected spin current. Considering both the driving spin-polarized charge current and the pumped spin current, we also describe the charge transport in the CoFeB/Au bilayer with the account of anomalous and inverse spin Hall effects. An electric potential difference between the lateral sides of the CoFeB/Au bilayer is calculated as a function of distance from the CoFeB|MgO interface. It is found that this transverse voltage signal in Au is large enough for experimental detection, which indicates significant efficiency of the proposed current-driven spin injector.

Spin-current pump in silicene**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274059, 11574045, and 11704165).

We report a theoretical study of pumped spin currents in a silicene-based pump device, where two time-dependent staggered potentials are introduced through the perpendicular electric fields and a magnetic insulator is considered in between the two pumping potentials to magnetize the Dirac electrons. It is shown that giant spin currents can be generated in the pump device because the pumping can be optimal for each transport mode, the pumping current is quantized. By controlling the relevant parameters of the device, both pure spin currents and fully spin-polarized currents can be obtained. Our results may shed a new light on the generation of pumped spin currents in Dirac-electron systems.

Adiabatic Spin Pump through a Molecular Antiferromagnet Ce3Mn8III

We show that the spin-dependent charging energy recently discovered in a molecular antiferromagnet enables an adiabatic spin pump, without requiring a magnetic field or tunable spin–orbit coupling, when the molecule is connected to two unpolarized electrodes with oscillating chemical potentials. Onsite Coulomb repulsion between electrons of opposite spin orientations causes a spin-dependent suppression of the current between the two leads, leading to a pumped spin current. Consequently, the Berry-phase-like geometric current is highly spin-polarized. We explore the space of driving parameters to demonstrate the generation of either a fully polarized or a pure spin current. Negative differential conductance is also predicted. These possibilities highlight the advantage of molecular antiferromagnets over ferromagnetic molecules and quantum dots in generating and manipulating low-power spin currents.

Magnetic Moment Orientation-Dependent Spin Dissipation in Antiferromagnets.

Spin interaction in antiferromagnetic materials is of central interest in the recently emerging antiferromagnetic spintronics. In this Letter, we explore the spin current interaction in antiferromagnetic FeMn by the spin pumping effect. Exchange biased FeNi/FeMn films, in which the Néel vector can be presumably controlled via the exchange spring effect, are employed to investigate the damping enhancement depending on the relative orientation between the Néel vector and the polarization of the pumped spin current. The correlation between the enhanced damping and the strength of the exchange bias suggests that the twisting of the Néel vector induces an additional spin dissipation, which verifies that the Slonczewski-type spin torque is effective even in antiferromagnetic materials.

Spin pumping and probe in permalloy dots-topological insulator bilayers

We present a ferromagnetic resonance (FMR) spin pumping experiment at room temperature in periodic arrays of permalloy nanodots of different radii deposited onto a 3D topological insulator Bi2Se3 film. We measure the dc voltage signal generated by spin-to-charge conversion of the pumped spin current due to the spin-orbit coupling in the bulk of Bi2Se3. In the nanostructured samples, two resonance peaks are observed, associated with Kittel and inhomogeneous edge modes, respectively. This more complex modal composition in comparison to continuous systems may provide additional advantages for development of prospective spintronic devices. We support our experimental results by theoretical calculations, which are based on micromagnetic modeling of the magnetization dynamics under FMR excitation in a nanodot. A numerical approach to the calculation of the spin-pumping voltage is proposed, and the efficiency of spin-to-charge conversion is estimated for two nanostructured samples with different dot sizes.

Crystal field effects on spin pumping

"Spin pumping" is the injection of spin angular momentum by a time-dependent magnetization into an adjacent normal metal proportional to the spin mixing conductance. We study the role of electrostatic interactions in the form of crystal fields on the pumped spin currents generated by insulators with exchange-coupled local moments at the interface to a metal. The crystal field is shown to render the spin currents anisotropic, which implies that the spin mixing conductance of insulator|normal metal bilayers depends on crystal cut and orientation. We interpret the interface "effective field" (imaginary part of the spin mixing conductance) in terms of the coherent motion of the equilibrium spin density induced by proximity in the normal metal.

Electric-field-induced antiferromagnetic resonance in antiferromagnetic insulators with spin-orbit coupling

We study theoretically spin dynamics in three-dimensional antiferromagnetic insulators with spin-orbit coupling. We focus on the antiferromagnetic insulators whose low-energy effective model possesses a topological term called the θ term. By solving the Landau-Lifshitz-Gilbert equation in the presence of the θ term, we show that the antiferromagnetic resonance can be realized by ac electric fields along with static magnetic fields. The antiferromagnetic resonance can be detected via the spin pumping from the Néel field and net magnetization. We calculate both contributions to the pumped spin current, and find that the magnitude of the ac electric field to cause the resonance state is very small (∼ 1 V/m). This indicates that spin currents can be generated efficiently. The mechanism of the antiferromagnetic resonance in this study is understood as the inverse process of the dynamical chiral magnetic effect.

Magnetopumping current in graphene Corbino pump

We study conductance and adiabatic pumped charge and spin currents in a graphene quantum pump with Corbino geometry in the presence of an applied perpendicular magnetic field. Pump is driven by the periodic and out of phase modulations of the magnetic field and an electrostatic potential applied to the ring area of the pump. We show that Zeeman splitting, despite its smallness, suppresses conductance and pumped current oscillations at zero doping. Moreover, quite considerable spin conductance and pumped spin current are generated at low dopings due to Zeeman splitting. We find that pumped charge and spin currents increase by increasing the magnetic field, with small oscillations, until they are suppressed due to the effect of nonzero doping and Zeeman splitting.

Adiabatically twisting a magnetic molecule to generate pure spin currents in graphene

The spin–orbit effect in graphene is too muted to have any observable significance with respect to its application in spintronics. However, graphene technology is too valuable to be rendered impotent to spin transport. In this communication we look at the effect of adiabatically twisting a single-molecule magnet embedded in a graphene monolayer. Surprisingly, we see that pure spin currents (zero charge current) can be generated from the system via quantum pumping. In addition we also see that spin-selective current can be pumped from the system. The pure spin current seen is quite resilient to temperature while disorder has a limited effect. Furthermore, the direction of these spin-pumped currents can be easily and exclusively controlled by the magnetization of the single-molecule magnet, with disorder having no effect on the magnetization control of the pumped spin currents.

Giant Spin Pumping and Inverse Spin Hall Effect in the Presence of Surface and Bulk Spin-Orbit Coupling of Topological Insulator Bi2Se3.

Three-dimensional (3D) topological insulators are known for their strong spin-orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for "topological spintronics." Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the prototypical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) line width. Theoretical modeling of spin pumping through the surface of Bi2Se3, as well as of the measured angular dependence of spin-charge conversion signal, suggests that pumped spin current is first greatly enhanced by the surface SOC and then converted into a dc-voltage signal primarily by the inverse spin Hall effect due to SOC of the bulk of Bi2Se3. We find that the FMR line width broadens significantly (more than a factor of 5) and we deduce a spin Hall angle as large as 0.43 in the Bi2Se3 layer.

Electrically controllable spin pumping in graphene via rotating magnetization

We investigate pure spin pumping in graphene by imposing a ferromagnet (F) with rotating magnetization on top of it. Using the generalized scattering approach for adiabatic spin pumping, we obtain the spin current pumped through magnetic graphene to the normal (N) region. This spin current which can be easily controlled by gate voltages, reaches sufficiently large values measurable in current experimental setups. The spin current reaches its maximum when one of the spins is completely filtered because of its vanishing density of states in the ferromagnetic part. In order to study the effect of the ferromagnetic part length on the pumped spin current, the N—F—N structure is considered. It is found that in contrast to the metallic ferromagnetic materials the transverse spin coherence length can be comparable to the length of F. Subsequently, due to the quantum interferences inside the middle F region, the spin current becomes an oscillatory function of in which J is the spin splitting and L is the length of F. Finally controllability of the pumped spin into two different normal sides in the N—F—N hybrid device gives rise to the spin battery effect.

Evaluation of Nonlinear Effect in High Power Spin Pumping in Polycrystalline Bi-Substituted Yttrium Iron Garnet (Bi:YIG)/Pt Bilayer Structure

We have demonstrated a quantitative evaluation of spin pumping at uniform excitation in linear and nonlinear regimes of microwave absorption in a polycrystalline Bi-substituted yttrium iron garnet (Bi:YIG)/Pt bilayer structure. The evaluation is performed based on a ratio of magnitude of pumped spin currents to amplitude of uniform precession which are estimated from inverse spin Hall effect voltage and microwave absorption intensity at various incident powers. The ratio shows additional contribution to spin pumping by spin waves generated from uniform precession via nonlinear coupling between them. We have also clarified the additional contribution by the generated spin waves at various frequencies based on the ratio.

Generation of large spin currents in graphene using adiabatic quantum pumping

We study adiabatic quantum pumping of Dirac fermions in a monolayer graphene in the large pumping amplitude regime in order to study how to generate a large spin polarized current. Spin current is generated by applying two periodic oscillating gate voltages to a monolayer graphene with exchange splitting induced by ferromagnetic proximity. We find that pumped charge and spin currents are sensitive functions of the Fermi energy and pure spin current and spin current with different degrees of polarization and large magnitudes are obtained in our scheme. We also study the effects of parameters of the system, such as the barrier separation and the exchange splitting, on the pumped currents. The spin current generated by this method can be as high as 80-100 nA. This shows the method can be used in the study of spin physics in graphene and the realization of graphene spintronic devices.

Spin Pumping without Three-Magnon Splitting in Polycrystalline Bi1Y2Fe5O12/Pt Bilayer Structure

Spin pumping at various precession frequencies has been investigated in a polycrystalline Bi-substituted yttrium iron garnet (Bi:YIG)/Pt bilayer structure by means of inverse spin Hall effect (ISHE) with comparison to a single crystal La-substituted yttrium iron garnet (La:YIG)/Pt bilayer structure which shows three-magnon splitting. As precession frequency decreases, the magnitude of pumped spin currents in the polycrystalline sample continuously increases to high magnitude comparable to the highest magnitude in the single crystal sample while that in the single crystal sample decreases below a threshold frequency of three-magnon splitting. By comparing ISHE voltage with microwave absorption intensity in the polycrystalline sample, we verify that the large magnitude of the pumped spin currents is maintained at low frequencies through the suppression of three-magnon splitting.

Thermal spin pumping mediated by magnons in the semiclassical regime

We microscopically analyze thermal spin pumping mediated by magnons, at the interfacebetween a ferromagnetic insulator and a non-magnetic metal, in the semiclassical regime.The generation of a spin current is discussed by calculating the thermal spintransfer torque, which breaks the spin conservation law for conduction electrons andoperates the coherent magnon state. Inhomogeneous thermal fluctuations betweenconduction electrons and magnons induce a net spin current, which is pumped into theadjacent non-magnetic metal. The pumped spin current is proportional to thetemperature difference. When the effective temperature of magnons is lower than that ofconduction electrons, localized spins lose spin angular momentum by emittingmagnons and conduction electrons flip from down to up by absorbing all theemitted momentum, and vice versa. Magnons at the zero mode cannot contribute tothermal spin pumping because they are eliminated by the spin-flip condition.Consequently thermal spin pumping does not cost any kind of applied magneticfields.

Pure spin current generation in monolayer graphene by quantum pumping

We study a method to generate pure spin current in monolayer graphene over a wide range of Fermi energy by adiabatic quantum pumping. The device consists of three gate electrodes and two ferromagnetic strips, which induce a spin-splitting in the graphene through the proximity effect. A pure spin current is generated by applying two periodic oscillating gate voltages. We find that the pumped pure spin current is a sensitive oscillatory function of the Fermi energy. Large spin currents can be found at Fermi energies where there are Fabry–Perot resonances in the barriers. Furthermore, we analyze the effects of the parameters of the system on the pumped currents. Our predicted pumped spin current can be of the order of 100 nA which is measurable using the current technology. The proposed method is useful in the realization of graphene spintronic devices.