Spin Current Injection Research Articles

Speed and efficiency of femtosecond spin current injection into a nonmagnetic material

We investigate femtosecond spin injection from an optically excited Ni top layer into an Au bottom layer using time-resolved complex magneto-optical Kerr effect (C-MOKE) measurements. Employing the C-MOKE formalism, we are able to follow layer-resolved demagnetization in Ni and the simultaneous spin injection into the adjacent Au film, both occurring within $\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. We confirm the ballistic to diffusive propagation of the spin transfer process with ab initio theory and superdiffusive transport calculations. In particular, our combined experimental-theoretical effort does allow us to quantify the so far elusive amount of spin injection, and therefore the spin injection efficiency at the interface.

Spin caloritronic nano-oscillator

Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here, we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in Y3Fe5O12/Pt bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the Y3Fe5O12 layer. This leads to excitation of auto-oscillations of the Y3Fe5O12 magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.

Thermographic measurements of the spin Peltier effect in metal/yttrium-iron-garnet junction systems

The spin Peltier effect (SPE), heat-current generation due to spin-current injection, in various metal (Pt, W, and Au single layers and Pt/Cu bilayer)/ferrimagnetic insulator (yttrium iron garnet: YIG) junction systems has been investigated by means of a lock-in thermography (LIT) method. The SPE is excited by a spin current across the metal/YIG interface, which is generated by applying a charge current to the metallic layer via the spin Hall effect. The LIT method enables the thermal imaging of the SPE free from the Joule-heating contribution. Importantly, we observed spin-current-induced temperature modulation not only in the Pt/YIG and W/YIG systems but also in the Au/YIG and Pt/Cu/YIG systems, excluding the possible contamination by anomalous Ettingshausen effects due to proximity-induced ferromagnetism near the metal/YIG interface. As demonstrated in our previous study, the SPE signals are confined only in the vicinity of the metal/YIG interface; we buttress this conclusion by reducing a spatial blur due to thermal diffusion in an infrared emission layer on the sample surface used for the LIT measurements. We also found that the YIG-thickness dependence of the SPE is similar to that of the spin Seebeck effect measured in the same Pt/YIG sample, implying the reciprocal relation between them.

Spin pumping into superconductors: A new probe of spin dynamics in a superconducting thin film

Spin pumping refers to the microwave-driven spin current injection from a ferromagnet into the adjacent target material. We theoretically investigate the spin pumping into superconductors by fully taking account of impurity spin-orbit scattering that is indispensable to describe diffusive spin transport with finite spin diffusion length. We calculate temperature dependence of the spin pumping signal and show that a pronounced coherence peak appears immediately below the superconducting transition temperature Tc, which survives even in the presence of the spin-orbit scattering. The phenomenon provides us with a new way of studying the dynamic spin susceptibility in a superconducting thin film. This is contrasted with the nuclear magnetic resonance technique used to study a bulk superconductor.

Enhancement of the spin Peltier effect in multilayers

The spin Peltier effect (SPE), heat-current generation as a result of spin-current injection, has been investigated in alternately stacked Pt/${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ multilayer films. The temperature modulation induced by the SPE in the $[\text{Pt}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}]\ifmmode\times\else\texttimes\fi{}n$ films was found to be significantly enhanced with increasing the number of Pt/${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ bilayers $n$. This SPE enhancement is much greater than that expected for a simple stack of independent Pt/${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ bilayers. The observed $n$ dependence of the SPE can be explained by introducing spin-current redistribution in the multilayer films in the thickness direction, in a manner similar to the enhancement of the spin Seebeck effect in multilayers.

Room temperature spin Kondo effect and intermixing in Co/Cu non-local spin valves

The anomalous low temperature suppression of the spin accumulation signal ΔRNL in non-local spin valves (NLSVs) based on common ferromagnet (FM)/normal metal (N) pairings has recently been shown to result from a manifestation of the Kondo effect. Local magnetic moments in the N due to even minor levels of FM/N interdiffusion depolarize the injected spin current, suppressing the effective spin polarization around and below the Kondo temperature TK. Previous studies have focused on FM/N combinations that happen to have low TK so that Kondo effects occur only well below 300 K. Here, we study NLSVs based on Co/Cu, a materials combination that is not only technologically relevant but also has a high TK, up to 500 K. Despite the negligible equilibrium solubility of Co in Cu, we find clear Kondo effects in both ΔRNL and Cu resistivity, due to Co/Cu intermixing that we probe via quantitative transmission electron microscopy. Most significantly, under certain conditions the spin Kondo effect suppresses the injected spin polarization even at room temperature, with important technological implications. Studies as a function of the Cu thickness and annealing temperature reveal complex trends in interdiffusion lengths and Kondo effects, which we interpret in terms of the interplay between diffusion kinetics and thermodynamics, as well as the thickness dependence of the Kondo effect.

Local injection of pure spin current generates electric current vortices

We show that local injection of pure spin current into an electrically disconnected ferromagnetic–normal-metal sandwich induces electric currents that run along the closed loops inside the device and are powered by the source of the spin injection. Such electric currents may significantly modify voltage distribution in spin-injection devices and induce long-range tails of spin accumulation.

Enhanced optical spin current injection in the hexagonal lattice with intrinsic and Rashba spin–orbit interactions

We study the photo-induced spin current injection in a hexagonal lattice with both intrinsic and Rashba spin–orbit interactions which is irradiated by a polarized light beam. It is found that the spin current injection rate could be enhanced as the graphene lattice is in the topological insulator state. Furthermore, the spin current injection rate could be remarkably modulated by the degree of polarization of light and its frequency.

Two-color multiphoton emission from nanotips

Two-color multiphoton emission from polycrystalline tungsten nanotips has been demonstrated using two-color laser fields. The two-color photoemission is assisted by a three-photon multicolor quantum channel, which leads to a twofold increase in quantum efficiency. Weak-field control of two-color multiphoton emission was achieved by changing the efficiency of the quantum channel with pulse delay. The result of this study complements two-color tunneling photoemission in strong fields, and has potential applications for nanowire-based photonic devices. Moreover, the demonstrated two-color multiphoton emission may be important for realizing ultrafast spin-polarized electron sources via optically injected spin current.

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.

Co/Fe multilayers with ultra-low damping and large negative anisotropy as the free layer for spin torque oscillator

We have investigated the characteristic properties of Co/Fe multilayers sputtered on Ru underlayer as the field generation layer (FGL) of spin torque oscillator (STO) for microwave assisted magnetic recording. The Co/Fe multilayers exhibit large uniaxial negative anisotropy (up to ∼8.7 × 106 erg/cm3), ultra-low damping constant (∼0.008), and small in plane coercivity Hc (∼5 Oe) that are required for stable STO precession over a wide range of applied magnetic fields and injected spin currents. The importance of Ru underlayer to achieve the above parameters has been addressed. We further show through theoretical calculations that a frequency of 30+ GHz is realistic using Co/Fe multilayers as the field generation layer in a STO comprising a perpendicularly magnetized polarizer and a planar FGL (10-nm Co/Fe multilayers) separated by a metallic spacer.

Estimation of transverse spin penetration length using second-harmonic measurement: Proposal of an experimental method

A theoretical description of spin current injection from a nonmagnetic layer into a magnetic one is presented, with the main emphasis on the description and determination of the penetration depth of spin current component transverse to the magnetization. This penetration depth also determines the depth of spin transfer torque generation. Physically, the spin current may be driven by an external electric field or by a temperature gradient. To determine the penetration depth we used ab initio calculations of channel and mixing conductances as well as of mixing transmission. The results are then used to determine the second harmonic voltage response, which in turn can be used to determine the penetration depth experimentally.

High Performance and Energy-Efficient On-Chip Cache Using Dual Port (1R/1W) Spin-Orbit Torque MRAM

This paper proposes a dual (1R/1W) port spin-orbit torque magnetic random access memory (1R/1W SOT-MRAM) for energy efficient on-chip cache applications. Our proposed dual port memory can alleviate the impact of write latency on system performance by supporting simultaneous read and write accesses. The spin-orbit device leverages the high spin current injection efficiency of spin Hall metal to achieve low critical switching current to program a magnetic tunnel junction. The low write current reduces the write power consumption, and the size of the access transistors, leading to higher integration density. Furthermore, the decoupled read and write current paths of the spin-orbit device improves oxide barrier reliability, because the write current does not flow through the oxide barrier. Device, circuit, and system level co-simulations show that a 1R/1W SOT-MRAM based L2 cache can improve the performance and energy-efficiency of the computing systems compared to SRAM and standard STT-MRAM based L2 caches.

Thermal spin current and spin accumulation at ferromagnetic insulator/nonmagnetic metal interface

Spin current injection and spin accumulation near a ferromagnetic insulator (FI)/nonmagnetic metal (NM) bilayer film under a thermal gradient is investigated theoretically. Using the Fermi golden rule and the Boltzmann equations, we find that FI and NM can exchange spins via interfacial electron-magnon scattering because of the imbalance between magnon emission and absorption caused by either non-equilibrium distribution of magnons or non-equilibrium between magnons and electrons. A temperature gradient in FI and/or a temperature difference across the FI/NM interface generates a spin current which carries angular momenta parallel to the magnetization of FI from the hotter side to the colder one. Interestingly, the spin current induced by a temperature gradient in NM is negligibly small due to the nonmagnetic nature of the non-equilibrium electron distributions. The results agree well with all existing experiments.

Manipulation of pure spin current in ferromagnetic metals independent of magnetization

Upon the injection of a pure spin current, a ferromagnet, similar to a nonmagnetic metal, also exhibits inverse spin Hall effect (ISHE). We show in Co/Cu/YIG, where the thin Cu layer allows transmission of spin current from YIG into Co but decouples the two ferromagnets, that the interaction between ISHE and ferromagnetic ordering in Co can be unambiguously investigated. By switching on and off the pure spin current contribution, we demonstrate that the ISHE in Co is independent of the direction of the Co magnetization, which clearly suggests that the ISHE in Co is dominated not by the extrinsic impurity scatterings, but from the intrinsic origin.

Magnetoelectric control of spin currents

The ability to control the spin current injection has been explored on a hybrid magnetoelectric system consisting of a (011)-cut ferroelectric lead magnesium niobate-lead titanate (PMNT) single crystal, a ferromagnetic FePt alloy, and a metallic Pt. With this PMNT/FePt/Pt structure we have been able to control the magnetic field position or the microwave excitation frequency at which the spin pumping phenomenon between FePt and Pt occurs. We demonstrate that the magnetoelectric heterostructure operating in the L-T (longitudinal magnetized-transverse polarized) mode couples the PMNT crystal to the magnetostrictive FePt/Pt bilayer, displaying a strong magnetoelectric coefficient of ∼140 Oe cm kV−1. Our results show that this mechanism can be effectively exploited as a tunable spin current intensity emitter and open the possibility to create an oscillating or a bistable switch to effectively manipulate spin currents.

Ultrafast and Gigantic Spin Injection in Semiconductors.

The injection of spin currents in semiconductors is one of the big challenges of spintronics. Motivated by the ultrafast demagnetization and spin injection into metals, we propose an alternative femtosecond route based on the laser excitation of superdiffusive spin currents in a ferromagnet such as Ni. Our calculations show that even though only a fraction of the current crosses the Ni-Si interface, the laser-induced creation of strong transient electrical fields at a ferromagnet-semiconductor interface allows for the injection of chargeless spin currents with record spin polarizations of 80%. Beyond that they are pulsed on the time scale of 100fs which opens the door for new experiments and ultrafast spintronics.

Inverse spin Hall effect from pulsed spin current inorganic semiconductors with tunable spin-orbit coupling.

Exploration of spin currents in organic semiconductors (OSECs) induced by resonant microwave absorption in ferromagnetic substrates is appealing for potential spintronics applications. Owing to the inherently weak spin-orbit coupling (SOC) of OSECs, their inverse spin Hall effect (ISHE) response is very subtle; limited by the microwave power applicable under continuous-wave (cw) excitation. Here we introduce a novel approach for generating significant ISHE signals in OSECs using pulsed ferromagnetic resonance, where the ISHE is two to three orders of magnitude larger compared to cw excitation. This strong ISHE enables us to investigate a variety of OSECs ranging from π-conjugated polymers with strong SOC that contain intrachain platinum atoms, to weak SOC polymers, to C60 films, where the SOC is predominantly caused by the curvature of the molecule's surface. The pulsed-ISHE technique offers a robust route for efficient injection and detection schemes of spin currents at room temperature, and paves the way for spin orbitronics in plastic materials.

Spin Circuit Representation for the Spin Hall Effect

Spin circuits with four component voltages and currents have been developed and used in the past to analyze various structures, which include non-collinear ferromagnets. Recent demonstrations of large spin orbit torques in heavy metals like Pt, Ta, and W open up new possibilities in spintronic applications by providing an alternative way to write information into a magnet. Here, we extend the four component (one charge and three spins) conductance matrix to include materials with spin Hall effect based on the standard diffusion equation. Our proposed spin circuit successfully reproduces standard results like spin Hall effect (SHE), inverse spin Hall effect, and spin Hall magnetoresistance. This circuit representation also makes it straightforward to analyze new configurations. We present two examples, namely, 1) the possibility of spin injection using giant SHE (GSHE) materials into semiconductors without tunneling barriers, and 2) the effect of spin ground on one surface to enhance spin current injection from the opposite surface in a thin GSHE sample. Finally, we provide an elemental conductance matrix for a small cubic structure which can be used as a building block to analyze any arbitrarily shaped GSHE material.

Probing excitations in insulators via injection of spin currents

We propose a spin transport experiment to measure the low-energy excitations in insulators with spin degrees of freedom, with a focus on detecting ground states that lack magnetic order. A general formalism to compute the spin-current from a metal with a non-equilibrium distribution of spins to an insulator is developed. It is applied to insulating states with and without long range magnetic order, and salient features in the spin-conductance are noted.