Electrical Spin Injection in a Ferromagnetic Metal/Insulator/Semiconductor Tunnel Heterostructure

We demonstrate experimentally the electrical spin injection from a ferromagnetic metal/tunnel barrier contact into a semiconductor III–V heterostructure. The injected electrons have an in-plane spin orientation. We show that by applying an oblique external magnetic field this spin orientation can be manipulated within the semiconductor, and a nonzero perpendicular spin component arises. This perpendicular component can be easily monitored by optical means (circular polarization of the emitted light). In a CoFe/AlO _x /(Al,Ga)As/GaAs heterostructure we observe injected spin polarization in access of 9% at 80 K and optimized structures have recently shown spin injection up to room temperature.     

自旋注入对有机半导体极化子电导的影响

在有机半导体自旋电子器件中,自旋从铁磁极注入到有机半导体后,自旋相上的极化子和自旋向下的极化子有不同的态密度,从而产生不同的电导.利用自旋漂移一扩散方程通过自洽计算得到了铁磁/有机半导体自旋注入结构中极化子自旋相关的电导和电流的自旋极化率.计算结果表明,极化子电导的自旋相关性是自旋注入引起的,和电流的自旋极化率密切相关;在自旋注入发生后,有机半导体内不同位置上极化子自旋态密度不同,由此产生的极化子电导也不相同,极化予电导是位置的函数.另外还发现,外电场会增强有机半导体电流的自旋极化率.     

Tunable spin-tunnel contacts to silicon using low-work-function ferromagnets

Magnetic tunnel junctions have become ubiquitous components appearing in magnetic random-access memory, read heads of magnetic disk drives and semiconductor-based spin devices. Inserting a tunnel barrier has been key to achieving spin injection from ferromagnetic (FM) metals into GaAs, but spin injection into Si has remained elusive. We show that Schottky barrier formation leads to a huge conductivity mismatch of the FM tunnel contact and Si, which cannot be solved by the well-known method of adjusting the tunnel barrier thickness. We present a radically different approach for spin-tunnelling resistance control using low-work-function ferromagnets, inserted at the FM/tunnel barrier interface. We demonstrate that in this way the resistance-area (RA) product of FM/Al2O3/Si contacts can be tuned over eight orders of magnitude, while simultaneously maintaining a reasonable tunnel spin polarization. This raises prospects for Si-based spintronics and presents a new category of ferromagnetic materials for spin-tunnel contacts in low-RA-product applications.     

Spin transport in spin filtering magnetic tunneling junctions

Taking into account spin-orbit coupling and s-d interaction, we investigate spin transport properties of the magnetic tunneling junctions with spin filtering barrier using Landauer-Büttiker formalism implemented with the recursive algorithm to calculate the real-space Green function. We predict completely different bias dependence of negative tunnel magnetoresistance (TMR) between the systems composed of nonmagnetic electrode (NM)/ferromagnetic barrier (FB)/ferromagnet (FM) and NM/FB/FM/NM spin filtering tunnel junctions (SFTJs). Analyses of the results provide us possible ways of designing the systems which modulate the TMR in the negative magnetoresistance regime.     

Reversible electrical switching of spin polarization in multiferroic tunnel junctions

Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.     

Magnetic and electrical characterization of “ferromagnet/insulator/silicon” diodes

This work is focused on the study of magnetic and electrical properties of ferromagnet/semiconductor heterostructures that can be used for spin injection into silicon. Three different studies are conducted whose principal results will be presented. In all these studies, a simple diode-like ferromagnet/insulator/semiconductor (FM/I/S) structure is used. The first study aimed to investigate whether a magnetic “dead” layer is obtained at the ferromagnet/oxide barrier that could lead to spin depolarization of the injected electrons. The results show the absence of such layer even after annealing at temperatures up to 723 K (450 °C). The second study focused on the mechanisms of electrical transport through the insulator barrier. Capacitance–voltage as well as current–voltage characteristics have been measured. The results underline the importance of controlling the ferromagnet deposition process in obtaining defect-free silicon–insulator interface, a prerequisite to spin conservative direct-tunnel transport process. In the third study, magnetic characterization of diodes that may be used for spin injection and collection were performed.     

Studying local surface electric conductivity of a ZnSe/CdSe/ZnSe heterostructure by scanning tunneling microscopy in the field electron emission regime

The surface morphology and local electric conductivity of a ZnSe/CdSe/ZnSe nanoheterostructure have been studied by scanning tunneling microscopy (STM) in the field electron emission regime. The homogeneity of the local conductivity distribution in a near-surface layer has been evaluated. The main parameters of a potential barrier in the local field contact are determined. It is shown that the STM probe can be used for creating local regions with nonequilibrium carrier concentration in a semiconductor.     

Transient Spectroscopy of Optical Spin Injection in ZnMnSe/ZnCdSe Quantum Structures

We show, by time-resolved magneto-photoluminescence (PL) spectroscopy in combination with selective laser excitation, that optical polarization of the ZnCdSe spin detector induced by spin injection from the ZnMnSe spin injector persists over a much longer time scale than the lifetime of the ZnMnSe excitons. This finding provides compelling experimental evidence that the dominant mechanism for the observed spin injection in the ZnMnSe/ZnCdSe structures should not be due to injection of the excitonic spins of the diluted magnetic semiconductor (DMS). It is rather due to e.g. a delayed spin injection arising from tunneling of individual carriers or/and trapped spins in ZnMnSe.     

Spin injection in spin FETs using a step-doping profile

We investigate the effect of a step-doping profile on the spin injection from a ferromagnetic metal contact into a semiconductor quantum well in spin field-effect transistors using a Monte Carlo model. The considered scheme uses a heavily doped layer at the metal-semiconductor interface to vary the Schottky barrier shape and enhance the tunneling current. It is found that spin flux (spin current density) is enhanced proportionally to the total current, and the variation of current spin polarization does not exceed 20%.     

Graphene as a tunnel barrier: graphene-based magnetic tunnel junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic.     

Introduction to spin-polarized ballistic hot electron injection and detection in silicon

Ballistic hot electron transport overcomes the well-known problems of conductivity and spin lifetime mismatch that plague spin injection attempts in semiconductors using ferromagnetic ohmic contacts. Through the spin dependence of the mean free path in ferromagnetic thin films, it also provides a means for spin detection after transport. Experimental results using these techniques (consisting of spin precession and spin-valve measurements) with silicon-based devices reveals the exceptionally long spin lifetime and high spin coherence induced by drift-dominated transport in the semiconductor. An appropriate quantitative model that accurately simulates the device characteristics for both undoped and doped spin transport channels is described; it can be used to recover the transit-time distribution from precession measurements and determine the spin current velocity, diffusion constant and spin lifetime, constituting a spin 'Haynes-Shockley' experiment without time-of-flight techniques. A perspective on the future of these methods is offered as a summary.     

Spin-polarized light-emitting diodes based on heterostructures with a GaAs/InGaAs/GaAs quantum well and ferromagnetic GaMnSb injection layer

Light-emitting diode structures with an InGaAs/GaAs quantum well and a ferromagnetic GaMnSb layer as a p-type semiconductor have been manufactured and investigated. A magnetic-field-induced circular polarization of electroluminescence in these structures has been obtained, the degree of which (0.012 at 0.37 T) is almost constant in the temperature interval of 10–50 K. Circular polarization is determined by the injection of spin-polarized holes from the ferromagnetic GaMnSb layer.     

Spin transport in a thin graphite flake

We have studied the spin transport on a 30-nm thick and several micrometer long oriented graphite flake using a spin-valve configuration with four ferromagnetic Co electrodes of different widths and several \upmum\upmu\hbox{m} separation. A 5-nm thin Pt layer has been introduced in between the ferromagnetic Co injector/detector and the graphite surface. In spite of the conductivity mismatch problem, efficient electrical spin injection and detection in graphite has been achieved. The magnetoresistance in the local and half-local electrodes shows clear maxima with symmetry around zero field. The spin transport can be detected up to 150 K.     

Modeling of a ferromagnetic two-dimensional electron gas device

We present a realistic modeling of ballistic electron transport in a hybrid ferromagnetic (FM) two-dimensional electron gas (2DEG) device, consisting of an FM gate on an AlGaAs-GaAs or AlSb-InAs high electron mobility transistor (HEMT) heterostructure. The carriers within the 2DEG are spin-polarized by a combination of magnetic and electrostatic barriers. The magnetic barriers are supplied by a composite FM gate, consisting of two domains made of magnetically hard and soft materials. This gate arrangement breaks the antisymmetry of the fringe B field, and results in a finite spin polarization of the 2DEG current. The B field strength is calculated by considering the pole strength at the gate surfaces and domain boundary, and is significantly weaker than normally assumed. We obtain parameters such as the electrostatic barrier height, Fermi level, and carrier concentration within the 2DEG by a finite-element Poisson calculation, which is self-consistent with the Fermi-Dirac distribution. We calculate the transmission probability and conductance through the 2DEG from these parameter values, assuming a single particle effective mass Hamiltonian and purely ballistic transport. We show that the spin polarization ratio P/sub G/ is extremely sensitive to the gate bias and HEMT doping concentration. However, the maximum P/sub G/ is extremely low for AlGaAs-GaAs (0.003%) and even for AlSb-InAs (0.12%) devices, despite a large Lande g factor. These values are many orders of magnitude smaller than previous predictions of close to 100% polarization, obtained by using simpler models.     

Spin-Dependent Fano Factor in FM/DNA/FM Molecular Junction

Based on a tight-binding model and a generalized Green’s function method in the Landauer?Büttiker formalism, the current-voltage characteristics, the noise power resulting of the spin-dependent current fluctuations and Fano factor of poly(G)-poly(C) DNA molecule sandwiched between ferromagnetic three-dimensional electrodes (FM/DNA/FM) are numerically investigated. It is found when the bias voltage increased, the current and noise power increase and the Fano factor decreases. Our results show the spin-dependent transport properties are significantly influenced by the electrode/molecule coupling strength so that the reinforcement of the electrode/molecule coupling strength give rise to increasing of the current and noise power so that the Fano factor decreased.     

Spin Injection Efficiency at the Source/Channel Interface of Spin Transistors

Almost all spintronic transistors (e.g., spin field-effect transistors, spin bipolar transistors, and spin-enhanced MOSFETs) require high efficiency of spin injection from a ferromagnetic contact into a semiconductor channel for proper operation. In this paper, we calculate the efficiency of spin injection from a realistic nonideal ferromagnetic contact into the semiconductor quantum wire channel of a spintronic transistor, taking into account the presence of an axial magnetic field (caused by either the ferromagnetic contact or external agents) and spin orbit interaction. In our calculations, the temperature is assumed to be low enough that phonon scattering is weak and transport is phase-coherent, although not ballistic because of elastic scattering caused by impurities and defects. We consider a single impurity in the channel and show that the conductance depends strongly on the exact location of this impurity because of quantum mechanical interference effects. This is a nuisance since it exacerbates device variability. The ldquosignrdquo of the impurity potential, i.e., whether it is attractive or repulsive, also influences the channel conductance. Surprisingly, at absolute zero temperature, the spin injection efficiency can reach 100% at certain gate biases, even though the ferromagnetic injector is nonideal. However, this efficiency drops rapidly with increasing temperature.     

Manipulation of spin currents in metallic systems

The transport properties of diffusive spin currents have been investigated in lateral ferromagnetic/non-magnetic metal hybrid structures. The spin diffusion processes were found to be strongly dependent on the magnitude of the spin resistances of connected materials. Efficient spin injection and detection are accomplished by optimizing the junction structures on the basis of the spin resistance circuitry. The magnetization switching of a nanoscale ferromagnetic particle and also room temperature spin Hall effect measurements were realized by using an efficient pure-spin-current injection.     

Charge–Spin Coupling at a Ferromagnet–Nonmagnet Interface

The topic of electrical spin injection from a ferromagnetic to a nonmagnetic material is presently attracting great interest and attention. A thermodynamic study of spin injection across a ferromagnetic–nonmagnetic material interface is presented. Using an entropy production calculation, the linear dynamic equations for interfacial transport of charge, heat, and spin magnetic moment are derived. A general equation for the fractional polarization of injected current is developed by matching boundary conditions at the interface. Polarization efficiency is sensitive to the intrinsic interface resistance, and to the resisivities and spin diffusion lengths of both materials. The physics of nonequilibrium spin diffusion across the interface is discussed, and the limiting case where resistance mismatch is important is identified. Example systems of interest are spin injection from a ferromagnetic metal to a nonmagnetic metal and from a ferromagnetic metal to a semiconductor. Charge–spin coupling and spin diffusion in one dimension, compared with higher dimension, are also discussed.     

Spin Injection: A Survey and Review

The study of the transport and relaxation of spin-polarized carriers in the solid state began about 30 years ago. Tunneling spectroscopy was applied to ferromagnet–insulator–superconductor junctions to demonstrate the polarization of interfacial currents. The use of a ferromagnetic material as an injector and/or detector of polarized carriers has since become a valuable tool, and spin injection has been applied to nonmagnetic metals, superconductors, and semiconductors. The spin injection phenomenology is reviewed in the context of two topics of continuing importance for basic and applied research: (i) the transmission of polarized carriers across ferromagnet/nonmagnetic material interfaces and (ii) carrier spin relaxation inside the nonmagnetic material.