Spin Current Injection Research Articles

Spin-current-induced charge current

We show that the injection of a pure spin current (not accompanied by charge current) into a ring can induce a circulating charge current in the ring, provided that transport coefficients of the ring are spin dependent and inhomogeneous. As an example, we consider a hybrid ferromagnet--normal-metal ring system and calculate the magnitude of the charge current induced by the pure spin current injection. This phenomenon may have relevance for spintronic applications.

Spin current injection by intersubband transitions in quantum wells

We show that a pure spin current can be injected in quantum wells by the absorption of linearly polarized infrared radiation, leading to transitions between subbands. The magnitude and the direction of the spin current depend on the Dresselhaus and Rashba spin–orbit coupling constants and light frequency and, therefore, can be manipulated by changing the light frequency and/or applying an external bias across the quantum well. The injected spin current should be observable either as a voltage generated via the anomalous spin-Hall effect, or by spatially resolved pump–probe optical spectroscopy.

Theory of spin injection into conjugated organic semiconductors

We present a theoretical model to describe electrical spin injection from a ferromagnetic contact into a conjugated organic semiconductor. In thermal equilibrium the magnetic contact is spin polarized, whereas the organic semiconductor is unpolarized. The organic semiconductor must be driven far out of local thermal equilibrium by an electric current to achieve significant spin current injection. However, if the injecting contact has metallic conductivity, its electron distribution cannot be driven far from thermal equilibrium by practical current densities. Thus, quasi-equilibration between the conjugated organic semiconductor and the metallic contact must be suppressed to achieve effective spin injection. This requires a spin-dependent barrier to electrical injection that may be due either to tunneling through the depletion region of a large Schottky barrier or to tunneling through a thin, insulating, interface layer. Schottky barrier formation on conjugated organic semiconductors differs from that on inorganic semiconductors inasmuch as contacts made to organic semiconductors often follow near-ideal Schottky behavior, thus permitting the energy barrier to electrical injection to be varied over a wide range by using metals with different work functions. In addition, insulating tunnel barriers to organic semiconductors based on organic molecules can be conveniently fabricated using self-assembly techniques.

Dynamics of optical injection of charge and spin currents in quantum wells

We develop a dynamical theory for the optical injection of charge and spin current originating from the interferences between two coherent laser pulses of frequencies omega and 2omega. Multiband Bloch equations which include one- and two-photon interband transitions are derived. They also account for ac Stark shifts and intersubband two-photon transitions. The model is used to describe the case of time-dependent charge and spin current injection in a symmetric GaAs/AlGaAs quantum well. A comparison to the bulk case is also made. The separate contributions of the populations and intersubband coherences to the charge current and THz emission are identified. The influence of the Stark shifts and the inter-valence-band two-photon transitions are calculated and discussed.

Non-local Hall resistance measured in submicron-scale non-magnetic/ferromagnetic junctions

We here demonstrate the measurement of the non-local Hall resistance which reflects the spatial distribution of non-equilibrium magnetization induced in the nano-scale normal metal Cu/ferromagnetic Ni–Fe alloy junction. The non-local Hall resistance seems to vary proportional to the perpendicular component of the injected spin current.

All-optical injection and control of spin and electrical currents in quantum wells

We show that quantum interference between one- and two-photon absorption can be used to inject spin currents, with or without an accompanying electrical current, in unbiased semiconductor quantum well structures. The directions in which the electrical and spin currents are injected can be coherently controlled, with a relative phase parameter of the optical fields as the control parameter. We characterize the currents for an unstrained quantum well and a quantum well under biaxial compressive strain using the Luttinger-Kohn model; we work out particular examples. If compressive strain is used to appropriately rearrange the subbands, then a degree of spin polarization of the spin currents higher than possible in bulk GaAs can be achieved and maintained even for photon energies well above the band gap.

Coulomb drag, magnetoresistance, and spin-current injection in magnetic multilayers

We study the effect of spin Coulomb drag on the magnetoresistance and the spin-current injection efficiency of a layered structure consisting of a nonmagnetic semiconductor sandwiched between two ferromagnetic electrodes of spin polarization p. The calculations are done within the framework of the drift–diffusion theory, which we generalize to include the spin trans-conductivity σ ↑↓. We find that for p close to 100% the spin drag enhances the magnetoresistance, while for smaller values of p it reduces it. A new approach to the measurement of σ ↑↓ is suggested.

Direct observation of optically injected spin-polarized currents in semiconductors.

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.

Quantum interference control of ballistic pure spin currents in semiconductors.

We demonstrate all-optical quantum interference injection and control of a ballistic pure spin current (without an accompanying charge current) in GaAs/AlGaAs quantum wells, consisting of spin-up electrons traveling in one direction and spin-down electrons traveling in the opposite direction. This current is generated through quantum interference of one- and two-photon absorption of approximately 100 fs phase-locked pulses that have orthogonal linear polarizations. We use a spatially resolved pump-probe technique to measure carrier movement of approximately 10 nm. Results agree with recent theoretical predictions.

Comparative study of spin injection into metals and semiconductors

We present an analysis of spin injection efficiency that is of general application and relevant to a wide range of spin-electronic devices. By applying simple band structure ideas to a single interface between a metallic ferromagnet and a three-dimensional semiconductor, two conflicting figures of merit are identified - spin accumulation and polarization of injected current - and their validity to the analysis of different device types is discussed. The injected spin accumulation is smaller than the all-metal injection case by a factor(m*/me)3/2(kT/EF)1/2e^εD/kT.Moreover, the injected spin current at the interface is reduced by the factor (m*/me)3/2(kT/EF)(γS/γF) e^εD/kT, whereγ for a particular material is the square root of its momentum scattering to spin-flip scattering ratio, m* and me are the effective and free masses, respectively, and εD is the donor binding energy. These results are consequent on the boundary condition that the spin channel electrochemical potentials are continuous at the interface. By inserting an insulating tunnel barrier between the ferromagnet and thesemiconductor, not only is this boundary condition removed and the spin polarization of the injected current restored to the all-metal magnitude, but alsothe spin accumulations in the metal and the semiconductor even have opposite signs. This implies that thin or discontinuous tunnel barriers have the worst spin injection efficiency of any configuration. We finally note that for injected spin current into metals with polarization approaching 100%, the Fermi surface is polarized to a depth which exceeds the equilibrium carrier depth by a factor lSD/λ, hence >>1.

Spin Injection and Spin Accumulation in Permalloy–Copper Mesoscopic Spin Valves

We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as nonmagnetic metal. Our multiterminal geometry allows us to experimentally distinguish different magnetoresistance signals, being (1) the spin valve effect, (2) the anomalous magnetoresistance (AMR) effect, and (3) Hall effects. We find that the AMR contribution of the Py contacts can be much larger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a “conventional” measurement geometry. However, these “contact” magnetoresistance signals can be used to monitor the magnetization reversal process, of the spin injecting and detecting Py contacts. In a “nonlocal” spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T = 4.2 K as well as at room temperature. We obtain spin diffusion lengths in Cu of 1 μm and 350 nm at T = 4.2 K and room temperature respectively.

Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve.

Finding a means to generate, control and use spin-polarized currents represents an important challenge for spin-based electronics, or 'spintronics'. Spin currents and the associated phenomenon of spin accumulation can be realized by driving a current from a ferromagnetic electrode into a non-magnetic metal or semiconductor. This was first demonstrated over 15 years ago in a spin injection experiment on a single crystal aluminium bar at temperatures below 77 K. Recent experiments have demonstrated successful optical detection of spin injection in semiconductors, using either optical injection by circularly polarized light or electrical injection from a magnetic semiconductor. However, it has not been possible to achieve fully electrical spin injection and detection at room temperature. Here we report room-temperature electrical injection and detection of spin currents and observe spin accumulation in an all-metal lateral mesoscopic spin valve, where ferromagnetic electrodes are used to drive a spin-polarized current into crossed copper strips. We anticipate that larger signals should be obtainable by optimizing the choice of materials and device geometry.

Optically injected spin currents in semiconductors.

We show that quantum interference of one and two photon absorption from a two color field allows one to optically inject ballistic spin currents in unbiased semiconductors. The spin currents can be generated with or without an accompanying electrical current and can be controlled using the relative phase of the two colors. We characterize the injected spin currents using symmetry arguments and an eight-band Kane model.

Waveguide diffusion modes and slowdown of D’yakonov-Perel’ spin relaxation in narrow two-dimensional semiconductor channels

We have shown that in narrow two-dimensional semiconductor channels the D'yakonov-Perel' spin relaxation rate is strongly reduced. This relaxation slowdown appears in special waveguide diffusion modes which determine the propagation of spin density in long channels. Experiments are suggested to detect the theoretically predicted effects. A possible application is a field effect transistor operated with injected spin current.