Spin-polarized Electron Injection Research Articles

Uncovering the origin of the anomalously high capacity of a 3d anode via in situ magnetometry.

Transition metals can deliver high lithium storage capacity, but the reason behind this remains elusive. Herein, the origin of this anomalous phenomenon is uncovered by in situ magnetometry taking metallic Co as a model system. It is revealed that the lithium storage in metallic Co undergoes a two-stage mechanism involving a spin-polarized electron injection to the 3d orbital of Co and subsequent electron transfer to the surrounding solid electrolyte interphase (SEI) at lower potentials. These effects create space charge zones for fast lithium storage on the electrode interface and boundaries with capacitive behavior. Therefore, the transition metal anode can enhance common intercalation or pseudocapacitive electrodes at high capacity while showing superior stability to existing conversion-type or alloying anodes. These findings pave the way for not only understanding the unusual lithium storage behavior of transition metals but also for engineering high-performance anodes with overall enhancement in capacity and long-term durability.

Spectral detection of spin-polarized ultra low-energy electrons in semiconductor heterostructures

The circularly polarized cathodoluminescence (CL) technique has been used to study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells (QWs). A polarized electron beam was created by the emission of optically oriented electrons from the p-GaAs(Cs,O) negative electron affinity (NEA) photocathode. The prepared beam was injected in a semiconductor QW target, which was activated by cesium and oxygen to reduce the work function. To study the spin-dependent injection, we developed a spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target placed parallel to each other on the opposite ends of the vacuum tube (photodiode). The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range of 0.5-5.0 eV and temperature in the range of 90-300 K was studied. The CL was polarized to 2 % by the injection of 20 % spin-polarized electron beam with the energy of 0.5 eV at room temperature. The asymmetry (Sherman function) of spin detection was estimated. It was shown that the dependence of the CL polarization degree on the injected electron energy is satisfactory described by the model that considers the electron spin relaxation in the heterostructure matrix and QWs. The results demonstrate that semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free-electron spin polarization.

Spin-Polarized Electron Injection into an InSb Semiconductor

The main stages of the study of spin-polarized electron injection into InSb semiconductor are considered. The characteristics of electromagnetic absorption and radiation due to spin-polarized electron injection into an InSb are given. The fundamental parameters of spin-polarized electrons, such as the relaxation time and spin diffusion length, are determined. The spin polarization of the conduction electrons in InSb is measured by direct detection.

Magnetotransport anisotropy in lattice-misfit-strained ultrathin La2/3Ca1/3MnO3 films epitaxially grown on (110)-oriented SrTiO3 and LaAlO3 substrates

(110)-Oriented manganite thin films are essential for studies on the interface effects concerning spin-polarized electron injection, especially the proximity effect with high temperature superconducting cuprates. However, due to the peculiarity of the (110) plane, an atomically smooth surface generally can only sustain up to a quite low film thickness. Therefore, anisotropy along the two crystallographically asymmetric [001] and [11¯0] in-plane directions in ultrathin (110)-oriented manganite films are worthy of a careful investigation. Here, studies on the magnetotransport anisotropy of (110)-oriented epitaxial La2/3Ca1/3MnO3 (LCMO) thin films of 20 nm thickness grown by pulsed laser deposition are reported. Both SrTiO3 (STO) and LaAlO3 (LAO) substrates were adopted to highlight the lattice-misfit-strain effects. While in LCMO/STO film, the magnetic easy axis is along the [11¯0] direction, in LCMO/LAO film the [001] direction is easier. Accordingly, the resistivity and magnetoresistance measured along the easy axes are lower. The correlation between the magnetic and transport anisotropy then has been undoubtedly demonstrated. For the LCMO films on STO, property evolutions with temperature and magnetic field are free from fine features, and thus clear and easily understood, making STO a more appropriate substrate for interface studies in (110)-oriented heterostructures.

Recent advances in organic spin-valve devices

Organic Spintronics has been considered to be the physics and applications of spin polarized electron injection, transport, manipulation and detection in organic diodes by the application of an external magnetic field. The prototype device is the organic spin-valve (OSV), which is based on an organic semiconductor spacer placed in between two ferromagnetic electrodes having different coercive fields, of which magnetoresistance changes with the applied field. Immense progress has been achieved in the past few years in fabricating, studying and understanding the underlying physics of these devices. We highlight the most significant advance in OSV research at the University of Utah, including the magnetoresistance response temperature and bias voltage dependencies; and show significant room temperature operation using LSMO/C 60/Co structure. We also report positive OSV-related magnetoresistance at low temperature, which was achieved using LSMO/polymer/Co OSV structure, where the polymer is a poly[phenylene-vinylene] derivative.

Spin injection effects on exciton formation in organic semiconductors

The effects of spin-polarized electron and hole injection from ferromagnetic contacts on the formation and distribution of singlet and triplet excitons in a conjugated organic semiconductor are modeled. Electron and hole transport in the semiconductor are described by spin-dependent device equations for a structure resembling an organic light emitting diode. The formation of electron-hole pairs at a given site is modeled as a Langevin process, and the subsequent local relaxation into the lowest energy exciton states is described by rate equations. Once formed, excitons may recombine in the semiconductor or diffuse through the material and recombine at the contact interfaces. The model calculations yield steady-state spatial profiles for singlet and triplet excitons. It is shown that spin-polarized injection increases the formation of singlet excitons, and that the diffusion of excitons has significant effects on the triplet exciton profile.

Spin-Polarized Hot Electron Injection into Two-Dimensional Electron Gas by Magnetic Tunnel Transistor

A magnetic tunnel transistor has been fabricated on top of a modulation-doped GaAs/AlGaAs heterostructure that provides a high-mobility two-dimensional electron gas. The emitter and base of the magnetic tunnel transistor consisted of a ferromagnetic metal, whereas the collector consisted of the two-dimensional electron gas. Spin polarized hot electrons from the emitter were filtered by the CoFe base layer and then injected into the collector, leading to a finite magnetocurrent ratio. The magnetocurrent ratio decreased as the hot electron energy was increased by increasing the bias voltage between the emitter and the base above a certain threshold voltage. We observed magnetocurrent ratios of ∼27% at 17 K and ∼2% at 300 K, indicating that spin-polarized hot electrons are injected into the two-dimensional electron gas from 17 K up to room temperature.

Mesoscopic rectification in a quantum dot with spin–orbit interaction

We investigate the conductance of an open quantum dot in which uniform Rashbaspin–orbit interaction (SOI) is present in the cavity region. The dot has a centraltriangular stopper (CTS) whose rotation angle controls the symmetry of the wholesystem. For a Fermi wavelength comparable to the linear dimension of the CTS, theSOI-dependence of the conductance is sensitive to both the direction of bias and therotation angle of the CTS. We propose a quantum ratchet which generates thedirected current against AC bias with time-average zero by using spin-polarizedelectron injection. The relationship between the symmetry of the dot and therectification effect is revealed, and is used as a mechanism for charge rectification.

Spin Hall effect and zitterbewegung in an electron waveguide

We study spin-resolved probability distributions for electrons in a multichannel waveguide in the presence of a spin-orbit interaction. For a spin-polarized electron injection, a zitterbewegung pattern is predicted in the probability distribution of electrons in the waveguide. For a spin-unpolarized injection, the spin-resolved electron probability in the waveguide shows spin accumulations. In addition to the spin Hall phenomenon---namely, accumulations of opposite spins at the lateral edges of the waveguide---we predict the existence of a regular stripe pattern of spin accumulations in the internal region of the waveguide. We show that the predicted zitterbewegung and spin Hall effect stem from the same mechanism and are formed from coherent states of electrons in the waveguide.

Spin polarization in photo- and electroluminescence of InAs and metal/InAs hybrid structures

We report the magnetic field dependence of the degree of circular polarization on electroluminescence (EL) from InAs Schottky diode with ferro- and non-ferromagnetic electrodes by measuring the σ+ and σ− components of the luminescence PL intensity. These data are compared with reference photoluminescence (PL) data from p-InAs substrate. We found an interesting circular polarization behaviour of the positive slope (0 < B < 5 T) and the negative slope (5 T < B < 10 T) reaching a maximum value of 12–15% at B ≈ 5 T in both non-magnetic Schottky diode (EL data) and p-InAs substrate (PL data). The positive dependence is presumably attributed to the Zeeman effect and the negative dependence to magnetic depopulation of Landau levels of the valence band. We have also found an interesting behaviour of the degree of circular polarization from PL data as a function of external magnetic field containing double peaks. The intensity of the higher field peak is quenched at higher temperatures while the lower field peak stays almost the same. We attribute the higher field and lower field peaks to the radiative recombination of electrons with acceptor trapped holes and free excitons, respectively. The degree of circular polarization of the ferromagnetic metal Schottky diode was found to be negative and alters its sign according to the external magnetic field, B, and its magnitude reaches a maximum value of 12% at B ≈ ±5 T. The clear difference between the ferro- and non-ferromagnetic samples indicates successful spin polarized electron injection from the ferromagnetic metal into the semiconductor. The net degree of circular polarization is estimated to be a much higher value of ≈20% which is translated to be ≈40% of spin injection efficiency assuming selection rules between heavy and light holes.

Band offsets for the epitaxial TiO2/SrTiO3/Si(001) system

We have used x-ray photoelectron spectroscopy with high energy resolution to determine band discontinuities at the two buried interfaces of the epitaxial TiO2 (anatase)/SrTiO3/Si(001) system. The valence band offsets are −2.1±0.1 eV and +0.2±0.1 eV at the SrTiO3/Si and TiO2/SrTiO3 heterojunctions, respectively. Assuming bulk band gaps for the SrTiO3 and TiO2 epitaxial films, the associated conduction band offsets are +0.1±0.1 eV and +0.1±0.1 eV. Si at the interface is in a flatband state, indicating a very low density of electronic states. These results suggest that spin-polarized electron injection from ferromagnetic Co-doped TiO2 anatase into Si should be facile.

Reduction of spin injection efficiency by interface defect spin scattering in ZnMnSe/AlGaAs-GaAs spin-polarized light-emitting diodes.

We report the first experimental demonstration that interface microstructure limits diffusive electrical spin-injection efficiency across heteroepitaxial interfaces. An inverse correlation be-tween spin-polarized electron injection efficiency and interface defect density is demonstrated for ZnMnSe/AlGaAs-GaAs spin-polarized light-emitting diodes that exhibit quantum well spin polarizations up to 85%. A theoretical treatment shows that the suppression of spin injection due to interface defects results from the contribution of the defect potential to the spin-orbit interaction, which increases the spin-flip scattering.

Spin-Polarized Electron Injection in Co/Cu/Fe Sandwich Structure

A material asymmetry Co/Cu/Fe junction structure has been prepared for studying the spin-polarized electron injection at 77K. The sample performance was demonstrated to be analogous to that of a bipolar transistor. The maximal value of the output pulse voltage between Cu and Fe layers could reach the order of several μV when the bias current between Co and Cu layers was 10μA. The interface roughness, photograph of material, magnetic loop and injection characteristic curves have been measured. Some important points on this topic have been discussed.