Source Of Spin-polarized Electrons Research Articles

An improved source of spin-polarized electrons based on spin exchange in optically pumped rubidium vapor

We have improved a polarized electron source in which unpolarized electrons undergo collisions with a mixture of buffer gas molecules and optically spin-polarized Rb atoms. With a nitrogen buffer gas, the source reliably provides spin polarization between 15% and 25% with beam currents >4 μA. Vacuum pump upgrades mitigate problems caused by denatured diffusion pump oil, leading to longer run times. A new differential pumping scheme allows the use of higher buffer gas pressures up to 800 mTorr. With a new optics layout, the Rb polarization is continuously monitored by a probe laser and improved pump laser power provides more constant high polarization. We have implemented an einzel lens to better control the energy of the electrons delivered to the target chamber and to preferentially select electron populations of higher polarization. The source is designed for studies of biologically relevant chiral molecule samples, which can poison photoemission-based GaAs polarized electron sources at very low partial pressures. It operates adjacent to a target chamber that rises to pressures as high as 10−4 Torr and has been implemented in a first experiment with chiral cysteine targets.

Improved preparation techniques for preparing high-performance GaAs photocathodes.

Efficiency and lifetime are always problems raised with photocathodes during operation. With the purpose of obtaining high-performance GaAs photocathodes with high sensitivity and long operational lifetime, it is necessary to investigate the preparation techniques during both the cleaning and the activation procedure. By comparison with the classical preparation techniques, the improved preparation techniques with an optimized chemical etching method and activation procedure are proposed. The experimental results show that the optimized chemical etching solution is more effective in removing oxide and carbon contamination, which can help photocathodes obtain higher sensitivity. On this basis, better long wavelength response and longer operational lifetime can be obtained with the help of the more competitive activation procedure. The proposed preparation techniques will be useful for applications as a source of spin-polarized electrons.

Electrically Controlled Spin Injection from Giant Rashba Spin-Orbit Conductor BiTeBr.

Ferromagnetic materials are the widely used source of spin-polarized electrons in spintronic devices, which are controlled by external magnetic fields or spin-transfer torque methods. However, with increasing demand for smaller and faster spintronic components utilization of spin–orbit phenomena provides promising alternatives. New materials with unique spin textures are highly desirable since all-electric creation and control of spin polarization is expected where the strength, as well as an arbitrary orientation of the polarization, can be defined without the use of a magnetic field. In this work, we use a novel spin–orbit crystal BiTeBr for this purpose. Because of its giant Rashba spin splitting, bulk spin polarization is created at room temperature by an electric current. Integrating BiTeBr crystal into graphene-based spin valve devices, we demonstrate for the first time that it acts as a current-controlled spin injector, opening new avenues for future spintronic applications in integrated circuits.

A Giant Bulk‐Type Dresselhaus Splitting with 3D Chiral Spin Texture in IrBiSe

Non-magnetic half-metals are a new class of materials that are metallic only for one spin direction. In contrast to conventional half-metals, this spin direction is not fixed in space and is always perpendicular to the momentum of an electron. IrBiSe is one of such materials. Using angle-resolved photoemission spectroscopy and band structure calculations, Zhonghao Liu et al. find a record-high Dresselhaus spin-orbit splitting, fully spin-polarized remnant Fermi surfaces and a chiral 3D spin-texture, all with no magnetism present (see article number 1900684). Promising applications include using IrBiSe as a source of spin-polarized electrons, and lightly doped IrBiSe is expected to generate electric-field-controlled spin-polarized currents, free from back scattering, and could host triplet superconductivity.

Ambipolar transport in Mn2CoAl films by ionic liquid gating

We demonstrate ambipolar transport and modulation of electronic properties of Mn2CoAl (MCA), one of the most promising candidates for spin gapless semiconductors (SGSs), by using ionic liquid gating in electronic double-layer transistors. The carrier concentration and mobility of MCA films were systematically changed with carrier polarity inversion from the p- to the n-type by the gating technique. The ambipolar transport is one of the most significant properties of SGSs and strongly promotes the gapless features of MCA. The present results pave the way for the use of MCA as a spin source of both spin-polarized electrons and holes.

Optical detection of transverse spin-Seebeck effect in permalloy film using Sagnac interferometer microscopy

Here, the authors investigate the possibility of building up nonequilibrium spin accumulation in a ferromagnetic thin film composed of a metallic alloy of nickel and iron, commonly known as permalloy. Analogously to thermoelectric generators, a temperature gradient in a ferromagnet is known to generate a distribution of accumulated electron spins over macroscopic distances, which may be used as an efficient source of spin-polarized electrons in future spintronic devices. To enable the detection and quantitative determination of this imbalance of electron spins, the authors have developed an entirely optical technique, based on the magneto-optical Kerr effect, using a Sagnac interferometer microscope with a spatial resolution of one micrometer and an absolute polarization angle resolution of ten nanoradians. This new technique is not only robust against electrical artifacts, but allows the user to measure previously inaccessible experimental geometries where the magnetization is not restricted to lying in the plane of the film.

Electron ionization and spin polarization control of Fe atom adsorbed graphene irradiated by a femtosecond laser

We investigate the structural properties and ionized spin electrons of an Fe–graphene system, in which the time-dependent density functional theory (TDDFT) within the generalized gradient approximation is used. The electron dynamics, including electron ionization and ionized electron spin polarization, is described for Fe atom adsorbed graphene under femtosecond laser irradiation. The theoretical results show that the electron ionization and ionized electron spin polarization are sensitive to the laser parameters, such as the incident angle and the peak intensity. The spin polarization presents the maximum value under certain laser parameters, which may be used as a source of spin-polarized electrons.

ChemInform Abstract: Spin Transport in Ferromagnet/III‐V Semiconductor Heterostructures

AbstractReview: 104 refs.

Development of an Active 238UF6 Gas Target

Abstract At the superconducting 130 MeV Darmstadt electron linac S-DALINAC a new source of spin-polarized electrons using a GaAs cathode has been installed, opening the path for experiments with polarized electron and photon beams for nuclear structure studies at low momentum transfers, e.g., the search for forward-backward asymmetries originating from parity non-conservation (PNC) in the photon-induced fission process of 238 U.Detailed studies of different properties, e.g., the energy dependence of fission modes, the population of fission isomers, or the search for (PNC) effects in the photon-induced fission process of 238 U, depends on high quality data, therefore needing high luminosities. An active gas target containing uranium may overcome the problem that large solid target thicknesses cause poor energy and angular resolution.A single Frisch-grid ionization chamber has been built to test a mixture of standard counting gases (e.g., argon) with depleted uraniumhexafluoride ( 238 UF 6 ) using a triple alpha source, evaluating signal quality and drift velocity. For mass fractions up to 2 percent of 238 U in the counting gas. The drift velocity increases with rising UF 6 content, while a good signal quality and energy resolution is preserved.

A possible approach towards spin-polarized transport through single molecule magnets: Mn 12 on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0)

The possibility to use the Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) system as a substrate for future spin-polarized transport measurements on Mn 12 single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn 12 monolayers was studied. The results demonstrate that Mn 12 can be deposited on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies.

A possible source of spin-polarized electrons: The inert graphene/Ni(111) system

We report on an investigation of spin-polarized secondary electron emission from the chemically inert system: graphene/Ni(111). An ordered passivation graphene layer (monolayer of graphite) was formed on Ni(111) surface via cracking of propylene gas. The spin polarization of secondary electrons obtained from this system upon photoemission is only slightly lower than the one from the clean Ni surface but does not change upon large oxygen exposure. These results suggest to use such passivated Ni(111) surface as a source of spin-polarized electrons stable against adsorption of reactive gases.

Electrical detection of spin transport in lateral ferromagnet–semiconductor devices

A longstanding goal of research in semiconductor spintronics is the ability to inject, modulate, and detect electron spin in a single device. A simple prototype consists of a lateral semiconductor channel with two ferromagnetic contacts, one of which serves as a source of spin-polarized electrons and the other as a detector. Based on work in analogous metallic systems, two important criteria have emerged for demonstrating electrical detection of spin transport. The first is the measurement of a non-equilibrium spin population using a non-local ferromagnetic detector through which no charge current flows. The potential at the detection electrode should be sensitive to the relative magnetizations of the detector and the source electrodes, a property referred to as the spin-valve effect. A second and more rigorous test is the existence of a Hanle effect, which is the modulation and suppression of the spin valve signal due to precession and dephasing in a transverse magnetic field. Here we report on the observation of both the spin valve and Hanle effects in lateral devices consisting of epitaxial Fe Schottky tunnel barrier contacts on an n-doped GaAs channel. The dependence on transverse magnetic field, temperature, and contact separation are in good agreement with a model incorporating spin drift and diffusion. Spin transport is detected for both directions of current flow through the source electrode. The sign of the electrical detection signal is found to vary with the injection current and is correlated with the spin polarization in the GaAs channel determined by optical measurements. These results therefore demonstrate a fully electrical scheme for spin injection, transport, and detection in a lateral semiconductor device.

Adjustable spin torque in magnetic tunnel junctions with two fixed layers

We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional fixed magnetic layer added above the magnetic free layer of a standard MTJ structure. This acts as a second source of spin-polarized electrons that, depending on the relative alignment of the two fixed layers, either augments or diminishes the net spin torque exerted on the free layer. The compound structure allows a quantitative comparison of spin torque from tunneling electrons and from electrons passing through metallic spacer layers, as well as analysis of Joule self-heating effects. This has significance for current-switched magnetic random access memory, where spin torque is exploited and, for magnetic sensing, where it is detrimental.

Inelastic scattering approach to the theory of a magnetic tunnel transistor source

The high efficiency of a magnetic tunnel transistor as a source of spin-polarized electrons has been proven recently [X. Jiang et al., Phys. Rev. Lett. $90,$ 256603 (2003)]. A concept of this device based on an active group of hot electrons controlling the collector current and experiencing predominantly inelastic scattering in the base is developed. It takes into account the connection between the injection and filtering of spin-polarized electrons inside the device. Explicit expressions for the parameters of the device in terms of the basic parameters of the system are presented.

Ferromagnetic resonant interband tunneling diode

We consider the spin-polarizing properties of a ferromagnetic resonant interband tunneling diode based on the type-II InAs/(AlSb)/GaMnSb/(AlSb)/InAs heterostructure system. The current–voltage characteristics are simulated using the effective bond-orbital method, in conjunction with self-consistent treatments of both the electrostatic potential and the magnetic exchange interaction. We find that the device can be employed as either a source of spin-polarized electrons, whose polarity varies with applied bias, or as a spin valve. For example, the polarization of electrons tunneling to the collector from an unpolarized emitter can exceed 75% in either direction.

Spin-polarizing properties of the InAs/(AlSb)/GaMnSb/(AlSb)/InAs ferromagnetic resonant interband tunneling diode

We theoretically investigate the low-temperature spin-dependent current-voltage characteristics of the ferromagnetic resonant interband tunneling diode (FRITD). The simulations are based on a transmission formulation of the effective bond-orbital method (EBOM), which self-consistently includes both the electrostatic potential and the magnetic exchange interaction. Despite negligible spin splitting in the InAs emitter and collector regions, this type-II InAs/(AlSb)/GaMnSb/(AlSb)/InAs structure can function either as a source of spin-polarized electrons, whose polarity depends on the applied bias, or as a spin valve. For unpolarized emitter electrons, the spin polarization of carriers transmitted to the collector can be as high as \ensuremath{\approx}90%. We consider both thicker-well structures, in which the interband tunneling occurs primarily via light-hole-like states, and thinner-well devices whose transport is dominated by heavy-hole transitions. Since the maximum FRITD operating temperature is limited primarily by the Curie temperature in the GaMnSb hole quantum well, the same favorable device performance may be achievable at 77 K by employing the digital incorporation of Mn into the center of a GaSb hole well.

Use of a gaas microprobe tip pumped by circularly polarized light as a source of spin‐polarized tunneling electrons

AbstractWe have studied the feasibility of using GaAs microprobe tips fabricated by anisotropic wet chemical etching followed by sulfur passivation as a source of spin‐polarized tunneling electrons. In our experiments, we studied the magnetic state of a nickel thin film by spin‐polarized STM, using a GaAs microprobe tip illuminated by circularly polarized light. We observed that the tunneling current was different depending on the sense of rotation of the circular polarization (and hence was proportional to the number of spin‐polarized tunneling electrons). We then made measurements of the thin film's hysteresis characteristics by the same technique and by the magnetic Kerr effect. We confirmed by time‐resolved photoluminescence measurements that surface states located at the surface of the GaAs microprobe tip can mediate the tunneling, which shortens the lifetime of electrons within the tip. This indicates that these surface states can affect the spin‐polarized tunneling signal. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(5): 21–28, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.1103

Bolt-on source of spin-polarized electrons for inverse photoemission

We have developed a portable spin-polarized electron gun which can be bolted on to an ultrahigh vacuum chamber. The gun has been successfully operated with an electron gun to target distance of about 150 mm. This allows accommodation of other surface science equipment in the same vacuum system. The spin-polarized electrons are obtained via photoemission from a negative electron affinity GaAs(001) surface with circularly polarized light. A transversely polarized beam is achieved with a 90° electrostatic deflector. A set of two three-element electrostatic tube lenses are employed to transport and to focus the electrons onto a target. The measured transmission through the electron optics is >70% for electron energies in the range 7–20 eV. This is achieved by using large diameter electron transport lenses. The energy resolution of the electron beam is measured to be better than 0.27 eV and the polarization is determined to be 25±5%.

Theory of scanning tunneling microscopy with spin-polarized electrons obtained from a semiconducting tip

A principle of spin-polarized scanning tunneling microscopy (STM) for studying surface magnetism is theoretically analysed. In this approach the source of spin-polarized electrons is a semiconducting (e.g. GaAs) tip illuminated by circularly polarized light. Equations describing the tunneling current flowing between the tip and a ferromagnetic sample are derived. It is found that the magnetic domains and the quantization axis of the surface spins can be resolved with this method.

Theory of scanning tunneling microscopy with spin-polarized electrons obtained from a semiconducting tip

Abstract A principle of spin-polarized scanning tunneling microscopy (STM) for studying surface magnetism is theoretically analysed. In this approach the source of spin-polarized electrons is a semiconducting (e.g. GaAs) tip illuminated by circularly polarized light. Equations describing the tunneling current flowing between the tip and a ferromagnetic sample are derived. It is found that the magnetic domains and the quantization axis of the surface spins can be resolved with this method.