Electrical Spin Injection in Perpendicular Magnetized FePt/MgO/GaAs Heterostructures at Room Temperature

We report zero-magnetic-field spin injection from FePt into GaAs at room temperature using FePt/MgO/GaAs-based light-emitting diode heterostructures. Experiments are performed on two samples with different compositions; Fe₆₁Pt₃₉ and Fe₅₇Pt₄₃. The polarizations of injected electrons at 0 T for these two samples are at least 1.5% and 3.3%, respectively. The higher zero-magnetic-field injected spin polarization is considered to be due to the better remanent perpendicular magnetization of the FePt layer in the sample with Fe₅₇Pt₄₃.     

Spin Manipulation Using Magnetic II–VI Semiconductors

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.     

Electrical Manipulation of Spin Injection into a Single InAs Quantum Dots

We demonstrate spin injection from a n-Zn_0.96Mn_0.04Se layer into individual InAs quantum dots (SQDs) in a p–i–n diode structure using cw polarization resolved magneto-micro photoluminescence spectroscopy. Interestingly, we find that the spin injection efficiency strongly varies from dot to dot. We obtain a single quantum dot circular polarization degree ranging from 2% to almost 50% (at B=4 T) at zero biasing and within the spectral range studied here, we found 2 maxima of the degree of the circular polarization at SQD energies separated by ∼33 meV. Importantly, we demonstrate that the spin injection efficiency can be manipulated by external forward biasing (U _ext).     

The effect of photocurrent amplification in an In2O3-GaSe heterostructure

The effect of photocurrent amplification in an In₂O₃-GaSe heterostructure with the barrier plane perpendicular to the base semiconductor layer was experimentally observed. At a reverse bias of U=10 V, the gain reached M≈82 and the absolute current sensitivity amounted to 30–32 A/W. An analysis of the current-voltage characteristics allowed a mechanism of the charge transfer through dielectric to be determined that is always operative on the surface of gallium monoselenide in the heterostructures fabricated by spraying. It is suggested that the mechanism of conductivity can be modified by changing the barrier plane orientation from parallel to perpendicular to the GaSe layers.     

Electric Field Effects on Spin Polarized Transport in FM/NMS and FM/NMS/FM Structures in Interaction Approximation

In this article, we have solved spin-dependent drift-diffusion equations analytically by considering a spin selective barrier between the magnet and semiconductor layer in interaction approximation and also taking into account the correlation and exchange effects. We have studied the electric field effects on the spin polarized transport in the ferromagnetic/nonmagnetic semiconductor (FM/NMS) and FM/NMS/FM structures in a degenerate regime. We have shown by increasing the conductivity of semiconductor up to ferromagnetic conductivity, semiconductor effective resistance becomes smaller and the spin injection efficiency will be increased. Also, the electric field enhances spin polarization density. Furthermore, in injection structures with interfacial barriers, the electric field enhances spin polarization considerably. In fact, the spin selective interfacial barrier acts as a spin filter, which permits electrons with a particular spin direction ↑(↓) pass through the interface. In addition, the calculated results in interacting approximation show that spin injection is increased. Finally, it is found that in FM/NMS/FM structures at low-field regime, the width of the semiconductor has the important role in spin transport.     

Observation of e2/h Conductance Steps in a Side-Gate Point Contact on In0.75Ga0.25As/In0.75Al0.25As Heterostructure

In 1996, it was suggested by K. J. Thomas et al. (Phys. Rev. Lett. 77, 135 (1996)) that 0.7(2e²/h) conductance structure in Quasi-1DEG (Q1DEG) at GaAs/AlGaAs heterostructure might be related to spin polarization at zero magnetic field. We have recently studied spontaneous spin-splitting in the 2DEGs formed at normal metamorphic In_0.75Ga_0.25As/In_0.75Al_0.25As heterojunctions grown on GaAs substrates and obtained the value of 10 meV as the zero-field splitting at Fermi level (Y. Sato et al., J. Appl. Phys. 89, 8017 (2001)). In this work, we attempted to observe spin-related phenomena in this heterojunction Q1DEGs at zero magnetic field. We observed e²/h conductance steps in low electron concentration side-gate point contact.     

Spin–Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin–orbit torque induced switching in the Heusler alloy Mn₂Ru_{1? x} Ga, it is found that efficient current‐induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy‐efficient spintronic devices.     

Magnetoelectric Spin-FET for Memory,Logic, and Amplifier Applications

We propose a ballistic magneto-electric device that permits conductance modulation with both electric and magnetic fields applied perpendicular to its current conduction channel. Fields are applied through the ferromagnetic gates deposited on top of a HEMT heterostructure that contains a 2DEG for current conduction. The minimal-coupling Hamiltonian with spatially uniform electrical potentials, and delta Zeeman splitting is solved in the weak-coupling limit for which the Rashba spin orbit coupling is not considered. Ballistic transmission of electrons through a periodic system of zero-gauge double-pair magnetoelectric barriers is studied. Manipulation of barriers’ geometrical symmetry and configuration leads to the conception of a spin-FET for non-volatile storage and digital logic operations. The linear modulation of electron spin polarization (|P|) is also studied for its relevance to electrical signal amplification. Perpendicular magnetization of the ferromagnetic gates permits modulation of both |P| and electron transmission (T) threshold, the latter is particularly useful for spin logic design.     

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.     

Local Definition of Spin Polarization in a Semiconductor by Micro-scale Current Loops

We present an approach to electrical control of the spin polarization in a diluted magnetic semiconductor (DMS) structure. A variable magnetic field induced by a micro-scale current loop magnetizes the Mn²⁺ ions in a CdMnTe/CdMgTe DMS quantum well, which via the sp-d exchange interaction polarizes photo-generated electron-hole pairs confined in the well. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The current-induced magnetic field and the current-generated heating of the spin system are quantitatively extracted by micro magneto-luminescence measurements.     

Detailed Measurements of Nuclear Spin Polarizations in a Single InAlAs Quantum Dot Through Overhauser Shift of Photoluminescence

We investigated optical pumping of nuclear spin polarizations in a single self-assembled In_0.75Al_0.25As/Al_0.3Ga_0.7As quantum dot. The nuclear spin polarization exhibits the abrupt jump and hysteresis in the excitation power dependence at a particular excitation polarization. Measurement of circular polarization rate of the photoluminescence reveals that the abrupt change of the nuclear spin polarization is created mainly by the spin flip-flop process between nuclei and an electron of a positive charged exciton in this single quantum dot. Model calculation explains well the experimentally observed bistable behavior in InAlAs quantum dot. By using this abrupt change, the sign and magnitude of electron and hole g-factors in z-direction are verified.      

Selective and wideband UV sensors

It is shown that thin-film photoelectric devices can be constructed on the basis of wide-gap A^IIB^VI semiconductor compounds grown on a narrow-gap quasi-single-crystal substrate. The potential barrier ΔE _v existing at the interface of a multilayer heterostructure blocks the contribution of the narrow-gap component to the total photocurrent. Based on these heterostructures, selective and wideband UV sensors requiring no additional filters are developed for the first time.     

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.     

Fabrication of Hollow CoP/TiOx Heterostructures for Enhanced Oxygen Evolution Reaction

Transition‐metal phosphides have flourished as promising candidates for oxygen evolution reaction (OER) electrocatalysts. Herein, it is demonstrated that the electrocatalytic OER performance of CoP can be greatly improved by constructing a hybrid CoP/TiO_x heterostructure. The CoP/TiO_x heterostructure is fabricated using metal–organic framework nanocrystals as templates, which leads to unique hollow structures and uniformly distributed CoP nanoparticles on TiO_x. The strong interactions between CoP and TiO_x in the CoP/TiO_x heterostructure and the conductive nature of TiO_x with Ti³⁺ sites endow the CoP–TiO_x hybrid material with high OER activity comparable to the state‐of‐the‐art IrO₂ or RuO₂ OER electrocatalysts. In combination with theoretical calculations, this work reveals that the formation of CoP/TiO_x heterostructure can generate a pathway for facile electron transport and optimize the water adsorption energy, thus promoting the OER electrocatalysis.     

Controlling the Charge and the Spin Polarization Distributions in (In,Ga,Mn)As-Based Diluted Magnetic Semiconductor Multilayered Structures

A self-consistent Luttinger–Kohn (LK) calculation is used to obtain the electronic structure of Mn-doped semiconductor multilayers. In the particular, case of a (In,Ga)As/GaAs heterostructure, the ternary alloy has a smaller gap than GaAs, and introduces strain due to the differences on the lattice parameters. The aim of this work is to understand how the charge and the spin polarization densities can be controlled by the parameters of the structure in such way that they appear concentrated either on the magnetic or in the non-magnetic region. We show that by changing the In concentration the valence band mismatch competes with the energy splitting due to the magnetic interaction, and we can manipulate the charge and the spin polarization densities in the structure.     

Symmetry‐Reduction Enhanced Polarization‐Sensitive Photodetection in Core–Shell SbI3/Sb2O3 van der Waals Heterostructure

Structural symmetry is a simple way to quantify the anisotropic properties of materials toward unique device applications including anisotropic transportation and polarization‐sensitive photodetection. The enhancement of anisotropy can be achieved by artificial symmetry‐reduction design. A core–shell SbI₃/Sb₂O₃ nanowire, a heterostructure bonded by van der Waals forces, is introduced as an example of enhancing the performance of polarization‐sensitive photodetectors via symmetry reduction. The structural, vibrational, and optical anisotropies of such core–shell nanostructures are systematically investigated. It is found that the anisotropic absorbance of a core–shell nanowire is obviously higher than that of two single compounds from both theoretical and experimental investigations. Anisotropic photocurrents of the polarization‐sensitive photodetectors based on these core–shell SbI₃/Sb₂O₃ van der Waals nanowires are measured ranging from ultraviolet (UV) to visible light (360–532 nm). Compared with other van der Waals 1D materials, low anisotropy ratio (I_max/I_min) is measured based on SbI₃ but a device based on this core–shell nanowire possesses a relatively high anisotropy ratio of ≈3.14 under 450 nm polarized light. This work shows that the low‐symmetrical core–shell van der Waals heterostructure has large potential to be applied in wide range polarization‐sensitive photodetectors.     

Electrical Electron Spin Injection with a p+-(Ga,Mn)As/n+-GaAs Tunnel Junction

We demonstrate electrical electron spin injection in a p⁺-(Ga,Mn)As/n⁺-GaAs tunnel junction with an n-GaAs/(In,Ga)As/p-GaAs light emitting diode (LED). By applying a reverse bias to the p⁺-(Ga,Mn)As/n⁺-GaAs junction (forward bias to the LED), we observed clear hysteresis in electroluminescence (EL) polarization. The magnitude of the EL polarization, which does not depend critically on the spacer layer thickness up to 800 nm, is found to be about five times greater than that of the hole spin injection.     

Homogeneous Fermion Superfluid with Unequal Spin Populations

For decades, the conventional view is that an s-wave BCS superfluid can not support uniform spin polarization due to a gap Δ in the quasiparticle excitation spectrum. We show that this is an artifact of the dismissal of quasiparticle interactions V _qp in the conventional approach at the outset. Such interactions can cause triplet fluctuations in the ground state and hence non-zero spin polarization at “magnetic field” h<Δ. The resulting ground state is a pairing state of quasiparticles on the “BCS vacuum”. For sufficiently large V _qp , the spin polarization of at unitarity has the simple form m ∝ μ ^1/2. Our study is motivated by the recent experiments on Fermi gases with spin imbalance in the strongly interacting regime.      

Lateral Shifts for Spin Electrons in a Hybrid Magnetic-Electric-Barrier Nanostructure Modulated by Spin-Orbit Couplings

We theoretically investigate how to modulate spin-dependent lateral shifts by the spin-orbit coupling (SOC) in a hybrid magnetic-electric-barrier (MEB) nanostructure, which can be experimentally realized by depositing a ferromagnetic (FM) stripe and a Schottky metal (SM) stripe on the top and bottom of the semiconductor heterostructure, respectively. Two kinds of ROCs, Rashba SOC (RSOC) and Dresselhaus SOC (DSOC), are taken into account fully. The Schrödinger equation of the spin electron in the hybrid MEB nanostructure is exactly solved by using the improved transfer-matrix method (ITMM), and the lateral shift and its spin polarization are numerically calculated with the help of the stationary phase method (SPM). Theoretical analysis indicates that the spin polarization effect in the lateral shift still exists in the hybrid MEB nanostructure when the SOCs are considered. Numerical simulations show that both magnitude and sign of the spin polarization effect in lateral shifts vary strongly with the strengths of RSOC and DSOC. These interesting features may offer an effective means to control the behavior of spin-polarized electrons in the semiconductor nanostructure, and such a hybrid MEB nanostructure serves as a SOC-manipulable spatial spin splitter for spintronic applications.     

Magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films grown on substrates with orthorhombically distorted unit cells

We have studied the structure and magnetoresistance of 40-nm-thick epitaxial La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented NdGaO₃ (NGO) substrates. The manganite layers were oriented so that the b axis was perpendicular to the substrate plane and occurred under the action of inhomogeneous biaxial mechanical stresses. The negative magnetoresistance of the LCMO films in the vicinity of the ferromagnetic spin ordering was about 71% (μ₀ H = 1 T). The observed azimuthal anisotropy of the magnetotransport properties of 40-nm-thick LCMO/(001)NGO films can be explained within the framework of a model of anisotropic magnetoresistance taking into account the presence of the preferred orientation of the spontaneous magnetization.