Experimentally, the intermetallic compound Mn$_{4}$FeGe$_{3}$ has been recently shown to exhibit enhanced magnetic properties and spin polarization compared to the Mn$_{5}$Ge$_{3}$ parent compound. The present {\em ab-initio} study focusses on the effect of Fe substitution on the electronic and magnetic properties of the compound. Our calculations reveal that the changes on the Fermi surface of the doped compound are remarkable and provide explanations for the enhanced spin-polarization observed. Finally, we show that it is indeed possible to tune the degree of spin-polarization upon Fe doping, thus making the Mn$_{1-x}$Fe$_{x}$Ge$_{3}$ intermetallic alloy very promising for future spintronic applications.
We report a systematic study of p-type polarization induced doping in graded AlGaN nanowire light emitting diodes grown on silicon wafers by plasma-assisted molecular beam epitaxy. The composition gradient in the p-type base is varied in a set of samples from 0.7 %Al/nm to 4.95 %Al/nm corresponding to negative bound polarization charge densities of 2.2x10^18 cm^-3 to 1.6x10^19 cm^-3. Capacitance measurements and energy band modeling reveal that for gradients greater than or equal to 1.30 %Al/nm, the deep donor concentration is negligible and free hole concentrations roughly equal to the bound …
The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to finely tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was foun…
Different ways to define and to calculate the degree of spin polarization in a ferromagnet are discussed, particularly with respect to spin-polarized tunneling and Andreev reflection at the boundary between a superconductor and ferromagnet. As an example, the degrees of spin polarization for different experiments in Fe and Ni are calculated in the framework of the local spin density approximation (LSDA) and used to illustrate the differences between various definitions of spin polarization.
First-principles calculations were carried out to investigate interfacial strain effects on spin injection and spin polarization of a magnetic tunnel junction consisting of half-metallic full-Heusler alloy Co2CrAl and ferroelectric perovskite NaNbO3. Spin-dependent coherent tunneling was calculated within the framework of non-equilibrium Green's function technique. Both spin polarization and tunnel magnetoresistance (TMR) are affected by the interfacial strain but their responses to compressive and tensile strains are different. Spin polarization across the interface is fully preserved under a…
Electrical tunability of spin polarization has been a focus in spintronics. Here, we report that the trigonal warping (TW) effect, together with spin-orbit coupling (SOC), can lead to two distinct magnetoelectric effects in low-dimensional systems. Taking graphene with Rashba SOC as example, we study the electronic properties and spin-resolved scattering of system. It is found that the TW effect gives rise to a terraced spin texture in low-energy bands and can render significant spin polarization in the scattering, both resulting in an efficient electric control of spin polarization. Our work …
The generally accepted view that spin polarization is induced by the asymmetry of the global crystal space group has limited the search for spintronics [1] materials to non-centrosymmetric materials. Recently it has been suggested that spin polarization originates fundamentally from local atomic site asymmetries [2], and therefore centrosymmetric materials may exhibit previously overlooked spin polarizations. Here by using spin- and angle-resolved photoemission spectroscopy (spin-ARPES), we report helical spin texture induced by local Rashba effect (R-2) in centrosymmetric monolayer PtSe$_2$ f…
We have used spin-polarized scanning tunneling spectroscopy to observe the spin-polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. Both of these magnetic adatoms exhibit stationary out-of-plane spin-polarization due to their direct exchange interaction with the substrate, but the sign of the exchange coupling between electron states of the adatom and the surface state of the Co island is opposite for the two: Fe adatoms exhibit parallel spin-polarization to the Co surface state while Cr adatoms exhibit antiparallel spin-polarization. First-principles calculations pr…
The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezore…
Using a combined approach of spin-resolved photoemission spectroscopy, band structure and photoemission calculations we investigate the influence of bulk defects and surface states on the spin polarization of Co2Mn(alpha)Si thin films with bulk L21 order. We find that for Mn-poor alloys the spin polarization at EF is negative due to the presence of Co_Mn antisite and minority surface state contributions. In Mn-rich alloys, the suppression of Co(Mn) antisites leads to a positive spin polarization at the Fermi energy, and the influence of minority surface states on the photoelectron spin polariz…
We present measurements of the transport spin polarization of Ni_xFe_{1-x} (0<x<1) using the recently-developed Point Contact Andreev Reflection technique, and compare them with our first principles calculations of the spin polarization for this system. Surpisingly, the measured spin polarization is almost composition-independent. The results clearly demonstrate that the sign of the transport spin polarization does not coincide with that of the difference of the densities of states at the Fermi level. Calculations indicate that the independence of the spin polarization of the composition is du…
In this work, we discuss the connections between pseudo spin, real spin of electrons in material and spin polarization of photo-emitted electrons out of material. By investigating these three spin textures for Bi$_2$Se$_3$ and SmB$_6$ compounds, we find that the spin orientation of photo-electrons for SmB$_6$ has different correspondence to pseudo spin and real spin compare to Bi$_2$Se$_3$, due to the different symmetry properties of the photo-emission matrix between initial and final states. We calculate the spin polarization and circular dichroism spectra of photo-emitted electrons for both …
Similar to light polarization that is selected by a superposition of optical basis, electron spin direction can be controlled through a superposition of spin basis. We investigate such a spin interference occurring in photoemission of the spin-orbit coupled surface state in Bi2Se3 by using spin- and angle-resolved photoemission spectroscopy combined with laser light source (laser-SARPES). Our laser-SARPES with three-dimensional spin detection and tunable laser polarization including elliptical and circular polarization enables us to directly visualize how the direction of the fully-polarized p…
The changes of polarizational, elastic and structural properties of mutiferroic BiFeO3 under [111] direction uniaxial stress are calculated using density functional theory within the Perdew-Burke- Ernzerhof revised for solids (PBEsol) + U approach, and compared with available measurements or predictions. A large ferroelectric polarization of 87.5 ?C=cm2 is found, agreeing with other theoretical and experimental values, and the polarization changes near-linearly within the uniaxial stress range of 8 GPa to -8 GPa. This property can be used to indirectly control the radiative recombination of lu…
Spin-orbit coupling (SOC) can induce spin polarization in nonmagnetic 3D crystals when the inversion symmetry is broken, as manifested by the bulk Rashba (R-1) and Dresselhaus (D-1) effects. We determine that these spin polarization effects originate fundamentally from specific atomic site asymmetries, rather than from the generally accepted asymmetry of the crystal space-group. This understanding leads to the recognition that a previously overlooked hidden form of spin polarization should exist in centrosymmetric materials. Although all energy bands must be doubly degenerate in centrosymmetri…
We investigate exciton spin memory in individual InAs/GaAs self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. The conservation of the linear polarization generated along the bright exciton eigenaxes of up to 90% and the conversion from circular- to linear polarization of up to 47% both demonstrate a very long spin relaxation time with respect to the radiative lifetime. Results are quantitatively compared with a…
We report a first-principle study on electronic structure and simulation of the spin-polarized scanning tunneling microscopy graphic of a benzene/Fe4N interface. Fe4N is a compound ferromagnet suitable for many spintronic applications. We found that, depending on the particular termination schemes and interface configurations, the spin polarization on the benzene surface shows a rich variety of properties ranging from cosine-type oscillation to polarization inversion. Spin-polarization inversion above benzene is resulting from the hybridizations between C pz and the out-of-plane d orbitals of …
By means of first principles calculations we demonstrate an effective method to tailor the local spin configuration of graphene on Co(0001) surface through nitrogen doping. Two different site occupancies of the N impurities are discussed with the focus on structural, electronic and magnetic properties. N induces opposite local spin polarization at the two sites through Zener exchange-type hybridization with Co substrate. In addition, the induced spin polarization is energy dependent and controllable by electric field. Consequently this structure can be applied as a spin injection source in gra…
The spin polarization of electrons trapped in InAs self-assembled quantum dot ensembles is investigated. A statistical approach for the population of the spin levels allows one to infer the spin polarization from the measure values of the addition energies. From the magneto-capacitance spectroscopy data, the authors found a fully polarized ensemble of electronic spins above 10 T when $\mathbf{B}\parallel[001]$ and at 2.8 K. Finally, by including the g-tensor anisotropy the angular dependence of spin polarization with the magnetic field $\mathbf{B}$ orientation and strength could be determined.
Recent experimental and theoretical work has shown that the double perovskite NaLaMnWO$_6$ exhibits antiferromagnetic ordering owing to the Mn $d$ states, and computational studies further predict it to exhibit a spontaneous electric polarization due to an improper mechanism for ferroelectricity [King \textit{et al., Phys.\ Rev.\ B}, 2009, \textbf{79}, 224428; Fukushima \textit{et al., Phys.\ Chem.\ Chem.\ Phys.}, 2011, \textbf{13}, 12186], which make it a candidate multiferroic material. Using first-principles density functional calculations, we investigate nine isostructural and isovalent $A…
We present measurements of pure spin current absorption on lateral spin valves. By varying the width of the absorber we demonstrate that spin current absorption measurements enable to characterize efficiently the spin transport properties of ferromagnetic elements. The analytical model used to describe the measurement takes into account the polarization of the absorber. The analysis of the measurements allows thus determining the polarization and the spin diffusion length of a studied material independently, contrarily to most experiments based on lateral spin valves where those values are ent…
We use micro-Raman and photoluminescence (PL) spectroscopy at 300K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate. The initially degenerate E^1_{2g} Raman mode is split into a doublet as a direct consequence of the strain applied to MoS2 through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48meV/(% of strain) for the monolayer and 46meV/% for the bilayer, whose indirect gap shifts by 86meV/%. We find a strong decrease o…
Chirality-dependent spin polarization in metals: linear and quadratic responses
The University of Tokyo（东京大学）；Institute for Molecular Science（分子科学研究所）；RIKEN Center for Emergent Matter Science（理化学研究所新兴物质科学中心）；National Institutes of Natural Sciences（国立自然科学研究院）；Institute of Science Tokyo（东京科学大学）；Osaka Metropolitan University（大阪公立大学）
Properties of a Nuclear-Spin-Polarization-Induced Quantum Ring (NSPI QR) are studied theoretically. In the proposed system a local nuclear spin polarization creates an effective hyperfine field which confines the electrons with the spins opposite to the hyperfine field to the regions of maximal nuclear spin polarization. We investigate the electron energy spectrum and the magnetic response of NSPI QR and their evolution in time due to the nuclear spin diffusion and relaxation.
Polarization is a key ingredient of all waves, including the electromagnetic wave, the acoustic wave, as well as the spin wave. Due to the fixed ferromagnetic order, the spin wave in ferromagnet is limited to the right circular polarization. The spin wave in antiferromagnet, however, is endowed with the full polarization degree of freedom because of the two identical magnetic sublattices. In the synthetic antiferromagnet, the two magnetic sublattices are spatially separated into two sublayers. The circular polarization of spin wave is partially locked to the magnetic sublattice of the antiferr…
A new method to measure the spin polarization of a ferromagnetic nanomagnet is proposed and experimentally demonstrated. A high precision, repeatability and simplicity are the features of the proposed method. The dependency of the spin polarization on the gate voltage and the bias current were observed.
Tuning exciton emission via ferroelectric polarization at a heterogeneous interface between a monolayer transition metal dichalcogenide and a perovskite oxide membrane
Electrically-induced electron spin polarization is imaged in n-type ZnSe epilayers using Kerr rotation spectroscopy. Despite no evidence for an electrically-induced internal magnetic field, current-induced in-plane spin polarization is observed with characteristic spin lifetimes that decrease with doping density. The spin Hall effect is also observed, indicated by an electrically-induced out-of-plane spin polarization with opposite sign for spins accumulating on opposite edges of the sample. The spin Hall conductivity is estimated as 3 +/- 1.5 Ohms**-1 m**-1/|e| at 20 K, which is consistent wi…
The generation of non-equilibrium electron spin polarization, spin transport, and spin detection are fundamental in many quantum devices. We demonstrate that a lattice of magnetic nanodots enhances the electron spin polarization in monolayer graphene via carrier exchange. We probed the spin polarization through a resistively-detected variant of electron spin resonance (ESR) and observed resonance amplification mediated by the presence of the nanodots. Each nanodot locally injects a surplus of spin-polarized carriers into the graphene, and the ensemble of all "spin hot spots" generates a non-eq…
The spin-driven polarizations of type-I multiferroics are veiled by the preexisting ferroelectric (FE) polarization. Using first-principles calculations combined with a spin model, we uncover two hidden but huge spin-driven polarizations in the room-temperature multiferroic BiFeO3. One is associated with the global inversion symmetry broken by a FE distortion and the other is associated with the local inversion symmetry broken by an antiferrodistortive octahedra rotation. Comparison with recent neutron scatterings reveals that first polarization reaches 3.0 $μ$C/cm$^2$, which is larger than in…
Chirality-induced spin selectivity (CISS) refers to the fact that electrons get spin polarized after passing through organic chiral molecules in a nanoscale device. In CISS, chiral molecules are commonly believed to be a spin filter through which one favored spin transmits and the opposite spin gets reflected, i.e., transmitted and reflected electrons exhibit opposite spin polarization. In this work, we point out that such a spin filter scenario contradicts the principle that equilibrium spin current must vanish. Instead, we find that both transmitted and reflected electrons present the same t…
The centrosymmetric materials with hidden spin polarization are considered to be the promising candidates for realization of energy efficient spintronics systems and devices. However, the control of hidden spin polarization and resulting transport behavior is not well understood. We hypothesized that inhomogeneous strain can be the external knob to study and control hidden spin polarization. In this work, we demonstrate a strain gradient mediated symmetry breaking to discover the hidden spin polarization in centrosymmetric Si lattice. The hidden spin polarization gives rise to magnetocrystalli…
We present a short critical overview of different microscopic models for nonrelativistic and relativistic magnetoelectric coupling including the so-called "spin current scenario", ab-initio calculations, and several recent microscopic approaches to a spin-dependent electric polarization in 3d oxides.
Electrically generated spin polarization is probed directly in bulk GaN using Kerr rotation spectroscopy. A series of n-type GaN epilayers are grown in the wurtzite phase both by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) with a variety of doping densities chosen to broadly modulate the transverse spin lifetime, T2*. The spin polarization is characterized as a function of electrical excitation energy over a range of temperatures. Despite weak spin-orbit interactions in GaN, a current-induced spin polarization (CISP) is observed in the material at temperatur…
We perform ab-initio calculations to investigate the spin polarization at the interface between a thiophene molecule and cobalt substrate. We find that the reduced symmetry in the presence of a sulfur atom (in the thiophene molecule) leads to a strong spatial dependence of the spin polarization of the molecule: The two carbon atoms far from the sulfur acquire a polarization opposite to that of the substrate, while the carbon atoms bonded directly to sulfur possess the same polarization as the substrate. We determine the origin of this peculiar spin interface property as well as its impact on t…
We consider a disordered two-dimensional electron gas with spin-orbit coupling placed in a perpendicular magnetic field and calculate the magnitude and direction of the electric-field-induced spin polarization. We find that in strong magnetic fields the polarization becomes an oscillatory function of the magnetic field and that the amplitude of these oscillations is parametrically larger than the polarization at zero magnetic field. We show that the enhanced amplitude of the polarization is a consequence of strong electron-hole asymmetry in a quantizing magnetic field.
Decrease of spin polarization in spintronics devices under an application of bias voltage is one of currently important problems which should be solved. We reveal unprecedented robustness of spin polarization in multi-layer graphene spin valves at room temperature. Surprisingly, the spin polarization of injected spins is constant up to a bias voltage of +2.7 V and -0.6 V in positive and negative bias voltage applications, which is dramatically superior to all spintronics devices. Our finding is induced by suppression of spin scattering due to an ideal interface formation. Furthermore, we first…
The charge-current-induced spin polarization is a key property of topological insulators for their applications in spintronics. However, topological surface states are expected to give rise to only one type of spin polarization for a given current direction, which has been a limiting factor for spin manipulations. Here we report that in devices based on the bulk-insulating topological insulator BiSbTeSe2, an unexpected switching of spin polarization was observed upon changing the chemical potential. The spin polarization expected from the topological surface states was detected in a heavily el…
We show that electric field can induce spin polarization and dichroism effects in angle resolved photoemission spectroscopy (ARPES) in spin orbit coupling systems. The physical origin behind the effects essentially is the same as the spin Hall effect induced by the electric field. Since the ARPES experiments have both energy and momentum resolutions, the spin Hall effect can be directly verified by the ARPES experiments for individual band even if there is no net spin current.
We have studied the spin dynamics of a high-mobility two-dimensional electron system in a GaAs/Al_{0.3}Ga_{0.7}As single quantum well by time-resolved Faraday rotation and time-resolved Kerr rotation in dependence on the initial degree of spin polarization, P, of the electrons. By increasing the initial spin polarization from the low-P regime to a significant P of several percent, we find that the spin dephasing time, $T_2^\ast$, increases from about 20 ps to 200 ps; Moreover, $T_2^\ast$ increases with temperature at small spin polarization but decreases with temperature at large spin polariza…
The spin polarization of ferromagnetic alkali-metal iron antimonides KFe4Sb12 and NaFe4Sb12 is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization Pt of 67% and 60% for the K and Na compound, respectively, is inferred which establishes these materials as a new class of highly spin polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation (LSDA) that predict nearly 100% spin polarization in the density of states. We discuss the impac…
