Large Spin Polarization Research Articles

Half-metallicity and spin-polarization transport properties in transition-metal atoms single-edge-terminated zigzag α-graphyne nanoribbons

By employing the non-equilibrium Green's function method and the spin-polarized density functional theory, we investigated the spin-resolved electronic transport properties of the zigzag α-graphyne nanoribbons (ZαGyNRs) passivated with 3d transition-metal atoms at one of the ribbon edge. Our results exhibit half-metallic behavior in response to the Fe-, Co- and Ni-doping at the edge of ZαGyNRs. The 3d transition-metal edge introduced in this paper can result in large spin polarization on currents of the ZαGyNRs devices, and some interesting phenomena such as half-metallicity, spin-polarization effect and negative differential resistance behavior can be observed. These theoretical results suggest a route to manipulate the spin-resolved electronic transport properties in ZαGyNRs-based spintronics devices.

Extraordinary waves in two dimensional electron gas with separate spin evolution and Coulomb exchange interaction

Hydrodynamics analysis of waves in two-dimensional degenerate electron gas with the account of separate spin evolution is presented. The transverse electric field is included along with the longitudinal electric field. The Coulomb exchange interaction is included in the analysis. In contrast with the three-dimensional plasma-like mediums the contribution of the transverse electric field is small. We show the decrease of frequency of both the extraordinary (Langmuir) wave and the spin-electron acoustic wave due to the exchange interaction. Moreover, spin-electron acoustic wave has negative dispersion at the relatively large spin-polarization. Corresponding dispersion dependencies are presented and analyzed.

Theory of Spin Torque Switching Current for the Double Magnetic Tunnel Junction

A single-domain model for spin-transfer torque switching of double magnetic tunnel junctions with perpendicularly magnetized layers is developed. An analytical formula is derived for the switching current threshold, I c0 . Surprisingly, the model predicts that the switching current can be reduced by a factor of 10 compared to single magnetic tunnel junctions, for sufficiently large spin-polarizations. This effect is explained in terms of the “enhanced antiparallel-to-parallel” spin torque, familiar from single magnetic tunnel junctions.

Electronic and magnetic properties of Re-doped single-layer MoS2: A DFT study

The properties of Re-doped single-layer MoS2 have been investigated by density functional theory (DFT). Four doping configurations are considered, and all of them show large spin polarization. We ascribe the spin polarization of these systems to the single occupation of split 5d orbitals under different crystal fields and the charge transfer from Re atom to single-layer MoS2. Furthermore, our results show that ferromagnetic ground states are favored in these doped systems when different spin orders are considered. These results would be helpful for future MoS2-based spintronic devices.

Magnetic properties of low-moment ferrimagnetic Heusler Cr2CoGa thin films grown by molecular beam epitaxy

Recently, theorists have predicted many materials with a low magnetic moment and large spin-polarization for spintronic applications. These compounds are predicted to form in the inverse Heusler structure; however, many of these compounds have been found to phase segregate. In this study, ordered Cr2CoGa thin films were synthesized without phase segregation using molecular beam epitaxy. The present as-grown films exhibit a low magnetic moment from antiferromagnetically coupled Cr and Co atoms as measured with superconducting quantum interface device magnetometry and soft X-ray magnetic circular dichroism. Electrical measurements demonstrated a thermally-activated semiconductor-like resistivity component with an activation energy of 87 meV. These results confirm spin gapless semiconducting behavior, which makes these thin films well positioned for future devices.

Magnetic MoS2 Interface Monolayer on a CdS Nanowire by Cation Exchange

MoS2 atomic layers have recently attracted much interest because of their two-dimensional structure as well as tunable optical, electrical, and mechanical properties for next generation electronic and electro-optical devices. Here we have achieved facile fabrication of MoS2 thin films on CdS nanowires by cation exchange in solution at room temperature and importantly observed their extraordinary magnetic properties. We establish the atomic structure of the MoS2/CdS heterostructure by taking atomic images of the MoS2/CdS interface as well as performing first-principles density functional geometry optimizations and STEM-ADF image simulations. Furthermore, our first principles density functional calculations for the MoS2/CdS heterostructure reveal that the magnetism in the MoS2/CdS heterostructure stems from the ferromagnetic MoS2 monolayer next to the MoS2/CdS interface. The ferromagnetism is attributed to the partial occupation of the Mo dx2-y2/dxy conduction band in the interfacial MoS2 monolayer caused by the mixed covalent-ionic bonding among the MoS2 and CdS monolayers near the MoS2/CdS interface. The present findings of the ferromagnetic MoS2 monolayer with large spin polarization at the MoS2-semiconductor interface suggest a new route for fabrication of the transition metal dichalcogenide-based magnetic semiconductor multilayers for applications in spintronic devices.

Direct coupling between charge current and spin polarization by extrinsic mechanisms in graphene

Spintronics---the all-electrical control of the electron spin for quantum or classical information storage and processing---is one of the most promising applications of the two-dimensional material graphene. Although pristine graphene has negligible spin-orbit coupling (SOC), both theory and experiment suggest that SOC in graphene can be substantially enhanced by extrinsic means, such as functionalization by adatom impurities. We present a theory of transport in graphene that accounts for the full spin-coherent dynamics of the carriers, including hitherto-neglected spin precession processes during resonant scattering with dilute impurities. We identify a novel "anisotropic spin precession scattering" process, specific to two dimensions and extrinsic SOC, which contributes to a large current-induced spin polarization in graphene. The theory also yields a comprehensive description of the spin relaxation mechanisms in impurity-decorated graphene: the Elliot-Yafet and Dyakonov-Perel relaxation rates are both present, and we find that the latter can dominate for some choices of carrier density and impurity strength. This provides theoretical foundations for designing future graphene-based integrated spintronic devices.

Compensated Ferrimagnetic Tetragonal Heusler Thin Films for Antiferromagnetic Spintronics.

Fully compensated ferrimagnets with tetragonal crystal structure have the potential for large spin-polarization and strong out-of-plane magnetic anisotropy; hence, they are ideal candidates for high-density-memory applications. Tetragonal Heusler thin films with compensated magnetic state are realized by substitution of Pt in Mn3-x Ptx Ga. Furthermore, the bilayer formed from compensated/uncompensated Mn-Pt-Ga layers is utilized to accomplish exchange bias up to room temperature.

Decoupling a hole spin qubit from the nuclearspins.

A huge effort is underway to develop semiconductor nanostructures as low-noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath. A hole spin may circumvent the nuclear spin noise. In principle, the nuclear spins can be switched off for a pure heavy-hole spin. In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark-state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is <1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to nanoelectronvolt energies; equivalently dephasing times up to a microsecond. The combination of large and strong optical dipole makes the single hole spin in a GaAs-based device an attractive quantum platform.

Interpreting current-induced spin polarization in topological insulator surface states

Several recent experiments on three-dimensional topological insulators claim to observe a large charge current-induced non-equilibrium ensemble spin polarization of electrons in the helical surface state. We present a comprehensive criticism of such claims, using both theory and experiment: First, we clarify the interpretation of quantities extracted from these measurements by deriving standard expressions from a Boltzmann transport equation approach in the relaxation-time approximation at zero and finite temperature to emphasize our assertion that, despite high in-plane spin projection, obtainable current-induced ensemble spin polarization is minuscule. Second, we use a simple experiment to demonstrate that magnetic field-dependent open-circuit voltage hysteresis (identical to those attributed to current-induced spin polarization in topological insulator surface states) can be generated in analogous devices where current is driven through thin films of a topologically-trivial metal. This result *ipso facto* discredits the naive interpretation of previous experiments with TIs, which were used to claim observation of helicity, i.e. spin-momentum locking in the topologically-protected surface state.

High spin polarization and the origin of unique ferromagnetic ground state in CuFeSb

CuFeSb is isostructural to the ferro-pnictide and chalcogenide superconductors and it is one of the few materials in the family that are known to stabilize in a ferromagnetic ground state. Majority of the members of this family are either superconductors or antiferromagnets. Therefore, CuFeSb may be used as an ideal source of spin polarized current in spin-transport devices involving pnictide and the chalcogenide superconductors. However, for that the Fermi surface of CuFeSb needs to be sufficiently spin polarized. In this paper we report direct measurement of transport spin polarization in CuFeSb by spin-resolved Andreev reflection spectroscopy. From a number of measurements using multiple superconducting tips we found that the intrinsic transport spin polarization in CuFeSb is high (∼47%). In order to understand the unique ground state of CuFeSb and the origin of large spin polarization at the Fermi level, we have evaluated the spin-polarized band structure of CuFeSb through first principles calculations. Apart from supporting the observed 47% transport spin polarization, such calculations also indicate that the Sb-Fe-Sb angles and the height of Sb from the Fe plane are strikingly different for CuFeSb than the equivalent parameters in other members of the same family thereby explaining the origin of the unique ground state of CuFeSb.

Effect of Annealing Temperature on the Microstructure and Magnetic Properties of MnGa Films

MnGa films are the promising magnetic recording materials and spintronic materials owing to their intrinsic properties, such as large magnetic anisotropy, high coercivity, moderate magnetization, and high spin polarization. In this paper, MnGa films with high coercivity and low surface roughness have been successfully fabricated onto MgO substrates by magnetron sputtering and post-annealing. Moreover, the effects of post-annealing temperature (T a) on crystalline structure, surface morphology, and magnetic performances of MnGa films have also been investigated. It is found that the crystallization temperature for MnGa films is 400 ∘C. With increasing T a, the crystallization degree enhances and an in-plane texture is formed. The grain size and surface roughness of MnGa films increase slowly when T a is below 500 ∘C, but they exhibit a rapid rise when T a is above 500 ∘C. As T a increases, the coercivity (H c) and remanence squareness ratio (S) for MnGa films improve monotonically, whereas saturation magnetization (M s) increases firstly and then drops. The increases in H c, S, and M s with T a are attributed to the grains’ growth and the improvement of crystallinity, and the decrease of M s at higher T a possibly is due to the partial oxidation of Mn.

Symmetry-selected spin-split hybrid states inC60/ferromagneticinterfaces

The understanding of orbital hybridization and spin-polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of C$_{60}$ deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001) and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play an intricate role in the spin-polarization of the hybrid orbitals. We find that a large spin-polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore bcc-Cr(001), bcc-Fe(001) and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single C$_{60}$ adsorbed on Cr(001) and Co/Pt(111) also confirm that both the symmetry of the substrate and of the molecular conformation have a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.

Spin response to localized pumps: Exciton polaritons versus electrons and holes

Polariton polarization can be described in terms of a pseudospin which can be oriented along the $x,\,y,$ or $z$ axis, similarly to electron and hole spin. Unlike electrons and holes where time-reversal symmetry requires that the spin-orbit interaction be odd in the momentum, the analogue of the spin-orbit interaction for polaritons, the so-called TE-TM splitting, is even in the momentum. We calculate and compare spin transport of polariton, electron, and hole systems, in the diffusive regime of many scatterings. After dimensional rescaling diffusive systems with spatially uniform particle densities have identical dynamics, regardless of the particle type. Differences between the three particles appear in spatially non-uniform systems, with pumps at a specific localized point. We consider both oscillating pumps and transient (delta-function) pumps. In such systems each particle type produces distinctive spin patterns. The particles can be distinguished by their differing spatial multipole character, their response and resonances in a perpendicular magnetic field, and their relative magnitude which is largest for electrons and weakest for holes. These patterns are manifested both in response to unpolarized pumps which produce in-plane and perpendicular spin signals, and to polarized pumps where the spin precesses from in-plane to out-of-plane and vice versa. These results will be useful for designing systems with large spin polarization signals, for identifying the dominant spin-orbit interaction and measuring subdominant terms in experimental devices, and for measuring the scattering time and the spin-orbit coupling's magnitude.

Inversion of Spin Signal and Spin Filtering in Ferromagnet|Hexagonal Boron Nitride-Graphene van der Waals Heterostructures.

Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.

Spin-filter and negative differential resistance effect in zigzag-edged bilayer graphene nanoribbon devices

By performing first-principle quantum transport calculation, the spin-dependent transport properties of zigzag-edged bilayer graphene nanoribbon based devices are investigated. There are four kinds of structures with different stacking sequences and treatment of dangling bonds considered in our work. It is shown that the devices are perfect spin-filters with extremely large spin polarization as well as substantial negative differential resistance effects, which are affected by the stacking sequences and edge structures. All these phenomena can be explained by the spin-resolved local density of states and the tranmission spectra.

First-principles study on the magnetism and electronic structure in 3d transition metal (X=Sc, V, Cr, Mn, Fe, Ni, Cu) doped CoO

We have studied the electronic structure and magnetism of the single transitional metal element X=Sc, V, Cr, Mn, Fe, Ni, Cu-doped CoO systems by first-principles calculations. At X=Sc, Cr, Cu, the binding energy of the doped systems is lower than pure CoO, suggesting that these systems are energetically stable. In the Sc, V, Cr, Mn, Fe, Ni, Cu-doped 2×2×2 CoO supercells, the total magnetic moments are 3.03, 5.64, 6.80, 7.70, 6.93, 2.30 and 1.96μB, respectively. At X=Cr and Fe, the doped CoO systems are half-metallic with a high spin polarization. The large magnetic moment and high spin polarization in the Cr and Fe-doped CoO are important for the design of the spintronic devices.

Partial spin polarization of a conductance in a bi-layer [formula omitted] heterostructure based nanowire for the rectangular and the smooth lateral confinement potentials

We simulate the electron transport in a vertical bi-layer nanowire in order to study an influence of the lateral confinement's shape on a spin polarization of wire's conductance. The active part of considered quantum wire constitutes a double inverted heterojunction In0.52Al0.48As/In0.53Ga0.47As which nanostructure can be fabricated in molecular beam epitaxy process while the lateral confinement potential can be finally formed by means of cleaved overgrowth or surface oxidization methods giving the desired rectangular and smooth lateral confinement. In calculations we take into account interaction between charge carriers using DFT within local spin density approximation. We show that if the magnetic field is perpendicular to the wire axis, the pseudogaps are opened in energy dispersion relation E(k) what in conjunction with spin Zeeman shift of spin-up and spin-down subbands may enhance the spin polarization of conductance with reference to a single layer wire. For nanowire with rectangular lateral confinement potential we found that the electron density has two maximums localized at wire edges in each layers. This modificates strongly all magnetosubbands giving up to four energy minimums in lowest subband and considerably diminishes widths of pseudogaps what translates into low maximal spin polarization of conductance, not exceeding 40%. This drawback is absent in wire with smooth lateral confinement. However, in order to gain a large spin polarization simultaneous tuning of magnetic field as well as the Fermi energies in both layers of nanowire are required.

ChemInform Abstract: Tetragonal Heusler‐Like Mn‐Ga Alloys Based Perpendicular Magnetic Tunnel Junctions

AbstractReview: 105 refs.

Oscillatory Tunneling Magnetoresistance in Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;/n-GaAs/Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Junction

Spintronics strives to revolutionize conventional electronics by integrating magnetic materials with semiconductor devices, such as the spin field effect transistor (SFET)[1], which not only improve the capabilities of electronic devices, but develop new functionalities. For electrodes of spin injection and detection in SFET device, half metallic Fe 3 O 4 is an attractive candidate because its high Curie temperature of 858 K, large spin polarization near 100% at the Fermi level and relatively high electronic conductivity at room temperature, which is believed to benefit the injection of spin carriers into the semiconductors.[2] For ferromagnetic metal(FM)/ Semiconductor system, Fe 3 O 4 / GaAs is a very promising system for the fabrication of magnetoelectronic devices due to the Schottky contact of the Fe 3 O 4 /GaAs interface[3,4], which is crucial for studying the behaviors of spin dependent transportation for the devices. It is reported that in a FM/I/NM/I/FM double tunnel junction, where NM is the normal metal, and I the insulating barrier, theories predicted an oscillation of the tunneling magnetoresistance (TMR) effect as a function of the NM layer thickness because the spin polarization of the tunneling electron oscillates as a result of the resonant tunneling. Furthermore, Quantum oscillation of spin polarization in GaAs channel was experimentally demonstrated. In this paper, we have presented a theoretical approach to the tunneling conductance and TMR in a Fe 3 O 4 /GaAs/Fe 3 O 4 magnetic double tunnel junction with both ballistic and diffusive components.