Papers by Keyword: Spin Polarization

Abstract: In this work, theoretical investigation in coherent manipulation throughout local density of states calculation for serially coupled double quantum dots embedded between ferromagnetic leads (FM-QD1-QD2-FM) by using the non-equilibrium Green's function approach. Since the local density of states are formulated incorporating the spin polarization and the type of spin configuration on the leads. Our model incorporates the inter-dot hopping, the intra-dot Coulomb correlation, the spin exchange energy and the coupling interactions between the quantum dots and leads. The results concerned to the parallel configuration at strong inter-dot coupling regime shows that the spin down electrons in the quantum dots may be more coupled coherently if the regime is tuned. The local density of states of the two dots for spin up electrons shows a broad hump with small splitting i.e. the case is decoherent for spin up electrons. In the case of weak interdot coupling it is obvious that the spin dependent density of states on the quantum dots show that the resonances are not well splitted. For the antiparallel configuration in the strong coupling regime, the spin dependent density of states of the double quantum dots show four peaks but with broaden and overlapping. In the case of weak coupling regime, the total spin dependent density of states, which have two peaks with certain board, one can conclude that the states are not coupled coherently. The case of the antiferromagnetic nature of the spin exchange interaction, our calculations for the parallel and antiparallel configurations (in strong and weak regimes) show a decoherence state.

Abstract: The spin injection efficiency in the ferromagnet/ organic semiconductors system (FM/OSE) was studied under an external electric-field. It is found that the spin injection efficiency can be strongly influenced by the spin-dependent electrical conductivity and the downstream spin diffusion length of polarons. With the increase of external electric-field, the downstream spin diffusion length increases and makes the spin-dependent electrical conductivity increase, too. So the spin injection efficiency is enhanced. When the external electric-field increases from 1 to 10 mV/μm at T=80K, the spin injection efficiency increases about 20%. It seems that the downstream spin diffusion length is an significant factor to affect the spin injection efficiency in the FM/ OSE under an external electric-field.

Abstract: The results of "ab-initio" calculations of spin - polarized electronic structure, total energy and the local atomic magnetic moments of some manganite surfaces are presented. A slab consisting of one, three, five or, in some cases, seven layers of Mn-O atoms. is used to model the CaMnO₃ (CMO), LaMnO₃ (LMO) (001) surface. Total energies calculations of magnetic properties of manganites surfaces were performed using density-functional theory (DFT) and the pseudopotential method. We have found that on the surface layers without structural optimization manganese atomic magnetic moment is higher than in the bulk and “dead layers”, where all the local moments are zero, have not been found. All the above ultrathin films appeared ferromagnetic semimetals with almost complete polarization of DOS near the Fermi level.

Abstract: A novel doping scheme for graphene was recently realized experimentally by creating different vacancy complexes doped with a transition metal (TM) atom [nanoLett. 12, 141 (2012)]. This provides a new reliable way to modifying the electronic structure and transport property of graphene. Here, we show, by performing first-principles calculations, that the defect complex of TM@V₄ (a TM atom doped tetra-vacancy) in zigzag graphene nanoribbons (ZGNRs) can lead to a 100% spin-polarized electron transport in a wide energy range around the Fermi energy. Analyses show that this is due to the particular atomic structure of the TM@V₄ complex regardless of the species of the TM atom.

Abstract: Ti-based Heusler compounds are rare reported widely up to now. In this paper, ternary full-Heusler compound Ti₂NiIn is systematically studied by first-principles calculations. The calculations show the compound has a complete spin polarization around the Fermi level in the total density of state. The majority spin shows strongly metallic character, while the minority spin has an insulating behavior in its band structure. The compound has a precisely total magnetic moment of-3.0μ_B per formula by first-principles calculations. This result complies well with the SlaterPauling (SP) rule. All the results show that Ti₂NiIn with Hg₂CuTi-type structure is a new half-meltal ferromagnet by first-principles calculations.

Abstract: Spin injection efficiency in the ferromagnet/ organic semiconductors system (FM/OSEs) was explored considering the spin dependence of the electric-conductivity induced by spin injection in the OSEs. It is known that the OSEs is spin polarized, once spin was injected from FM layer to OSEs layer. The up-spin polarons and the down-spin polarons have different density. The spin dependence of the electric-conductivity is so induced. In the literature, it was usually supposed that the electric-conductivity in the spin polarized OSEs is spin independent. So, it is crucial to reflect the physics in the spin injection. Our work shows that the spin-dependent electrical-conductivity is one of the significant factors which affect the spin injection efficiency. The spin injection efficiency increases obviously with the rising of the spin-dependent electrical-conductivity in the same spin injection system. And the effect becomes larger, when the polaron proportion increases. Furthermore, the effects of interfacial electrochemical-potential proportion on the spin injection efficiency in the heterojunction are also included.

Abstract: I theoretically investigate the spin Polarization and transmission of the electrons in a nanostructure consisting barriers with periodic, parallel and also anti parallel magnetization .also I investigate polarization when distance between barriers is constant, or is increased, or is decreased periodically. These observable quantities are found to be strongly affected by both the magnetic configuration and the number of the periodic magnetic barriers. When the number of periods increases, in parallel magnetization for periodic increasing distance the polarization is enhanced so in parallel configuration it is better that distance between barriers to be increasing periodically. I investigate Polarization in these configuration in both delta function approximation and modulated magnetic barriers in x direction. This Polarization can be used in spintronics device.

Abstract: The spin-polarized transport is investigated in a magnetic tunnel junction which consists of two ferromagnetic electrodes separated by a magnetic barrier and a nonmagnetic metallic spacer placed in distance above the two dimensional electron gas (2DEG) in presence of an inhomogeneous external modulated magnetic field and a perpendicular wave vector dependent effective potential. Based on the transfer matrix method and the nearly-free-electron approximation the dependence of the conductance and spin polarization on the Fermi energy of the electrons are studied theoretically the. strong oscillations with large amplitude investigated in spin polarization in terms of the Fermi energy due to the inhomogeneous magnetic field. The conductance in terms of the Fermi energy shows no oscillation in low energy but has a strong pick in middle region. this results may be useful for the development of spin electronic devices based on coherent transport, or may be used as a tunable spin-filter.

Abstract: The electronic structure and magnetic properties of Ni/Al2O3/Ni tunnel junctions with O-terminated and Al-terminated interface models are investigated by first-principles discrete variational method with the local-spin-density approximation. The results show that the interface atomic has an important effect on interface electronic state and spin polarization as well as TMR ratio. For the O-terminated and Al-terminated interface models, spin polarization at Fermi level of Ni layers exhibit negative. It is found that absolute value of spin polarization as well as TMR ratio of Al-terminated interface models is much larger than that of O-terminated interface, which shows over-oxidization of Al layer could deteriorate magnetoresistance properties.

Abstract: A series models of Ni3Fe/Al2O3/Ni3Fe magnetic tunnel junction with Al-terminated interfaces have been established for investigating the influence of ferromagnetic layer thickness on the electronic structure, employing first-principle methods based on local spin-density approximation theory. The spin polarization of the interfacial Ni3Fe monolayer shows a maximum value as the thickness of ferromagnetic layer increases. The Al monolayers at the ferromagnetic/insulating interface and the O monolayer in the interior of insulating layer are also studied in terms of the change of spin polarization with the ferromagnetic layer thickness. In addition, we have found that the structure of Ni3Fe monolayer has a great influence on the spin polarization.