Spin-dependent Transport Behavior Research Articles

Ferromagnetic exchange field-controlled band dispersions of non-Dirac electrons

Using model analysis and first-principles calculations, we demonstrate that intrinsic ferromagnetic field of materials can effectively modulate the non-Dirac band dispersions. The four-bands k·p model illustrates that rotating magnetization from in-plane to out-of-plane lifts the degeneracy of band dispersions at Γ point, and spin components of lower or upper two bands tend to become identical as the enhancement of exchange field, which results in non-trivial topology. Moreover, we exemplify these phenomena in stanene-based systems, namely stanene/hematene heterostructure and half-hydrogen-passivated stanene, respectively. Some interesting spin-dependent transport behavior, such as large magnetoresistance and quantum anomalous hall effect, are achieved. These findings enrich the physics of non-Dirac electrons and provide promising routes for realizing effective manipulation of band dispersions via spin freedom.

Electronic and transport properties of Heusler alloy based magnetic tunneling junctions: A first principles study

In this work, employing density functional theory based electronic structure calculations, we search for an alternative to MgO as a spacer layer in a magnetic tunneling junction(MTJ), with half-metallic(HM) Co2MnSb as an electrode. First, we demonstrate the possibility of designing an all-Heusler alloy based MTJ with semi-conducting(SC) TiCoSb alloy as the spacer material. We probe the robustness of the HM properties of the Mn–Sb/Co interface and show that the HM property is preserved, even with various disorders and defects. The spin-dependent transport behavior indicates that these properties depend sensitively on the heterojunction interfaces and thickness of the spacer material. Further, we study the transport properties of the heterojunctions of Co2MnSb alloy with the well-studied insulator MgO as well as the less-explored systems like, NaCl and AlN. In these three insulating materials, the smallest complex band decay coefficient is associated with Δ1 symmetry, unlike TiCoSb. This feature enables more desirable symmetry-based spin-filtering properties at the Γ point in the 2D Brillouin zone. We further calculate the resistance area (RA) product for all the heterojunctions, important for the realization of highly sensitive magnetic sensors and it is found that the other spacer layers yield a RA product, several orders less in magnitude compared to that of TiCoSb. Our results indicate that NaCl and AlN may be promising as an alternative to MgO as a spacer material. With the chemical compatibilities resulting into minimal interface buckling and an ultra-low RA product, TiCoSb may also be a promising new material for a MTJ with Co2MnSb as electrode, specifically in relation to overcoming the variability and current-injection challenges.

Trans-polyacetylene based organic spin valve for a multifunctional spin-based device: A first principle analysis

Carbon-based materials are the promising candidates for the design of multifunctional spintronic devices, owing to their fascinating inherent attributes such as the ballistic transport nature, long spin coherence time, etc. Here, we report the spin-dependent electron transport properties of a trans-polyacetylene channel sandwiched between two semi-infinite hydrogen-passivated zigzag graphene nanoribbon (ZGNR) electrodes within the framework of spin-polarized density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalisms. The proposed device exhibits an excellent spin-dependent transport behavior within the bias window of [-0.5 V, 0.5 V]. In parallel configuration (PC), the device offers an ideal spin filtration efficiency of almost 100% within the bias range of [-0.4 V, 0.4 V], and in antiparallel configuration (APC), it exhibits negative differential resistance (NDR), dual-spin rectification, and dual-spin filtration effects. Also, a high tunnel magnetoresistance (TMR) of 1.2× 10 5 % is achieved. To validate the aforementioned effects, the transmission spectra along with the area enclosed by the curve, eigenvalues, eigenstates, and quantum conductance have also been analyzed. In addition, the impact of hybridization and torsion defects on the spin-dependent transport phenomenon through trans-polyacetylene has been investigated. Furthermore, the modelled device is observed to exhibit the spin Seebeck effect in PC under the influence of temperature gradient. The proposed device may stand as a good contender for various spintronic and spin caloritronic applications. • First principle based analysis of a trans-polyacetylene based multifunctional spin-based device. • The device displays a perfect spin filtration efficiency and dual rectification effects. • A high tunnel magnetoresistance (TMR) of 1.2× 10 5 % is achieved. • Thermal Spin Seebeck and spin filtering effects are observed in the proposed model. • A potential candidate in the field of spintronic and spin caloritronic applications.

Observation of magnetoconductivity with terahertz probes for ferromagnetic Fe films

Ultrafast characterization of spin-dependent transport behavior in ferromagnetic systems without contact probes has been of strong demand, recently. We have experimentally investigated spin-dependent complex conductivity for ferromagnetic Fe films of various thickness by means of terahertz time-domain transmission spectroscopy. Comparison of the transmitted terahertz wave amplitude and the spin-dependent conductivity reveals that the magnetization state of films effectively determines the complex conductivity. Non-invasive observation of spin-dependent conductivity by contact-free terahertz probe method is proven to be promising in further investigating spintronic materials, particularly on an ultrafast timescale.

Tunnelling magnetoresistance and spin filtration effect in functionalized graphene sheet with CrO2 as electrode: An ab-initio study

Spin-dependent transport behavior of functionalized graphene sheet with CrO2 as half-metallic magnet (HMM) electrodes are explored using DFT (density functional theory) in a combination with non-equilibrium Green’s function formalism (NEGF). Functionalized graphene sheet with Hydrogen and Fluorine atoms are examined as ultra-thin tunnel barriers in the magnetic tunnel junction devices. Computations based on the DFT for functionalization of graphene sheet with H and F atoms show opening of the band gap in the graphene sheet. A band gap opening of 3.53 eV, 3.16 eV, and 2.64 eV have been observed when Graphene sheet is functionalized with H atoms on both sides, F atoms on both sides, and both F and H atoms on each side of the graphene sheet respectively. The results of all the three functionalized graphene sheet configurations have been compared. On comparison of the results for the three configurations, it is observed that a large value of tunneling magnetoresistance (TMR) around 344 × 103% is obtained for the H-Graphene-F device configuration. This suggests that the H-Graphene-F device configuration acts as a perfect ultra-thin insulating barrier for the spin-based devices. Negative differential resistance (NDR) behavior have been observed for F-Graphene-F and H-Graphene-F device configurations in parallel magnetization (PM) case. All the three configurations show perfect spin filtration efficiency of around 100%. Applied voltage dependent transmission spectrum have been plotted for all the three device configurations to validate the current-voltage behaviour. These unique and interesting nature of all the three devices suggest their applications in spintronics domain.

Spin-dependent transport behavior in C60 and Alq3 based spin valves with a magnetite electrode (invited)

The spin-dependent transport behavior in organic semiconductors (OSs) is generally observed at low temperatures, which likely results from poor spin injection efficiency at room temperature from the ferromagnetic metal electrodes to the OS layer. Possible reasons for this are the low Curie temperature and/or the small spin polarization efficiency for the ferromagnetic electrodes used in these devices. Magnetite has potential as an advanced candidate for use as the electrode in spintronic devices, because it can achieve 100% spin polarization efficiency in theory, and has a high Curie temperature (850 K). Here, we fabricated two types of organic spin valves using magnetite as a high efficiency electrode. C60 and 8-hydroxyquinoline aluminum (Alq3) were employed as the OS layers. Magnetoresistance ratios of around 8% and over 6% were obtained in C60 and Alq3-based spin valves at room temperature, respectively, which are two of the highest magnetoresistance ratios in organic spin valves reported thus far. The magnetoresistance effect was systemically investigated by varying the thickness of the Alq3 layer. Moreover, the temperature dependence of the magnetoresistance ratios for C60 and Alq3-based spin valves were evaluated to gain insight into the spin-dependent transport behavior. This study provides a useful method in designing organic spin devices operated at room temperature.

Spin dependent transport behavior in small world networks

Density functional theory (DFT) combined with the non equilibrium Green’s function formalism (NEGF) is applied to perform spin polarized transport calculations on small world network (SWN) systems consisting of atomic wires. Including the spin property in SWN structures leads to interesting electrical properties. It is revealed that the emerging spin polarization depends mainly on the SWN geometry given by the asymmetric distribution of loops joining the arbitrary atoms on the main chain. The spin-asymmetric behavior which yields the spin polarization is found to be largely determined by those loops which are close to the electrodes. However, spin polarization may vanish for a specific SWN structure due to symmetry.

Origin of large positive magnetoresistance in the hard-gap regime of epitaxial Co-doped ZnO ferromagnetic semiconductors

We studied the electrical transport properties of single-crystal epitaxial Co-doped ZnO films grown by molecular-beam epitaxy. Upon increasing temperature, transport mechanisms changed from hard-gap resistance to Efros variable range hopping. The magnetic field and the temperature dependence of magnetoresistance revealed that large positive magnetoresistance observed in the hard-gap regime originated from the shrinkage of electron wave functions. However, no sign of spin-dependent transport behavior was found.

Spin-flip effect on electrical transport in magnetic quantum wire systems

We investigate the spin-flip effect on electronic transport in a nanostructure composed of two nonmagnetic (NM) leads separated by a periodic spacer. The spacer is composed of one-dimensional heterostructure formed by a sequence of magnetic (A) and nonmagnetic (B) sites periodically juxtaposed (as in a typical periodic quantum dot (QD)). The calculations are based on the tight-binding model and transfer matrix method, which compute the current–voltage characteristic within the Landauer–Büttiker formalism. Our main goal is to assess the contribution of the spin-flip scattering to the transport properties of such systems. The spin-dependent transport behavior can be controlled via a gate magnetic field and an applied voltage in the ballistic regime. Our results show that the conductance strongly depends on the configurations of the magnetic QD. The application of the predicted results may be useful in designing spin-valve devices, such as spin-polarized molecular transistors.

Spin filtering and spin diode devices in quantum wire systems

Theoretical studies on the spin-dependent transmission and current–voltage characteristic in a mesoscopic system, which consists of two semi-infinite ferromagnetic (FM) leads (as source and drain) separated by a typical periodic quantum dot (QD) are presented. The calculations are based on the tight-binding model and transfer matrix method, and investigate the magnetoresistance (MR) and the spin polarization within the Landauer–Büttiker formalism. The spin-dependent transport behavior can be controlled via a gate voltage and an applied bias in the ballistic regime. The numerical results are shown for a periodic polymer chain with nonmagnetic (NM) and FM leads, and also, with two FM leads. The first system (NM/QD/FM) acts as a spin-filter device in a special condition also contact as a spin diode device. The application of the predicted results may be useful in designing molecular spin-polarized transistors in the future.

Electronic transport through domain walls in ferromagnetic nanowires: Coexistence of adiabatic and nonadiabatic spin dynamics

We study the effect of a domain wall on the electronic transport in ferromagnetic quantum wires. Due to the transverse confinement, conduction channels arise. In the presence of a domain wall, spin up and spin down electrons in these channels become coupled. For very short domain walls or at high longitudinal kinetic energy, this coupling is weak, leads to very few spin flips, and a perturbative treatment is possible. For very long domain wall structures, the spin follows adiabatically the local magnetization orientation, suppressing the effect of the domain wall on the total transmission, but reversing the spin of the electrons. In the intermediate regime, we numerically investigate the spin-dependent transport behavior for different shapes of the domain wall. We find that the knowledge of the precise shape of the domain wall is not crucial for determining the qualitative behavior. For parameters appropriate for experiments, electrons with low longitudinal energy are transmitted adiabatically while the electrons at high longitudinal energy are essentially unaffected by the domain wall. Taking this co-existence of different regimes into account is important for the understanding of recent experiments.

Transition from negative magnetoresistance behavior to positive behavior in Co20(Cu1−xGex)80 ribbons

We report a transition of the giant magnetoresistance (GMR) behavior in nanocrystalline Co20(Cu1−xGex)80 ribbons from negative to positive, as the semiconductor Ge substitutes for the Cu matrix. The growth of the hexagonal Co3Ge2 compound leads to a change of the physical origin of the GMR. The normal spin-dependent transport behavior in the CoCu granular system evolves into Coulomb blockade behavior of electronic tunneling in ribbons with a Co/Co3Ge2/Co junctionlike configuration.