Spin-down Conduction Research Articles

Optimization of the tunneling magnetoresistance and spin-valley polarization in complex magnetic silicene structures

Aperiodic order is ubiquitous in nature and quite relevant in science and technology. There are extensive works in aperiodic structures studying fundamental characteristics in physical properties, such as fractality, self-similarity, and fragmentation. However, there are fewer reports in which aperiodicity signifies an improvement in physical quantities with practical applications. Here, we show that the aperiodicity of fractal or self-similar type optimizes the tunneling magnetoresistance and spin-valley polarization of magnetic silicene structures, raising the prospects of spin-valleytronics. We reach this conclusion by studying the spin-valley-dependent transport properties of complex (Cantor-like) magnetic silicene structures within the lines of the transfer matrix method and the Landauer–Büttiker formalism. We find that the self-similar arrangement of magnetic barriers in conjunction with structural asymmetry reduces the conductance oscillations typical of periodic magnetic silicene superlattices and more importantly makes the K′-spin-down conductance component dominant, resulting in nearly perfect positive and negative spin-valley polarization states accessible by simply reversing the magnetization direction. The tunneling magnetoresistance is not as prominent as in periodic magnetic silicene superlattices; however, it is better than in single magnetic junctions. Furthermore, the optimization of the spin-valley-dependent transport properties caused by the complex structure is superior than the corresponding one reported in typical aperiodic structures, such as Fibonacci and Thue–Morse magnetic silicene superlattices.

First principles study of Co and Ni based half Heusler alloys

The structural stability, electronic structure, mechanical, magnetic and thermal properties of Co and Ni based half-Heusler (HH) alloys are studied. The most stable phase for CoCrSb, NiCrSb, NiCrBi, CoMnSb, CoMnBi and CoCrBi alloys is found to be γ-phase, whereas α-phase is the most stable phase for NiMnBi and NiMnSb among the three different phases (α, β, γ). As the pressure is increased, structural phase transition from γ-phase to α-phase is predicted for NiCrSb, CoCrBi, NiCrBi, CoMnBi, γ-phase to β-phase for CoMnSb and α-phase to β-phase for NiMnSb, at the pressures of 66.96 GPa, 16.91 GPa, 92.50 GPa, 14.58 GPa, 79.23 GPa and 71.47 GPa respectively. Density of states plot shows that there is no energy gap in spin-up state of all these HH alloys and in the spin-down state, the valence and conduction states are separated by a semiconducting energy gap. Thus, these compounds are half metallic. The spin polarized calculations predicted that these Heusler alloys are ferromagnetic nature. The total magnetic moment per unit cell (μcell) is found to be closer to the integer value of 2 μB, 3 μB and 4 μB. The 100% spin polarization at EF confirms the half-metallic nature of these materials. The dynamical stability of novel HH alloys has also been investigated by computing phonon density of states and it shows that these alloys (except CoCrBi and CoMnBi) are dynamically stable.

Full range modulation of giant magnetoresistance in graphene–like zigzag nanoribbons via dual edge disorders

The effects of dual edge disorders on the giant magnetoresistance RM in graphene-like zigzag nanoribbons of width number n (n-ZNRs) are studied systematically using the tight-binding model with non-equilibrium Green's functions. Manipulation of RM in the full range from −100% to 100% can be realized if the disorder potentials on the lower and upper edges have the same strength but opposite signs. This occurs because the spin-up and spin-down conductance may vanish together not only in the parallel and but also in the antiparallel magnetic configuration, benefiting from the symmetry of the energy band structure in honeycomb lattice. The disorder potential ratio and positions can also effectively modulate RM as well as the ZNR width and edge magnetization. Distinguished patterns of RM behavior are observed in n-ZNRs of even and odd n, respectively. The patterns are expected insensitive to the width of the domain wall and remain qualitatively similar as soon as the relative positions of the dual disorders hold fixed.

Theoretical Investigation of Nonequilibrium Spin Transport Through a Triple Site Quantum Wire System

A triple lattice site quantum wire (QW) system is chosen to investigate nonequilibrium spin transport properties theoretically. We calculate the initial charge of the QW from the exactly derived ground state. Based on the Keldysh formalism, we describe theoretical methods and derive analytical formulas for nonequilibrium spin transport of the QW systems within Hartree-Fock approximation when Coulomb interactions are present. We report the numerical results of the nonequilibrium differential spin conductance, spin transport current, and electronic charge distribution of the triple site QW system. When only including Coulomb repulsions between the spin-up and spin-down electrons, the series of peaks and valleys in the conductance characteristics appears due to the spin resonant tunneling and spin blockade. With the increase of Coulomb interaction energy U, the conductance peaks start to split into two corresponding to the spin-up and spin-down conductance. Near the resonant points, the spin current polarization and the in-site spin charge polarizations take place. When additionally including spin-spin interactions, the differences between the spin-up and spin-down conductance characteristics are reduced, implying that the spin split is relaxed by spin-spin interactions. The spin splits, the spin current, and spin charge polarizations are relaxed at high temperatures due to the thermal fluctuations.

Spin transfer and proximity effects in case of FePt (L1) nanoparticles coated with P3HT

Nanocomposites based on half-metallic FePt (L10) magnetic nanoparticles coated with the semiconducting conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) show a significant reduction in the magnetic coercivity. This study adopts a physical approach based on chemical potential equalization at the interface. The underlying charge/spin transfer mechanism unveils an imbalance: only spin-down polarized electrons are allowed to be transferred from the semiconductor to the half-metal (spin-down) conduction band, while spin-up states remain blocked at the interface. This process determines an excess of spin-up states on the P3HT side, and due to a RKKY mechanism, this effective spin system becomes ferromagnetic polarized. Due to this proximity effect, the conjugate polymer becomes exchange coupled to the hard magnetic FePt (L10) phase, thus reducing the coercivity of the half-metal. These processes make this type of composite suitable for magnetic recording applications.

Structure, magnetic and electronic transport properties in antiperovskite cubic γ′-CuFe3N polycrystalline films

The structure, magnetic and electronic transport properties of the facing-target reactively sputtered polycrystalline CuFe3N films have been investigated systematically. The CuFe3N film has a cubic antiperovskite structure. The saturation magnetization satisfied the modified Bloch's spin wave theory. At 300 K, the saturation magnetization of the 80 nm-thick CuFe3N films is 402 emu/cm3, which is smaller than 800 emu/cm3 of the 80 nm-thick polycrystalline Fe4N films. The CuFe3N films are metallic. The anomalous Hall effect in the “dirty region” of σxx < 104 S/cm can be explained by the intrinsic mechanism and the side jump in the proper scaling, which is similar to Fe4N. However, the sign of magnetoresistances in CuFe3N is opposite to that of Fe4N below 20 K because the doped-Cu affects the competition among the Lorentz force, spin-orbit coupling, weak localization and s-d exchange interaction. The small anisotropic magnetoresistance is dominated by the spin-down conduction electron. The transformation from the two-fold to one-fold symmetry in the planar Hall resistivity of the 10-nm-thick-CuFe3N films is related to the competition between the temperature-dependent charge scattering and magnetic-field-dependent spin scattering.

Spin-resolved transport properties in molybdenum disulfide superlattice

The effects of the perpendicular magnetic field on the multi-barriers system of 5-barriers to 100-barriers are studied. Considering the low-energy Hamiltonian of the system with spin-orbit coupling we modulate the external magnetic field as the Zeeman energy on the orbital magnetic moment. The transport of an incident electron passing through the superlattice created by the top-gated monolayer MoS2 based on the transfer matrix method is investigated and the spin-up and the spin-down transmissions and the spin-polarization for 5-barriers to 100-barriers are obtained. Comparing the results show that in the case of 30-barriers, we have both reliable transmissions for the system and a 100% spin-polarization so that the spin-flip can occur and gives us a desired spintronic device. We also sketched the variations of the transmissions and spin-polarization versus the relative barrier-widths over the well-widths, (D/W), which show the optimum value is 1 by 2, respectively. Finally, we calculate the spin-up and spin-down conductance and total conductivity of the system which shows the so-called resonant tunneling peaks and amplifying the internal spin-orbit coupling with the external magnetic field as the Zeeman energy.

New mechanism of plasmons specific for spin-polarized nanoparticles

Here it is experimentally shown that Co nanoparticles with a single-domain crystal structure support a plasmon resonance at approximately 280 nm with better quality than gold nanoparticle resonance in the visible. Magnetic nature of the nanoparticles suggests a new type of these plasmons. The exchange interaction of electrons splits the energy bands between spin-up electrons and spin-down electrons. It makes it possible for coexistence of two independent channels of conductivity as well as two independent plasmons in the same nanoparticle with very different electron relaxation. Indeed, the density of empty states in a partially populated d-band is high, resulting in a large relaxation rate of the spin-down conduction electrons and consequently in low quality of the plasmon resonance. In contrast, the majority electrons with a completely filled d-band do not provide final states for the scattering processes of the conduction spin-up electrons, therefore supporting a high quality plasmon resonance. The scattering without spin flip is required to keep these two plasmons independent.

Spin Transport in Electric-Barrier-Modulated Ferromagnetic/Normal/Ferromagnetic Monolayer Zigzag MoS2 Nanoribbon Junction

In this paper, with the three-band tight-binding model and non-equilibrium Green’s function technique, we investigate spin transport in electric-barrier-modulated Ferromagnetic/Normal/Ferromagnetic (F/N/F) monolayer (ML) zigzag MoS2 nanoribbon junction. The results demonstrate that once the double electric barriers structure emerges, the oscillations of spin conductances become violent, especially for spin-down conductance, the numbers of resonant peaks increase obviously, thus we can obtain [Formula: see text] spin polarization in the low energy region. It is also found that with the intensity of the exchange field enhancement, the resonant peaks of spin-up and spin-down conductances move in the opposite direction in a certain energy region. As a consequence, the spin-down conductance can be filtered out completely. The findings here indicate that the present structure may be considered as a good candidate for spin filter.

The role of mesoscopic structuring on the intermixing of spin-polarised conduction channels in thin-film ferromagnets for spintronics

The separation of spin-up and spin-down conduction channels is fundamental to electronic transport in ferromagnets and essential for spintronic functionality. The spin states available for conduction are defined by the ferromagnetic material, but additional physical factors can affect scattering and modify the spin-dependence of conduction. Here the effect of mesoscopic structuring, arising during the growth of ferromagnetic thin films, on the electronic transport was investigated. Resistivity and anisotropic magnetoresistance were measured in a series of Ni80Fe20 thin films as a function of nominal film thickness from up to . The observed thickness dependence of the resisivity and magnetic anisotropy of resistivity are interpreted using a model that accounts for the macroscopic structuring from the growth of the films and incorporates a structural dependence of the spin-flip scattering. The model shows good agreement for both the thickness dependence of the resistivity and the reduction of the anisotropic magnetoresistivity. The latter indicating that increasing mixing of the conducting spin channels occurs in ultra-thin films, mainly a consequence of macroscopic structuring of the films.

Anisotropic magnetoresistance in facing-target reactively sputtered epitaxial γ′-Fe4N films

The negative anisotropic magnetoresistance that comes from the spin-down conduction electrons are observed in facing-target sputtered epitaxial γ′-Fe4N films with different film thicknesses, substrates and orientations. Anisotropic magnetoresistance of γ′-Fe4N films on LaAlO3(100) is larger than other substrates. The magnitude of anisotropic magnetoresistance in (100)-oriented films is always larger than (110)-oriented films. The anisotropic magnetoresistance is intimately related to the magnetocrystalline anisotropy. Fourier coefficient C2θ and C4θ of cos2θ and cos4θ terms strongly depend on the measuring temperature. No significant influence of magnetic field on C2θ and C4θ appears. The marked change of C2θ and appearance of C4θ at low temperatures are from crystal field splitting of d orbitals induced by the lattice change due to the tensile stress from substrate and the compressive stress from decreased temperatures.A. Magnetic materials; A. Nitrides; B. Epitaxial growth; D. Electrical properties

Scattering of conduction electrons in Fe(t x , Å)/Cr(10 Å) superlattices with ultrathin iron layers

IR magnetoreflection spectra, diagonal σxx and off-diagonal σxy components of the effective optical conductivity tensor, and magnetic properties of Fe(tx, A)/Cr(10 A) superlattices have been studied. The abrupt decrease in the amplitude of dissipative function −ωImσxy(ω) (ω is the cyclic frequency of light wave) in the superlattices with ultrathin Fe layers (tFe = 3.2, 2.6, 2.1 A) has been analyzed. It has been found that the magnetorefractive effect in nanostructures with ultrathin iron layers is due to scattering of conduction electrons by magnetic interfacial layers formed in the Cr matrix with complete consumption of deposited iron atoms. The parameters of the interfacial scattering of electrons in the spin-up (└) and spin-down (┌) conduction channels have been discussed.

Conductance of ferro- and antiferro-magnetic single-atom contacts: A first-principles study

We present a first-principles study on the spin dependent conductance of five single-atom magnetic junctions consisting of a magnetic tip and an adatom adsorbed on a magnetic surface, i.e., the Co-Co/Co(001) and Ni-X/Ni(001) (X = Fe, Co, Ni, Cu) junctions. When their spin configuration changes from ferromagnetism to anti-ferromagnetism, the spin-up conductance increases while the spin-down one decreases. For the junctions with a magnetic adatom, there is nearly no spin valve effect as the decreased spin-down conductance counteracts the increased spin-up one. For the junction with a nonmagnetic adatom (Ni-Cu/Ni(001)), a spin valve effect is obtained with a variation of 22% in the total conductance. In addition, the change in spin configuration enhances the spin filter effect for the Ni-Fe/Ni(001) junction but suppresses it for the other junctions.

Tunable spin-dependent Andreev reflection in a four-terminal Aharonov-Bohm interferometer with coherent indirect coupling and Rashba spin-orbit interaction

Using the nonequilibrium Green’s function method, we theoretically study the Andreev reflection(AR) in a four-terminal Aharonov-Bohm interferometer containing a coupled double quantum dot with the Rashba spin-orbit interaction (RSOI) and the coherent indirect coupling via two ferromagnetic leads. When two ferromagnetic electrodes are in the parallel configuration, the spin-up conductance is equal to the spin-down conductance due to the absence of the RSOI. However, for the antiparallel alignment, the spin-polarized AR occurs resulting from the crossed AR (CAR) and the RSOI. The effects of the coherent indirect coupling, RSOI, and magnetic flux on the Andreev-reflected tunneling magnetoresistance are analyzed at length. The spin-related current is calculated, and a distinct swap effect emerges. Furthermore, the pure spin current can be generated due to the CAR when two ferromagnets become two half metals. It is found that the strong RSOI and the large indirect coupling are in favor of the CAR and the production of the strong spin current. The properties of the spin-related current are tunable in terms of the external parameters. Our results offer new ways to manipulate the spin-dependent transport.

Conductance of single-atom magnetic junctions: A first-principles study

We present a first-principles investigation to show that the contact conductance of a half conductance quantum (G0/2) found previously does not generally hold for single-atom magnetic junctions composed of a tip and an adatom adsorbed on a surface. The contact conductance of the Ni-Co/Co(111) junction is approximately G0/2, while for the Co-Co/Co(111), Ni-Ni/Ni(111), and Ni-Ni/Ni(001) junctions the contact conductances are 0.80G0, 1.55G0, and 1.77G0, respectively. The deviation from G0/2 is mainly caused by the variation of the spin-down conductance largely determined by the minority d orbitals, as the spin-up one changes little for different junctions.

SPIN TRANSPORT FOR A QUANTUM WIRE WITH WEAK DRESSELHAUS SPIN-ORBIT COUPLING

We investigate theoretically the electron transport properties of a quantum wire (QW) non-adiabatically connected to two normal leads with weak Dresselhaus spin-orbit coupling (DSOC). Using the scattering matrix method and Landauer–Büttiker formula within the effective free-electron approximation, we have calculated the spin-dependent conductances G↑/↓ and spin polarization Pz of a hard-wall potential confined QW. It is demonstrated that regardless of the existence of DSOC G↑/↓ and Pz present oscillation structures near the subband edges of QW, and the number of quantized conductance plateaus is determined by the number of propagation modes in two leads. Moreover, the DSOC induces splitting of spin-up and spin-down conductance plateaus as well as the existence of spin polarization (Pz ≠ 0), and the enhancement of Dresselhaus strength destroys the conductance plateaus for the wide QW case. The above results indicate that the spin-dependent conductances and Pz are strongly dependent on the Dresselhaus strength which is the physical basis for spin transistor.

First-principles study of magnetism and electronic structureof Sb-containing half-Heusler alloys

Using the first-principles density functional theory, we calculate the crystal structures, magnetisms and electronic structures of Sb-containing half-Heusler alloys XYSb(X=Ni, Pd, Pt; Y=Mn, Cr). The calculation results show that alloy NiMnSb is half-metal and the others are metals at equilibrium lattice constant. The contribution of the spin magnetic moment of Y element to the total moment is largest for all alloys. It is found that the Fermi level of the minority spin band shifts closer to the bottom of spin-down conduction band with atomic number of X element reducing. The Fermi level moves up due to the compressive strain, away from p bands of Sb atom. Under the compresive stress, PtMnSb, PdMnSb and NiCrSb can induce metal half-metal transitions.

High spin-filter efficiency in a Co ferrite fabricated by a thermal oxidation

Co ferrites fabricated by a thermal oxidation have been tested as a spin filter. Spin-filter efficiencies of 44% and 4.3% were confirmed at 10 K and RT, respectively, using a magnetic tunnel junction of Pt/CoFe2O4/MgO/Co. By increasing the bias voltage, the tunneling magnetoresistance (TMR) at 10 K increases then decreases. This is interpreted as a tunneling via spin-down conduction band in the Co ferrite. Since the electron at RT is attributed to a hopping conduction on the defect states in the Co ferrite, the reduction of the defects will be the key to achieving a higher spin-filter effect.

Fabrication of a Spin-filter Device with Co Ferrite

A magnetic tunnel junction (MTJ) spin-filter device of Pt/Co ferrite/MgO/Co demonstrated -19.8 % tunnel magnetoresistance (TMR) and 27.5% spin-filter efficiency at 10 K. By increasing the bias voltage, the TMR at 10 K first increased and then decreased, exhibiting a maximum at 0.17 V. This anomalous dependence of TMR on bias was interpreted as tunneling via the spin-down conduction band in the Co ferrite. At room temperature (RT), the tunneling mechanism was thought to originate from hopping in the defect states due to structural and chemical defects in the Co ferrite. To achieve higher spin-filter effect at RT, it is important to reduce the defects in the Co ferrite barrier.

An approach based on the free-electron theory to calculate electron conductance of perfect metallic nanowires

For the first time, the quantum free-electron theory is utilized for determining the quantized electrical conductance in a perfect metallic multi-atom nanowire. In this formulation, both spin-up and spin-down conduction electrons are considered so that our final result comes from a series combination of the contributions to the conductance from the two possible types of spin-oriented electron, assuming a symmetric distribution of the involved counter-ions. In addition, several aspects related to the atom-lead coupling are discussed.