Majority Spin Electrons Research Articles

Carrier-induced spin switching in Co/Graphene/Ni: A first principles study

Using the full potential linearized augmented plane wave (FLAPW) method, we report the theoretical results that the carrier induced switching of magnetic interaction between two magnetic layers in Co(111)/Graphene/Ni(111) (Co/Gr/Ni) is predicted. The Co/Gr/Ni shows an antiferromagnetic (AFM) ground state when there are no external carriers. The antiferromagnetic interaction is still observed for hole carriers. Very interestingly, however, the magnetic exchange interaction between Ni and Co layers can be manipulated in such a way as to change an AFM to a ferromagnetic (FM) state by injecting external electrons if the electron carrier concentration is larger than 0.1 × 1014 per cm2. Overall, we propose that a potential spin switching by external carriers or electric field can be realized in Co/Gr/Ni. Besides, the calculated DOS feature indicates that the Co/Gr/Ni system may manifest quite different transport properties when a bias voltage is applied. For instance, the current parallel to the film surface can be completely spin polarized from minority spin electrons. In contrast, the current perpendicular to the film surface will be positively spin polarized from majority spin electrons.

Half-metallicity, magnetic moments, and gap states in oxygen-deficient magnetite for spintronic applications

The electronic structure near oxygen vacancies in half-metallic magnetite has been calculated using first principles methods. Oxygen vacancies are responsible for the existence of gap states occupied by majority and minority spin electrons. We discuss whether these defects modify the spin magnetic moments, the magnetization, the magnetic coupling between Fe ions, and the half-metallic behaviour of magnetite. These results, which contribute to remove stumbling blocks to magnetite-based spintronic devices, could be useful to analyze the conductivity, the magnetotransport and magnetic properties, the electron and optical spectra of actual magnetite electrodes.

Mechanism of large magnetoresistance inCo2MnSi/Ag/Co2MnSidevices with current perpendicular to the plane

Fully epitaxial current-perpendicular-to-plane giant magnetoresistance (MR) devices with half-metallic ${\text{Co}}_{2}\text{MnSi}$ (CMS) electrodes and a Ag spacer were fabricated to investigate the relationship between the chemical ordering in CMS and its MR properties, including bulk and interface spin-asymmetry coefficients $\ensuremath{\beta}$ and $\ensuremath{\gamma}$. CMS/Ag/CMS annealed at $550\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ shows the largest MR ratio: 36.4% and 67.2% at RT and 110 K, respectively. An analysis based on Valet-Fert's model reveals large spin asymmetry $(\ensuremath{\gamma}>0.8)$ at the CMS/Ag interface, which contributes predominantly to the large MR ratio observed. First-principles ballistic conductance calculations for (001)-CMS/Ag/CMS predict a high majority-spin electron conductance, which could be the origin of the large $\ensuremath{\gamma}$ observed in this study.

Nonlinear Giant Magnetoresistance in Dual Spin Valves

Giant magnetoresistance (GMR) arises from differential scattering of the majority and minority spin electrons by a ferromagnet (FM) so that the resistance of a heterostructure depends on the relative magnetic orientation of the FM layers within it separated by nonmagnetic spacers. Here, we show that highly nonequilibrium spin accumulation in metallic heterostructures results in a current-dependent nonlinear GMR which is not predicted within the present understanding of GMR. The behavior can be explained by allowing the scattering asymmetries in an ultrathin FM layer to be current dependent.

Spin-dependent current modulation in organic spintronics

We investigate the spin-dependent current modulation in a model organic semiconductor sandwiched by two ferromagnetic electrodes. When the conductance band of the system is activated by an applied bias voltage, the majority-spin electrons are successively blocked within the organic semiconductor and form nonequilibrium polarons. This majority-spin blockage will modulate the minority-spin current due to the effective spin-spin coupling mediated by the electron-phonon interaction. This study suggests that the spin-blockage induced current modulation is a rather robust phenomenon in organic spintronics.

Spin-polarized electronic structures and transport properties of Fe-Co alloys

The electrical resistivities of Fe-Co alloys owing to random alloy disorder are calculated using the Kubo-Greenwood formula. The obtained electrical resistivities agree well with experimental data quantitatively at low temperature. The spin polarization of Fe50Co50 estimated from the conductivity (86%) has opposite sign to that from the densities of the states at the Fermi level (−73%). It is found that the conductivity is governed mainly by s electrons, and the s electrons in the minority spin states are less conductive due to strong scattering by the large densities of the states of d electrons than the majority spin electrons.

Giant Magneto-Optical Kerr Effects in Ferromagnetic Perovskite BiNiO3 with Half-Metallic State

We theoretically show that the giant magneto-optical Kerr effects are induced by the electronic structure of half-metallic ferromagnetism in the perovskite BiNiO3 with the orthorhombic structure by first-principles calculation. A Kerr rotation is up to 1.28° at 1.87 eV, which is comparative to the recognized maximum polar Kerr rotation of 1.35° at 1.75 eV in the Heusler compound PtMnSb. The strong p−d exchange interaction causes the half-metallic ferromagnetism, i.e., the majority-spin electrons are semiconducting and the minority-spin electrons are metallic, which finally leads to the large Kerr rotation in the half-metallic ferromagnetic perovskite BiNiO3.

Proximity effect between an unconventional superconductor and a ferromagnet with spin bandwidth asymmetry

We study the proximity effect within a junction made of an unconventional superconductor (US) and a ferromagnet $(F)$ in the clean limit with high barrier transparency. Superconductivity in the US side is described by an extended Hubbard model with intersite attractive interaction, while metallic ferromagnetism in the $F$ side is assumed to be originated by a relative change in the bandwidths of electrons with opposite spin. The effect of this mass-split mechanism is analyzed in conjunction with the usual Stoner-like one, where one band is rigidly shifted with respect to the other, due to the presence of a constant exchange field. Starting from the numerical solution of the Bogoliubov-de Gennes equations, we show that the two above mentioned mechanisms for ferromagnetism lead to different features as concerns the formation at the interface of dominant and subdominant superconducting components, as well as their propagation in the ferromagnetic side. This considerably affects the opening of gaplike structures in the local density of states for majority and minority spin electrons, leading to distinct effects as one moves toward the half-metallic regime, where the density of the minority carriers becomes vanishing.

Tight-binding theory of manganese and iron oxides

The electronic structure is found to be understandable in terms of the free-atom term values and universal interorbital coupling parameters, since self-consistent tight-binding calculations indicate that the Coulomb shifts of the $d$-state energies are small. The special-points averages over the bands are seen to be equivalent to the treatment of local octahedral clusters, so that all properties are given by algebraic formulas in terms of these parameters. The cohesive energy per manganese for MnO, ${\text{Mn}}_{2}{\text{O}}_{3}$, and ${\text{MnO}}_{2}$, in which manganese exists in the valence states ${\text{Mn}}^{2+}$, ${\text{Mn}}^{3+}$, and ${\text{Mn}}^{4+}$, is very nearly the same and dominated by the transfer of manganese $s$ electrons to the oxygen $p$ states. There are small corrections, 1.37 eV per Mn in all cases, from the couplings of the minority-spin states. Transferring one majority-spin electron from an upper cluster state to a nonbonding oxygen state adds 1.67 eV to the cohesion for ${\text{Mn}}_{2}{\text{O}}_{3}$ and the two transfers add twice that for ${\text{MnO}}_{2}$. The electronic and magnetic properties are consistent with this description and appear to be understandable in terms of the same parameters.

Ballistic conductance of oxygen and sulfur-incorporated zigzag Au atomic wires

We study the effects of oxygen and sulfur impurity atoms on the electronic structure and ballistic conductance of zigzag-shaped Au monoatomic wires by a first-principles calculation based on the embedded Green's-function technique. In contrast to the straight wires in which the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ $(\mathrm{S}\phantom{\rule{0.2em}{0ex}}3p)$ states form doubly degenerate sharp $\ensuremath{\pi}$ resonant levels at the Fermi energy $({E}_{F})$, one of them is broadened due to symmetry lowering accompanied by zigzag deformations, shifts to lower energies, and exhibits a Fano-type resonance dip in the ballistic conductance. The other state, which does not couple with the $\mathrm{Au}\phantom{\rule{0.2em}{0ex}}6s$-like band, remains a sharp resonant peak at ${E}_{F}$ in the density of states and in the conductance. It splits into two peaks corresponding to majority and minority spin electrons when spin polarization is taken into consideration.

Spin-dependent Mass Enhancement under a Magnetic Field in the Periodic Anderson Model

In order to study the mechanism of the mass enhancement in heavy fermion compounds in the presence of magnetic field, we study the periodic Anderson model using the fluctuation exchange approximation. The resulting value of the mass enhancement factor z^{-1} can become up to 10, which is significantly larger than that in the single-band Hubbard model. We show that the difference between the magnitude of the mass enhancement factor of up spin (minority spin) electrons z^{-1}_up and that of down spin (majority spin) electrons z^{-1}_down increases by the applied magnetic field B//z, which is consistent with de Haas-van Alphen measurements for CeCoIn_5, CeRu_2Si_2 and CePd_2Si_2. We predict that z^{-1}_up >z^{-1}_down in many Ce compounds, whereas z^{-1}_up < z^{-1}_down in Yb compounds.

Measuring spin dependent hot electron transport through a metal-semiconductor interface using spin-polarized ballistic electron emission microscopy

Spin-polarized ballistic electron transport has been studied in $\mathrm{Fe}∕\mathrm{Si}(001)$ Schottky diodes using ballistic electron emission microscopy (BEEM). Spin dependent scattering of polarized ballistic electrons injected from an Fe coated Au tip into the Fe films has been shown to affect the BEEM current. The spin dependent attenuation lengths were determined by measuring this effect with Fe thickness and found to be $1.8\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the minority spin electrons and $2.5\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the majority spin electrons at a tip bias of $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. In addition, the attenuation lengths were measured as a function of tip bias, which indicated that the $\mathrm{Fe}∕\mathrm{Si}(001)$ interface band structure has an effect on the hot electron transport through the diode.

Electric Field Effects on Spin Transport in Defective Metallic Carbon Nanotubes

On the basis of first-principles calculations, we investigate transport properties of spin-polarized electrons in defective metallic single-wall carbon nanotubes (SWCNTs) under homogeneous transverse electric fields. Either vacancies or carbon adatoms are introduced in (10,10) SWCNT and are shown to play a role of quasi-localized magnetic impurities. The applied transverse electric fields change the relative position of the energy levels of the defects with respect to the Fermi energy so that the spin-polarized conductances are shown to be tunable. For some impurities, the orientation of the majority spin electrons in conducting channels at the Fermi energy can be switched to the opposite spin by an experimentally attainable electric field. Our results suggest that pure carbon or organic nanomagnets could be realized in SWCNTs, and their spin transport properties are controllable by transverse electric fields.

Theory of bulk and surface quasiparticle spectra for Fe, Co, and Ni

The correlated electronic structure of iron, cobalt and nickel is investigated within the dynamical mean-field theory formalism, using the newly developed full-potential LMTO-based LDA+DMFT code. Detailed analysis of the calculated electron self-energy, density of states and the spectral density are presented for these metals. It has been found that all these elements show strong correlation effects for majority spin electrons, such as strong damping of quasiparticles and formation of a density of states satellite at about -7 eV below the Fermi level. The LDA+DMFT data for fcc nickel and cobalt (111) surfaces and bcc iron (001) surface is also presented. The electron self energy is found to depend strongly on the number of nearest neighbors, and it practically reaches the bulk value already in the second layer from the surface. The dependence of correlation effects on the dimensionality of the problem is also discussed.

Spin blockade at semiconductor/ferromagnet junctions

We study theoretically extraction of spin-polarized electrons at nonmagnetic semiconductor/ferromagnet junctions. The outflow of majority-spin electrons from the semiconductor into the ferromagnet leaves a cloud of minority-spin electrons in the semiconductor region near the junction, forming a local spin-dipole configuration at the semiconductor/ferromagnet interface. This minority-spin cloud can limit the majority-spin current through the junction, creating a pronounced spin blockade at a critical current. We calculate the critical spin-blockade current in both planar and cylindrical geometries and discuss possible experimental tests of our predictions.

A two-photon photoemission study of spin-dependent electron dynamics

We describe a spin- and femtosecond time-resolved two-photon photoemission experiment with an overall energy resolution of 90 meV and an angular resolution of ±2.5° at an electron count rate of a few kHz. As a model system we have studied image-potential states in front of a three monolayer iron film on Cu(1 0 0). With our setup an exchange splitting of both, the n = 1 and n = 2 image-potential state is clearly discernible. A strong dependence of the spin polarization in the image-potential states on the polarization of the pump light can be observed. Time-resolved measurements reveal that the lifetime of excited majority-spin electrons exceeds that of minority-spin electrons by a factor of 1.4.

Structural and magnetic properties of Co∕α-Al2O3∕Fe magnetic tunneling junction system: Ab initio investigations

Using the density functional theory calculations, the structural effects on the spin polarizations and change of tunneling magnetoresistance for Co∕α-Al2O3∕Fe magnetic tunneling junction were predicted. Hybridization of 3d orbitals of ferromagnetic atoms and O 2p orbital resulted in the covalent bond formations of Co–O and Fe–O at the interface and, subsequently, layer fluctuations of interfacial slabs occurred to the extent of 34.5% and 37.2% of the interlayer distances, 1.94Å for Co and 1.91Å for Fe. The magnetic moment of Co-interface atoms was enhanced to 2.03μB, while that for the Fe-interface atoms was reduced to 1.59μB from the corresponding bulk magnetization. Large asymmetric distributions of layer decomposed density of states for majority and minority spin electrons led the spin polarizations of 35.2% and 65.9% for top and bottom ferromagnetic electrodes, respectively. The change of magnetoresistance was calculated to be 37.7%.

Mn-dopedScN: A dilute ferromagnetic semiconductor with local exchange coupling

We present local spin density functional calculations for Mn-doped $\mathrm{ScN}$ using the linear muffin-tin orbital method in a supercell approach, including structural relaxation. Mn is found to induce a localized state in middle of the band gap of $\mathrm{ScN}$ with ${t}_{2g}$ character which spin splits and leads to a net magnetic moment of about $2--3{\ensuremath{\mu}}_{B}$ per Mn. The defect is a deep acceptor with 0/- transition state at about $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level. The magnetic moment increases when in the negative charge state by acquiring more ${e}_{g}$ majority spin electrons and will thus persist even in the presence of the commonly found unintentional background $n$-type doping of $\mathrm{ScN}$. Up to fourth nearest neighbors exchange interactions are found to be ferromagnetic. The experimentally observed high solubility of Mn in $\mathrm{ScN}$, presumably related to their common crystal structure, is favorable for this dilute magnetic semiconductor. Within mean field theory the Curie temperature is estimated to be above $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for 3% Mn. While this may be overly optimistic because of the neglect of disorder, the $n$-type character of $\mathrm{ScN}$ adds further interest to this unusual magnetic semiconductor because the spin-split conduction bands are nondegenerate and thus essentially free from spin-orbit coupling induced spin relaxation.

A spin-polarized scheme for obtaining quasi-particle energies within the density functional theory

We discuss an efficient scheme for obtaining spin-polarized quasi-particle excitation energies within the general framework of the density functional theory (DFT). Our approach is to correct the DFT eigenvalues via the electrostatic energy of a majority or minority spin electron resulting from its interaction with the associated exchange and correlation holes by using appropriate spin-resolved pair correlation functions. A version of the method for treating systems with localized orbitals, including the case of partially filled metallic bands, is considered. Illustrative results on Cu are presented.

Spin dependent transport: GMR & TMR

The discovery of giant magnetoresistance in 1988 opened the large research field of ‘spintronics’. Twenty years later, a large number of devices makes use of the electron's spin, in addition to its charge, to control electronic transport properties. The physical origin of spintronic phenomena is the different conduction properties of the majority and minority spin electrons in a ferromagnetic metal. At an interface involving a ferromagnetic conductor, this leads to spin dependent conduction or tunneling properties. Here we present an overview of magnetotransport phenomena in structures involving metallic layers. To cite this article: A. Schuhl, D. Lacour, C. R. Physique 6 (2005).