Minority Spin Electrons Research Articles

Electronic structure of the Co(0001)/MoS2 interface and its possible use for electrical spin injection in a single MoS2 layer

The ability to perform efficient electrical spin injection from ferromagnetic metals into two-dimensional semiconductor crystals based on transition metal dichalcogenide monolayers is a prerequisite for spintronic and valleytronic devices using these materials. Here, the hcp Co(0001)/MoS2 interface electronic structure is investigated by first-principles calculations based on the density functional theory. In the lowest energy configuration of the hybrid system after optimization of the atomic coordinates, we show that interface sulfur atoms are covalently bound to one, two or three cobalt atoms. A decrease of the Co atom spin magnetic moment is observed at the interface, together with a small magnetization of S atoms. Mo atoms also hold small magnetic moments which can take positive as well as negative values. The charge transfers due to covalent bonding between S and Co atoms at the interface have been calculated for majority and minority spin electrons and the connections between these interface charge transfers and the induced magnetic properties of the MoS2 layer are discussed. Band structure and density of states of the hybrid system are calculated for minority and majority spin electrons, taking into account spin-orbit coupling. We demonstrate that MoS2 bound to the Co contact becomes metallic due to hybridization between Co d and S p orbitals. For this metallic phase of MoS2, a spin polarization at the Fermi level of 16 % in absolute value is calculated, that could allow spin injection into the semiconducting MoS2 monolayer channel. Finally, the symmetry of the majority and minority spin electron wave functions at the Fermi level in the Co-bound metallic phase of MoS2 and the orientation of the border between the metallic and semiconducting phases of MoS2 are investigated, and their impact on spin injection into the MoS2 channel is discussed.

Spin-dependent electron transport through a Mn-phthalocyanine molecule — A steady-state density functional theory (SS-DFT) study

We generalize the recently proposed steady-state density functional theory (SS-DFT) to spin-dependent cases and theoretically investigate the electronic and transport properties of a Mn-phthalocyanine molecule sandwiched between two graphene nanoribbon leads. The junction filters spin-up (minority spin) electrons while allowing spin-down (majority spin) electrons to pass with a filtering efficiency of about 99.5% at low biases. The spin-down electrons are found to tunnel through the junction via the HOMO orbital of the Mn-phthalocyanine molecule. Detailed analysis of the spin-dependent electron tunneling mechanism as well as the electronic/magnetic properties of the junction is presented.

First Principles Investigation of Electronic Property and High Pressure Phase Stability of SmN

An investigation of electronic property and high pressure phase stability of SmN has been conducted using first principles calculations based on density functional theory. The electronic properties of SmN show a striking feature of a half metal, the majority-spin electrons are metallic and the minority-spin electrons are semiconducting. It was found that SmN undergoes a pressure-induced phase transition from NaCl-type (B1) to CsCl-type structure (B2) at 117 GPa. The elastic constants of SmN satisfy Born conditions at ambient pressure, indicating that B1 phase of SmN is mechanically stable at 0 GPa. The result of phonon spectra shows that B1 structure is dynamically stable at ambient pressure, which agrees with the conclusion derived from the elastic constants.

Coexistence of site- and bond-centered electron localization in the high-pressure phase ofLuFe2O4

Magnetic-electronic hyperfine interaction parameters of spectral components are obtained from in situ $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy pressure studies of the mixed-valence $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ multiferroic, up to $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and on recovered high-pressure phase samples. Temperature-dependent M\ossbauer spectra of the low-pressure phase show that $\mathrm{F}{\mathrm{e}}^{2+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$ sites are discernible, consistent with known site-centered charge order in the triangular (frustrated) Fe sublattice network. Magnetic spectra of the high-pressure phase, stabilized in a rectangular Fe sublattice network at $Pg8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, exhibit fingerprints of iron in an intermediate valence state only. Temperature-dependent resistivity pressure studies evidence thermally activated small polaron motion in the high-pressure phase. These experimental signatures, complemented by ab initio calculations of electronic structure, are considered evidence of asymmetric dimer formation ${\mathrm{Fe}}^{(2+\mathrm{\ensuremath{\Delta}}+)}\ensuremath{\Leftrightarrow}{\mathrm{Fe}}^{(3\ensuremath{-}\mathrm{\ensuremath{\Delta}})+}$, where the minority-spin electron deconfinement coefficient is $\mathrm{\ensuremath{\Delta}}=0.3\ensuremath{-}0.4$. Bragg satellites discerned in electron diffraction patterns of the metastable high-pressure phase possibly stem from this admixture of site- and bond-centered localization (intermediate-state charge order) in a magnetic background. This breaks inversion symmetry and potentially renders $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ in its high-pressure phase as a new charge order instigated (electronic) ferroelectric.

Orbital origin of the electrical conduction in ferromagnetic atomic-size contacts: Insights from shot noise measurements and theoretical simulations

With the goal to elucidate the nature of spin-dependent electronic transport in ferromagnetic atomic contacts, we present here a combined experimental and theoretical study of the conductance and shot noise of metallic atomic contacts made of the 3d ferromagnetic materials Fe, Co, and Ni. For comparison, we also present the corresponding results for the noble metal Cu. Conductance and shot noise measurements, performed using a low-temperature break junction setup, show that in these ferromagnetic nanowires: (i) there is no conductance quantization of any kind, (ii) transport is dominated by several partially-open conduction channels, even in the case of single-atom contacts, and (iii) the Fano factor of large contacts saturates to values that clearly differs from those of monovalent (nonmagnetic) metals. We rationalize these observations with the help of a theoretical approach that combines molecular dynamics simulations to describe the junction formation with nonequilibrium Green's function techniques to compute the transport properties within the Landauer-Buettiker framework. Our theoretical approach successfully reproduces all the basic experimental results and it shows that all the observations can be traced back to the fact that the d bands of the minority-spin electrons play a fundamental role in the transport through ferromagnetic atomic-size contacts. These d bands give rise to partially open conduction channels for any contact size, which in turn lead naturally to the different observations described above. Thus, the transport picture for these nanoscale ferromagnetic wires that emerges from the ensemble of our results is clearly at variance with the well established conduction mechanism that governs the transport in macroscopic ferromagnetic wires, where the d bands are responsible for the magnetism but do not take part in the charge flow.

Effect of magnetic field on positron lifetimes of Fe, Co and Ni

Positron lifetime spectra of Fe, Co and Ni were measured under magnetic field using a 22Na source. Very small but distinguishable difference of positron lifetime upon magnetic field reversal was observed suggesting the existence of two bulk lifetimes associated with majority and minority spin electrons. Using two spin-dependent Fe bulk lifetimes, the difference Doppler broadening of annihilation radiation spectra between majority and minority spin electrons were also examined. Agreement between experiment and theory indicates that spin-polarized positron annihilation spectroscopy may have potential in investigation of spin-aligned electron momentum distribution.

Spin-dependent thermal transport perpendicular to the planes of Co/Cu multilayers

We report measurements of the cross-plane thermal conductivity of periodic Co/Cu multilayers using time-domain thermoreflectance. The cross-plane thermal conductivity increases from $\ensuremath{\sim}18$ $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1\phantom{\rule{0.16em}{0ex}}}{\mathrm{K}}^{\ensuremath{-}1}$ at remanence to $\ensuremath{\sim}32$ $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1\phantom{\rule{0.16em}{0ex}}}{\mathrm{K}}^{\ensuremath{-}1}$ at saturation fields. This giant magnetothermal resistance (GMTR) effect is consistent with predictions based on the Wiedemann-Franz law. We discuss the role of a spin-dependent temperature, known as spin heat accumulation, in GMTR experiments and develop a three-temperature model capable of predicting the time evolution of the temperatures of majority-spin electrons, minority-spin electrons, and phonons subsequent to pulsed laser heating.

Spin Flows in Magnetic Semiconductor/Insulator/Superconductor Tunneling Junction

Tunneling currents along the c-axis of the majority and minority spin electrons have been studied for a magnetic semiconductor (MS)/insulator (I)/superconductor (S) tunneling junction consisting of a Ga1−xMnxAs MS with x = 1/32, a nonmagnetic I with a realistic dimension, and a HgBa2Ca2Cu3O8.4 (Hg-1223) high-Tc S. The normalized charge and spin currents, QT,Cμ′Vex and QT,Sμ′Vex, and the flows of the majority ↑ and minority ↓ spin electrons, QT,↑μ′Vex and QT,↓μ′Vex, have been calculated at a fixed external voltage Vex, as a function of the magnetic moment μ′ (≡μ/μB) per a Mn atom which is deduced from the band structure calculations. It is found that the tunneling due to the minority spin electron dominates when μ′<2.4, but such a phenomenon is not found for μ′>2.4. We have pointed out that the present MS/I/S tunneling junction seems to work as a switching device in which the ↑ and ↓ spin flows can be easily controlled by the external magnetic field.

Charge and Spin Currents in Ferromagnet-Insulator-Superconductor Tunneling Junctions Using Hg-1223 High-Tc Superconductor

Charge and spin currents along the c-axis in ferromagnet-insulator-superconductor (F/I/S) tunneling junctions have been studied within the framework of the tunneling Hamiltonian model. As a superconductor S, HgBa2Ca2Cu3O8+δ (Hg-1223) with δ=0.4 copper-oxide high-Tc superconductor has been selected, and as a ferromagnet F, Fe metal with bcc structure has been selected for simplicity. The electronic structures of above materials have been calculated on the basis of the band theory using the spin-polarized self-consistent-field data for the atomic orbital energies and the universal tight-binding parameters (UTBP) for the interactions. For the η↑ and η↓(=1-η↑) defined in the present paper, which are tunneling probabilities of the majority and the minority spin electrons, it is shown that the condition η↑=η↓ means the standard F/I/S tunneling junction with a nonmagnetic insulating layer, and the condition η↑≠η↓means the F/I/S tunneling junction with a magnetic insulating layer showing a detectable magnetization. We have found that the charge current and the differential conductance nearly remain the same as the change of η↑, but the spin current is largely changed due to the change of η↑. As an experimental method to detect the change of the spin current, the validity of an X-ray magnetic circular dichroism (XMCD) has been pointed out.

Hybrid DFT calculation ofFe57NMR resonances and orbital order in magnetite

The crystal structure and charge and orbital order of magnetite below the Verwey temperature are calculated using a first-principles hybrid density functional theory (DFT) method. The initial atomic positions in the crystal-structure calculation are those recently refined from x-ray diffraction data for the $Cc$ space-group unit cell [Senn, Wright, and Attfield, Nature (London) 481, 173 (2012)]. Fermi contact and magnetic dipolar contributions to hyperfine fields at $^{57}\mathrm{Fe}$ nuclei obtained from hybrid DFT calculations are used to obtain NMR resonance frequencies for magnetite for a range of external magnetic field directions in a relatively weak field. NMR frequencies from hybrid density functional theory calculations are compared to NMR data [M. Mizoguchi, J. Phys. Soc. Jpn. 70, 2333 (2001)] for a range of applied magnetic field directions. NMR resonance frequencies of $B$-site Fe ions show large relative variations with applied field direction owing to anisotropic hyperfine fields from charge and orbital ordered Fe $3d$ minority-spin electrons at those sites. Good agreement between computed and measured NMR resonance frequencies confirms the pattern of charge and orbital order obtained from calculations. The charge and orbital order of magne-tite in its low-temperature phase obtained from hybrid DFT calculations is analyzed in terms of one-electron bonds between Fe ions. The Verwey transition in magnetite therefore resembles Mott-Peierls transitions in vanadium oxides which undergo symmetry-breaking transitions owing to electron-pair bond formation.

First spin-resolved electron distributions in crystals from combined polarized neutron and X-ray diffraction experiments.

Since the 1980s it has been possible to probe crystallized matter, thanks to X-ray or neutron scattering techniques, to obtain an accurate charge density or spin distribution at the atomic scale. Despite the description of the same physical quantity (electron density) and tremendous development of sources, detectors, data treatment software etc., these different techniques evolved separately with one model per experiment. However, a breakthrough was recently made by the development of a common model in order to combine information coming from all these different experiments. Here we report the first experimental determination of spin-resolved electron density obtained by a combined treatment of X-ray, neutron and polarized neutron diffraction data. These experimental spin up and spin down densities compare very well with density functional theory (DFT) calculations and also confirm a theoretical prediction made in 1985 which claims that majority spin electrons should have a more contracted distribution around the nucleus than minority spin electrons. Topological analysis of the resulting experimental spin-resolved electron density is also briefly discussed.

Iron orbital occupancies upon valence mixing of charge-ordered GdBaFeIIFeIIIO5

Mossbauer spectra of charge-ordered/valence-mixed GdBaFe2O5 have been analyzed in order to determine the orbital occupancies of the minority-spin d electron as a function of temperature. In the charge-ordered state below the Verwey temperature TV, it occupies the Fe2+dxz orbital, which is ordered. Above TV, the electron partially occurs also at the trivalent iron and spreads over two additional orbitals, \(d_{x^{2}-y^{2}}\) and \(d_{z^{2}}\). In the valence-mixing state (above Tp) also orbital dyz participates. The study illustrates how the Mossbauer electric-field gradient, center shift and internal field jointly reflect the distribution of the minority-spin electron culprit behind the Verwey transition.

First-principles calculations of electronic and magnetic properties of CeN: The LDA + U method

Electronic and magnetic properties of CeN are investigated using first-principles calculations based on density functional theory (DFT) with the LDA + U method. Our results show that CeN is a half-metal. The majority-spin electron band structure has metallic intersections, whereas the minority-spin electron band structure has a semiconducting gap straddling the Fermi level. A small indirect energy gap occurs between X and W. The calculated magnetic moment is 0.99 μB per unit cell.

Large positive spin polarization and giant inverse tunneling magnetoresistance in Fe/PbTiO3/Fe multiferroic tunnel junction

We perform first-principles electronic structure and spin-dependent transport calculations of a multiferroic tunnel junction (MFTJ) with an epitaxial Fe/PbTiO3/Fe heterostructure. We predict a large positive spin-polarization (SP) and an intriguing giant inverse tunneling magnetoresistance (TMR) ratio in this tunnel junction. We demonstrate that the tunneling properties are determined by ferroelectric (FE) polarization screening and electronic reconstruction at the interface with lower electrostatic potential. The intricate complex band structure of PbTiO3, in particular the lowest decay rates concerning Pb 6pz and Ti 3dz2 states near the Γ¯ point, gives rise to the large positive SP of the tunneling current in the parallel magnetic configuration. However, the giant inverse TMR ratio is attributed to the minority-spin electrons of the interfacial Ti 3dxz+3dyz orbitals which have considerably weight in the extended area around the Γ¯ point at the Fermi energy and causes remarkable contributions to the conductance in the antiparallel magnetic configuration.

Electronic structure and half-metallic ferromagnetism in (C, Si, Ge and Sn) doped alkaline-earth sulfides: A first principles approach

The electronic structure and magnetic properties of AeS1−xMx (Ae=Mg, Ca and Sr; M=C, Si, Ge and Sn; x=0.125, 0.25, 0.50 and 1.00) in the rock salt structure was studied using full potential linear augmented plane wave (FP-LAPW) method within the spin density functional theory. The calculation reveals that the non-magnetic dopants can induce stable ferromagnetic state in alkaline-earth sulfides. Also it is found that the alkaline-earth sulfides exhibit half-metallicity with an integer magnetic moment of 2.00μB/f.u., with group IV elements as dopants (except for Si, Ge and Sn in MgS). The calculated results show that the minority spin electrons are metallic in nature, while the majority spin electrons show an energy gap around the Fermi level. It also found that the HM gap decreases with the increase of atomic size and concentration of the dopant atoms. The half-metallicity originates from the spin-polarized np like states of dopant atoms in these compounds.

Ab initio study of symmetrical tilt grain boundaries in bcc Fe: structural units, magnetic moments, interfacial bonding, local energy and local stress

We present first-principle calculations on symmetric tilt grain boundaries (GBs) in bcc Fe. Using density functional theory (DFT), we studied the structural, electronic and magnetic properties of Σ3(111) and Σ11(332) GBs formed by rotation around the [110] axis. The optimized structures, GB energies and GB excess free volumes are consistent with previous DFT and classical simulation studies. The GB configurations can be interpreted by the structural unit model as given by Nakashima and Takeuchi (2000 ISIJ 86 357). Both the GBs are composed of similar structural units of three- and five-membered rings with different densities at the interface according to the rotation angle. The interface atoms with larger atomic volumes reveal higher magnetic moments than the bulk value, while the interface atoms with shorter bond lengths have reduced magnetic moments in each GB. The charge density and local density of states reveal that the interface bonds with short bond lengths have more covalent nature, where minority-spin electrons play a dominant role as the typical nature of ferromagnetic Fe. In order to understand the structural stability of these GBs, we calculated the local energy and local stress for each atomic region using the scheme of Shiihara et al (2010 Phys. Rev. B 81 075441). In each GB, the interface atoms with larger atomic volumes and enhanced magnetic moments reveal larger local energy increase and tensile stress. The interface atoms constituting more covalent-like bonds with reduced magnetic moments have lower local energy increase, contributing to the stabilization, while compressive stress is generated at these atoms. The relative stability between the two GBs can be understood by the local energies at the structural units. The local energy and local stress analysis is a powerful tool to investigate the structural properties of GBs based on the behavior of valence electrons.

Structural, electronic and magnetic properties of quaternary half‐metallic Heusler alloy CoFeCrAl

AbstractWe have investigated the structural, electronic and magnetic properties in quaternary CoFeCrAl Heusler alloy. Rietveld refinement and Mössbauer spectroscopy of samples quenched from 973 K show absence of the A2 phase and presence of a highly ordered B2‐type structure. Full‐potential linearized augmented plane wave calculations using the experimental lattice constant show that there is an energy gap of 0.41 eV around the Fermi level in the bands for minority spin electrons. Partial substitution of Co by Fe in Co2CrAl thus brings excellent structural ordering while retaining the high Curie temperature, spontaneous magnetization and half‐metallicity necessary for application as spin valves. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab initio calculation of spin-dependent electron–phonon coupling in iron and cobalt

The spin-dependent coupling between electrons and phonons in ferromagnetic Fe and Co is calculated from first principles in a collinear-spin formalism. The added spin polarization is fundamental for the correct representation of the phonons, but also for obtaining good transport properties, and permits decomposition (e.g. of the resistivity) into the contributions of majority and minority spin. In Fe the minority spin coupling is only about 50% more important, but in Co the coupling between phonons and minority spin electrons is an order of magnitude larger than that with the majority spin, and both are strongly anisotropic.

Structural, electronic, magnetic and spin dependent transport properties of Fe/CaS/Fe (001) heterostructures

Structural, electronic, and magnetic properties of Fe/CaS (001) interfaces and Fe/CaS/Fe (001) heterostructures have been studied by means of a self-consistent Green's function technique for surface and interfaces implemented within the tight-binding linear muffin-tin orbital formalism. Spin dependent transport properties of the Fe/CaS/Fe (001) tunnel junctions with thin and intermediate barriers, in the current-perpendicular-to-plane geometry, have been determined by means of Kubo-Landauer approach implemented within the tight-binding linear muffin-tin orbital formalism. A small charge rearrangement is evidenced at the Fe/CaS (001) interfaces. The iron interfacial magnetic moments are enhanced over the bulk value. A small exchange coupling with the sign depending on the Fe/CaS (001) interface geometric structure and the strength decaying exponentially with the barrier is evidenced. Interfacial charge transfer, interface iron magnetic moments, and tunneling currents are sensitive to the interfacial structure. Interface resonant states have a decisive role in the tunneling process and the main contribution to the current in the ferromagnetic state of the junction is given by the minority-spin electrons.

Spin-filtering efficiency of ferrimagnetic spinels CoFe2O4and NiFe2O4

We assess the potential of the ferrimagnetic spinel ferrites CoFe2O4 and NiFe2O4 to act as spin filtering barriers in magnetic tunnel junctions. Our study is based on the electronic structure calculated by means of first-principles density functional theory within different approximations for the exchange correlation energy. We show that, in agreement with previous calculations, the densities of states suggest a lower tunneling barrier for minority spin electrons, and thus a negative spin-filter effect. However, a more detailed analysis based on the complex band-structure reveals that both signs for the spin-filtering efficiency are possible, depending on the band alignment between the electrode and the barrier materials and depending on the specific wave-function symmetry of the relevant bands within the electrode.