Minority Spin Electrons Research Articles

Mössbauer and Computational Study of an N2-Bridged Diiron Diketiminate Complex: Parallel Alignment of the Iron Spins by Direct Antiferromagnetic Exchange with Activated Dinitrogen

This work reports Mössbauer and DFT studies of the diiron-N2 complex LMeFeNNFeLMe (L = beta-diketiminate), 1a. Complex 1a, formally diiron(I), has a system spin S = 3 with an isolated MS = +/-3 quasi-doublet as a ground state; the MS = +/-2 doublet is >100 cm-1 higher in energy. Complex 1a exhibits at 4.2 K a large, positive magnetic hyperfine field, Bint = +68.1 T, and an effective g value of 16 +/- 2 along the easy magnetization axis of the ground doublet; this value is significantly larger than the spin-only value (g = 12). These results have been rationalized by DFT calculations, which show that each Fe site donates significant electron density into the pi* orbitals of dinitrogen, resulting in a configuration best described as two high-spin FeII (Sa = Sb = 2) bridged by triplet N22- (Sc = 1). In this description the minority spin electron of each iron is accommodated by two nonbonding, closely spaced 3d orbitals, z2 and yz (z is perpendicular to the diketiminate planes, x is along the Fe...Fe vector). Spin-orbit coupling between these orbital states generates a large unquenched orbital momentum along the iron-iron vector. The S = 3 ground state of 1a results from strong antiferromagnetic direct exchange couplings of the Fe spins (Sa = Sb = 2) to the N22- spin (Sc = 1) and can be formulated as ((Sa,Sb)Sab = 4, Sc = 1), S = 3>; H = J(Sa + Sb).Sc with J approximately 3500 cm-1.

Residual Kondo effect in quantum dot coupled to half-metallic ferromagnets

We study the Kondo effect in a quantum dot coupled to half-metallic ferromagneticelectrodes in the regime of strong on-dot correlations. Using the equation of motiontechnique for non-equilibrium Green functions in the slave boson representation, we showthat the Kondo effect is not completely suppressed for anti-parallel magnetization of theleads. In the parallel configuration, there is no Kondo effect but there is an effect associatedwith elastic co-tunnelling which, in turn, leads to similar behaviour of the local (on-dot)density of states (LDOS) as the usual Kondo effect. Namely, the LDOS shows thetemperature-dependent resonance at the Fermi energy, which splits with the bias voltageand the magnetic field. Moreover, unlike for non-magnetic or not fully polarizedferromagnetic leads, only the minority spin electrons can form such resonance in thedensity of states. However, this resonance cannot be observed directly in the transportmeasurements, and we give some clues how to identify the effect in such systems.

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.

Negative spin polarization of theFe3O4∕γ−Al2O3interface measured by spin-resolved photoemission

We report on spin-resolved photoemission spectroscopy measurements at the interface between a $25\text{\ensuremath{-}}\mathrm{nm}$-thick ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}\phantom{\rule{0.3em}{0ex}}(111)$ thin film and a $2\text{\ensuremath{-}}\mathrm{nm}$-thick $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}(111)$ layer. The ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ layer remains stoichiometric even after being covered by $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and exhibits a negative spin polarization of $\ensuremath{\sim}\ensuremath{-}40%$, after remanence correction. This value should be considered as a lower bound for the spin polarization and demonstrates that ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ remains a spin polarized material even after incorporation in a bilayer.

Spin drift diffusion studies of magnetoresistance effects in current-perpendicular-to-plane spin valves with half-metallic insertions

We present a theoretical analysis of spin transport and magnetoresistance (MR) of a penta-layer device, consisting of a basic pseudo-spin-valve trilayer with half-metallic (HM) insertions. Our analysis is based on two models, i.e., the Fert-Valet equation in the limit of infinite spin relaxation length $\ensuremath{\lambda}$, and the spin-dependent drift-diffusion equation, with finite $\ensuremath{\lambda}$. In both cases, we found that MR decreases with increasing HM or ferromagnetic (FM) layer resistivity, when the spin polarization of the HM or FM material is lower than some critical value. We derived the critical value analytically, and explained the MR effect in terms of the relative contribution of FM and HM layers in providing spin-dependent scattering. More interestingly, the presence of spin relaxation causes MR to be always suppressed at high HM/FM resistivity, irrespective of the spin polarization of the HM/FM layers. This phenomenon may be explained by introducing an effective spin-flip resistance, which provides a less resistive path to minority spin electrons via spin flipping. Finally, we show the validity of our analysis in the presence of realistically long contact leads, provided the lead conductivity is sufficiently high. Thus, the insertion of the HM layers results in an interplay between the resistivities of the HM/FM and spacer layers, and the effects of spin relaxation, leading to a MR behavior which is hitherto undescribed.

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%.

Fermi surfaces and electronic structure of the Heusler alloyCo2TiSn

The electronic structure of the Heusler alloyCo2TiSn is investigated here, with particular attention paid to its potential as a half-metallic ferromagnet.Ab initio calculations are performed using a plane wave pseudopotential code in the frameworkof density functional theory. These accurate calculations are done with convergence tolerances of10−5 and 10−4 eV on the total energy and Fermi energy, respectively. The alloy is found not to be ahalf-metal. Minority spin electrons undergo distinctly hole-like dispersion at theΓ pointin k space while the majority spin bands are metallic with a multiply connected tube-like Fermisurface. Further, the computed minority band gap and spin polarization at the Fermi levelare larger when the calculation is performed using the generalized gradient approximation.

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).

Theoretical Investigation of Electric and Magnetic Properties of Benzene–Vanadium Sandwich Complex Chain

We investigate the electric and magnetic properties of a benzene–vanadium complex chain [V(C6H6)]∞. By performing first principles calculation based on the spin-polarized density functional theory, we find that this system shows a half metallic ferromagnetic behavior, i.e., majority-spin (spin-up) electrons have a semiconducting band gap, while minority-spin (spin-down) electrons are metallic. We suggest that this ferromagnetic order is due to a double-exchange mechanism.

Anionogenic Ferromagnets

Magnetism in molecules and solids is understood to originate from atoms in that part of the periodic table where a particular value of the angular momentum appears first (i.e., the 2p, 3d, and 4f series). In contrast to the many magnetic compounds containing transition metal or lanthanide atoms, ferromagnetism based on atoms from the 2p series is very rare. We report density functional calculations that show the existing compound rubidium sesquioxide is a ferromagnet with an estimated Curie temperature of 300 K, unprecedented in p-electron magnetism. The magnetic moment is carried by the anion. Rubidium sesquioxide is a conductor, but only for the minority spin electrons (a so-called "half-metal"). Half-metals play an important role in spintronics, that is, electronics that exploits the electron spin. Since the magnetic moment resides on a light element (oxygen), spin-orbit interactions are considerably reduced compared to other half-metals. Consequently spin relaxation is expected to be suppressed by up to 2 orders of magnitude in comparison with materials presently used in spintronics.

Spin-Dependent Electron Dynamics in Front of a Ferromagnetic Surface

Exchange splitting and dynamics of image-potential states in front of a 3 monolayer iron film on Cu(100) have been studied with time-, energy-, and spin-resolved bichromatic two-photon photoemission. For the first image-potential state n=1 we observe an exchange splitting of 56 +/- 10 meV and spin-dependent lifetimes of 16 +/- 2 fs for majority-spin and of 11 +/- 2 fs for minority-spin electrons, respectively. The time-resolved studies of both the population and the linewidth of image-potential states manifest that at the magnetic surface not only inelastic but also quasielastic scattering processes are spin dependent.

Role of interface bonding in spin-dependent tunneling (invited)

Measured positive values of the spin polarization of the tunneling current from 3d ferromagnetic metals are commonly explained by the dominant s-electron contribution based on symmetry considerations for bulk materials, ignoring the influence of the interfaces. In this work, three different models are considered which suggest that the spin polarization is primarily determined by the electronic and atomic structures of the ferromagnet/insulator interfaces rather than by the bulk properties. A simple tight-binding model demonstrates that the existence of interface states and their contribution to the tunneling current depend on the degree of hybridization between the orbitals on metal and insulator atoms. The decisive role of the interface bonding is further supported by considering spin-dependent tunneling from oxidized Co surfaces through vacuum and in Co∕Al2O3∕Co tunnel junctions within the first-principles Green’s-function approach. For the oxidized Co surface it is found that the Co–O bonding at the surface removes the conducting orbitals forming the bulk Bloch states from the Fermi level, creating an additional tunneling barrier for minority-spin electrons. For the Co∕Al2O3∕Co junctions, two types of the interface O atoms are distinguished: those which saturate Al bonds and those which are adsorbed by Co. The latter bind strongly to Co creating interface states which enhance the tunneling current in the majority-spin channel. In both cases, the spin polarization changes sign and becomes positive, evidencing the crucial role of the interface structure and bonding.

Electric and Magnetic Properties of Co-filled Carbon Nanotube

We investigate the electric and magnetic properties of a (3,3) single-walled carbon nanotube filled with a linear Co nanowire. We carry out first-principle calculations based on the spin-polarized density functional theory, and find that in the stable structure, it shows half metallic ferromagnetic behavior, i.e., the majority-spin electrons show metallic behavior while the minority-spin electrons have a semiconducting bandgap.

An ab-initio theoretical investigation of the soft-magnetic properties of permalloys

We study Ni 80 Fe 20 -based permalloys with the relativistic spin-polarized Korringa–Kohn–Rostoker electronic structure method. Treating the compositional disorder with the coherent potential approximation, we investigate how the magnetocrystalline anisotropy, K, and magnetostriction, λ , of Ni-rich Ni–Fe alloys vary with the addition of small amounts of non-magnetic transition metals, Cu and Mo. From our calculations we follow the trends in K and λ and find the compositions of Ni–Fe–Cu and Ni–Fe–Mo where both are near zero. These high permeability compositions of Ni–Fe–Cu and Ni–Fe–Mo match well with those discovered experimentally. We monitor the connection of the magnetic anisotropy with the number of minority spin electrons N ↓ . By raising N ↓ via artificially increasing the band-filling of Ni 80 Fe 20 , we are able to reproduce the key features that underpin the magnetic softening we find in the ternary alloys. The effect of band-filling on the dependence of magnetocrystalline anisotropy on atomic short-range order in Ni 80 Fe 20 is also studied. Our calculations, based on a static concentration wave theory, indicate that the susceptibility of the high permeability of the Ni–Fe–Cu and Ni–Fe–Mo alloys to their annealing conditions is also strongly dependent on the alloys’ compositions. An ideal soft magnet appears from these calculations.

Aspects of Aqueous Iron and Manganese (II/III) Self-Exchange Electron Transfer Reactions

Ab initio methods were applied to the calculation of the reorganization energy λ and the electronic coupling matrix element VAB for the outer-sphere Fe(OH2)6II/III and Mn(OH2)6II/III self-exchange electron transfer (ET) reactions. For the Fe case, we find an appreciable effect on VAB depending on whether the minority spin electron occupies the dxy orbital or a mixture of dxz/dyz orbitals in the FeII ion. While these two possible nearly isoenergetic electron accepting states alter the magnitude and distance dependence of VAB, they do not affect the internal reorganization energy λI to any significant level. The magnitude and distance dependence of VAB are found to be strongly dependent on encounter orientation, as expected. VAB values for corner-to-corner encounter orientations are substantially larger at any given ET distance considered than those for face-to-face encounter orientations. Values of the decay parameter β are in good agreement with well-accepted values. The adiabaticity criterion is tied to orientation and distance dependence of VAB.

Magnetic and transport properties of Fe V1− atom bridge constructed between an STM tip and a solid surface

We have investigated the magnetic and transport properties of an atom bridge made from magnetic materials, which is an atom-scale wire constructed between a scanning tunneling microscopy tip and a solid surface , with the aid of ab initio calculations. In the case of Fe β V 1− β alloy atom bridges, we have found that the value of the mean magnetic moment is similar to that of the corresponding alloy bulk, and the quantized conductance contribution from both the majority and minority spin electrons changes as β changes. These properties are different from the case of Fe 1− α Ni α alloy atom bridge.

Spin-dependent tunneling from clean and oxidized Co surfaces

Abstract Transmission through a sufficiently thick vacuum barrier is factorized in the product of two “surface transmission functions” and a vacuum decay factor. Based on this factorization, we study the spin polarization of the tunneling current from clean and oxidized (1 1 1) FCC Co surfaces through vacuum into Al. The conductance is calculated using the principal-layer Green's function approach within the tight-binding LMTO scheme. We find that for typical vacuum barrier thicknesses the tunneling current from the clean surface is dominated by minority-spin electrons. A monolayer of oxygen on top of the surface completely changes the shape of k || -resolved transmission and makes the tunneling current almost 100% majority-spin polarized.

Electronic stability of magnetic Fe/Co superlattices with monatomic layer alternation.

We report a surprising observation that the growth of the [Fe(1 ML)/Co(1 ML)](n) superlattice of L1(0) structure on Cu(100) is stable only up to six atomic layers (n=3), which cannot be rationalized by stress arguments. Instead, first-principles calculations reveal a transition from the L1(0) to the B2 structure due to the effect of dimensionality on the stability of the electronic structure of the superlattice. Whereas the majority-spin electrons are energetically insensitive to the layer thickness, the minority-spin electrons induce the transition at n=3.

Ferromagnetism and metallic state in digital (Ga,Mn)As heterostructures

We present an extensive density functional theory study of the electronic, magnetic and transport properties of GaAs and AlAs digital ferromagnetic heterostructures. These can be obtained by $\delta$-doping with Mn the GaAs layers of a GaAs/AlAs superlattice. Our analysis spans a range of Mn concentrations and considers the presence of compensating defects such as As antisites. In the defect-free case all the heterostructures studied present an half-metallic electronic structure. In contrast when As antisites are present the half-metallic state is destroyed and the heterostructures behave as dirty planar metals. In this case they show a large $p$-type metallic conductance in the Mn plane mainly due to majority spin electrons, and an $n$-type hopping-like conductance in the GaAs planes mainly due to minority spin electrons. This suggests that if the As antisites can be kept far from the Mn planes, spatial separation of the different spin currents can be achieved. Finally we show that in the case of AlAs/(Ga,Mn)As digital ferromagnetic heterostructures the AlAs/GaAs valence band offset produces an additional confining potential for the holes responsible for the ferromagnetism. Therefore the ferromagnetic coupling between the Mn ions becomes larger and more robust to the presence of As antisites.