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The total angular momentum of a closed system is a conserved quantity, which should remain constant in time for any excitation experiment once the pumping signal has been extinguished. Such conservation, however, is never satisfied in practice in any real-time first-principles description of the demagnetization process. Furthermore, there is growing experimental evidence that the same takes place in experiments. The missing angular momentum is usually associated to lattice vibrations, which are not measured experimentally and are never considered in real-time simulations. Here we critically analyze the issue and conclude that current state-of-the-art simulations violate angular momentum conservation already at the electronic level of description. This shortcoming originates from an oversimplified description of the spin-orbit coupling, which includes atomic contributions but neglects completely that of itinerant electrons. We corroborate our findings with time-dependent simulations using model tight-binding Hamiltonians, and show that indeed such conservation can be reintroduced by an appropriate choice of spin-orbit coupling. The consequences of our findings on recent experiments are also discussed.

In the reset process of phase-change memories, the active material is brought rapidly above its melting temperature by Joule heating. Atomic migration in the liquid state due to the high electric field can lead to alloy demixing and eventually to device failure. The electromigration force F responsible for ionic migration is proportional to the electric field E, via the effective charge Z∗ (F = eZ*E, where e is electron charge). The determination of Z∗ is thus of great relevance for the electrothermal modeling of devices. We show that a direct first-principles calculation of the effective charges in metallic liquids is possible by computing the atomic forces in the presence of both an electric field and an electronic current within a nonequilibrium Green's function method based on density functional theory. We present results on the effective charges for GeTe and Sb2Te3 in their liquid state obtained with a calculation including wind forces.

Static (fixed-boundary) protected areas are key ocean conservation strategies, and marine higher predator distribution data can play a leading role toward identifying areas for conservation action. The Falkland Islands are a globally significant site for colonial breeding marine higher predators (i.e., seabirds and pinnipeds). However, overlap between marine predators and Falkland Islands proposed Marine Managed Areas (MMAs) has not been quantified. Hence, to provide information required to make informed decisions regarding the implementation of proposed MMAs, our aims were to objectively assess how the proposed MMA network overlaps with contemporary estimates of marine predator distribution. We collated tracking data (1999–2019) and used a combination of kernel density estimation and model-based predictions of spatial usage to quantify overlap between colonial breeding marine predators and proposed Falkland Islands MMAs. We also identified potential IUCN Key Biodiversity Areas (pKBAs) using (1) kernel density based methods originally designed to identify Important Bird and Biodiversity Areas (IBAs) and (2) habitat preference models. The proposed inshore MMA, which extends three nautical miles from the Falkland Islands, overlapped extensively with areas used by colonial breeding marine predators. This reflects breeding colonies being distributed throughout the Falklands archipelago, and use being high adjacent to colonies due to central-place foraging constraints. Up to 45% of pKBAs identified via kernel density estimation were located within the proposed MMAs. In particular, the proposed Jason Islands Group MMA overlapped with pKBAs for three marine predator species, suggesting it is a KBA hot spot. However, tracking data coverage was incomplete, which biased pKBAs identified using kernel density methods, to colonies tracked. Moreover, delineation of pKBA boundaries were sensitive to the choice of smoothing parameter used in kernel density estimation. Delineation b

Static (fixed-boundary) protected areas are key ocean conservation strategies, and marine higher predator distribution data can play a leading role toward identifying areas for conservation action. The Falkland Islands are a globally significant site for colonial breeding marine higher predators (i.e., seabirds and pinnipeds). However, overlap between marine predators and Falkland Islands proposed Marine Managed Areas (MMAs) has not been quantified. Hence, to provide information required to make informed decisions regarding the implementation of proposed MMAs, our aims were to objectively assess how the proposed MMA network overlaps with contemporary estimates of marine predator distribution. We collated tracking data (1999–2019) and used a combination of kernel density estimation and model-based predictions of spatial usage to quantify overlap between colonial breeding marine predators and proposed Falkland Islands MMAs. We also identified potential IUCN Key Biodiversity Areas (pKBAs) using (1) kernel density based methods originally designed to identify Important Bird and Biodiversity Areas (IBAs) and (2) habitat preference models. The proposed inshore MMA, which extends three nautical miles from the Falkland Islands, overlapped extensively with areas used by colonial breeding marine predators. This reflects breeding colonies being distributed throughout the Falklands archipelago, and use being high adjacent to colonies due to central-place foraging constraints. Up to 45% of pKBAs identified via kernel density estimation were located within the proposed MMAs. In particular, the proposed Jason Islands Group MMA overlapped with pKBAs for three marine predator species, suggesting it is a KBA hot spot. However, tracking data coverage was incomplete, which biased pKBAs identified using kernel density methods, to colonies tracked. Moreover, delineation of pKBA boundaries were sensitive to the choice of smoothing parameter used in kernel density estimation. Delineation b

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

A central paradigm in conservation biology is that population bottlenecks reduce genetic diversity and population viability. In an era of biodiversity loss and climate change, understanding the determinants and consequences of bottlenecks is therefore an important challenge. However, as most studies focus on single species, the multitude of potential drivers and the consequences of bottlenecks remain elusive. Here, we combined genetic data from over 11,000 individuals of 30 pinniped species with demographic, ecological and life history data to evaluate the consequences of commercial exploitation by 18th and 19th century sealers. We show that around one third of these species exhibit strong signatures of recent population declines. Bottleneck strength is associated with breeding habitat and mating system variation, and together with global abundance explains much of the variation in genetic diversity across species. Overall, bottleneck intensity is unrelated to IUCN status, although the three most heavily bottlenecked species are endangered. Our study reveals an unforeseen interplay between human exploitation, animal biology, demographic declines and genetic diversity.

Using first-principles calculations, we study the electronic and transport properties of rutheniumterpyridine molecules sandwiched between two Au(111) electrodes. We analyse both single and packed molecular devices, more amenable to scaling and realistic integration approaches. The devices display all together robust negative differential resistance features at low bias voltages. Remarkably, the electrical control of the spin transport in the studied systems implies a subtle distribution of the magnetisation density within the biased devices and highlights the key role of the Au(111) electrical contacts.

The data base for this study is represented by 46 DSDP-ODP drillsites that penetrated the sediments of the various Mediterranean basins for several hundred meters. Sedimentation rates were calculated for three discrete, well defined stratigraphic intervals extending from 5.33 Ma to time zero. A great dispersal of values, ranging from less than 5 up to 30 cm/1000 y is recorded in the Pleistocene, with maxima measured in the Nile cone, as a result of a high sediment supply from the longest river in the world, and in the Marsili basin, the deepest and youngest subbasin of the rapidly subsiding Tyrrhenian back-arc basin. In the late Pliocene the values recorded on structural highs are approximately half of those from basinal settings, whereas drillsites on basin margin often display depositional gaps. Starved basin conditions characterize the early Pliocene from 5.33 to 3.9 Ma, where sedimentation rates are consistently low, both on highs and on lows. We interpret this as a response to the rapid sea-level rise following the Messinian «salinity crisis» (Pliocene transgression). During the substantial draw-down when the water level in the playa lakes was two to three kilometers lower than world-wide sea-level, the major tributaries of the Mediterranean overincised their riverbed, and sedimentary aprons were formed in the deep basins. No sediment supply was available soon after the rapid sea-level rise, and approximately one million years elapsed before a new equilibrium was re-estabilished.

We present a time-dependent study of electron transport through a strongly correlated quantum dot, which combines adiabatic lattice density functional theory in the Bethe ansatz local-density approximation (BALDA) to the Hubbard model, with the multiple-probe battery method for open-boundary simulations in the time domain. In agreement with the recently proposed dynamical picture of Coulomb blockade, a characteristic driven regime, defined by regular current oscillations, is demonstrated for a certain range of bias voltages. We further investigate the effects of systematically improving the approximation for the electron–electron interaction at the dot site (going from non-interacting, through Hartree-only to adiabatic BALDA) on the transmission spectrum and the I–V characteristics. In particular, a negative differential conductance is obtained at large bias voltages and large Coulomb interaction strengths. This is attributed to the combined effect of the electron–electron interaction at the dot and the finite bandwidth of the electrodes.

We investigate the possibility of the electrical control of spin transfer in monoatomic chains incorporating spin impurities. Our theoretical framework is the mixed quantum-classical (Ehrenfest) description of the spin dynamics, in the spirit of thes-d model, where the itinerant electrons are described by a tight-binding model while localized spins are treated classically. Our main focus is on the dynamical exchange interaction between two well-separated spins. This can be quantified by the transfer of excitations in the form of transverse spin oscillations. We systematically study the effect of an electrostatic gate bias Vg on the interconnecting channel and we map out the long-range dynamical spin transfer as a function of Vg. We identify regions of Vg giving rise to significant amplification of the spin transmission at low frequencies and relate this to the electronic structure of the channel.

class of molecular ribbons, with almost-ideal charge transmission, that is weakly dependent on the anchoring structure or electrode crystalline orientation and easy to synthesize has been identified. Charge transport through two sets of aromatic nanoribbons, based on the pyrene and perylene motifs, has been investigated using density functional theory combined with the nonequilibrium Green's function method. The effects of wire length and multiple terminal thiolate groups at the junction with gold leads have been examined. For the oligopyrene series, an exponential drop in the conductance with the increase of the wire length is found. In contrast, the oligoperylene series of nanoribbons, with dual thiolate groups, exhibits no visible length dependence, indicating that the contacts are the principal source of the resistance. Between the Au(001) leads, the transmission spectra of the oligoperylenes display a continuum of highly conducting channels and the resulting conductance is nearly independent of the bias. The predictions are robust against artefacts from the exchange-correlation potential, as evidenced from the self-interaction corrected calculations. Therefore, oligoperylene nanoribbons show the potential to be the almost-ideal wires for molecular circuitry.

Ab initio computational methods for electronic transport in nanoscaled systems are an invaluable tool for the design of quantum devices. We have developed a flexible and efficient algorithm for evaluating I-V characteristics of atomic junctions, which integrates the nonequilibrium Green's function method with density functional theory. This is currently implemented in the package SMEAGOL. The heart of SMEAGOL is our scheme for constructing the surface Green's functions describing the current-voltage probes. It consists of a direct summation of both open and closed scattering channels together with a regularization procedure of the Hamiltonian and provides great improvements over standard recursive methods. In particular it allows us to tackle material systems with complicated electronic structures, such as magnetic transition metals. Here we present a detailed description of SMEAGOL together with an extensive range of applications relevant for the two burgeoning fields of spin and molecular electronics.

Ab initio computational methods for electronic transport in nanoscaled systems are an invaluable tool for the design of quantum devices. We have developed a flexible and efficient algorithm for evaluating I-V characteristics of atomic junctions, which integrates the nonequilibrium Green's function method with density functional theory. This is currently implemented in the package SMEAGOL. The heart of SMEAGOL is our scheme for constructing the surface Green's functions describing the current-voltage probes. It consists of a direct summation of both open and closed scattering channels together with a regularization procedure of the Hamiltonian and provides great improvements over standard recursive methods. In particular it allows us to tackle material systems with complicated electronic structures, such as magnetic transition metals. Here we present a detailed description of SMEAGOL together with an extensive range of applications relevant for the two burgeoning fields of spin and molecular electronics.

We report a detailed theoretical study of the bonding and conduction properties of an hydrogen molecule joining either platinum or palladium electrodes. We show that an atomic arrangement where the molecule is placed perpendicular to the electrodes is unstable for all distances between electrodes. In contrast, the configuration where the molecule bridges the electrodes is stable in a wide range of distances. In this last case the bonding state of the molecule does not hybridize with the leads and remains localized within the junction. As a result, this state does not transmit charge so that electronic transport is carried only through the antibonding state. This fact leads to conductances of 1G(0) at most, where G(0)=2e(2)/h. We indeed find that G is equal to 0.9 and 0.6G(0) for Pt and Pd contacts, respectively.

A critical analysis of point-contact Andreev reflection (PCAR) in ferromagnet/superconductor ballistic junctions is presented, based on tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. By accurately modeling the band structure of the bulk materials, we show that to reproduce the measured differential conductance of Cu/Pb and Co/Pb nanocontacts(1,2) one needs a detailed understanding of interface parameters which goes beyond simple averages of bulk matrix elements. As an example of how such a modification of surface parameters can affect spin-polarized transport, we explore the effect of an enhancement of the surface magnetization and show that this can significantly improve agreement with experiment. We also demonstrate that in general the spin polarization P of the junction may be different from the bulk polarization 15 of the magnetic metal used. For bulk Co we find (P) over bar = -0.4, whereas for the Co/Pb interface P = +0.4 and for the Co/Ir interface P = -0.01.

We report a detailed theoretical study of the bonding and conduction properties of an hydrogen molecule joining either platinum or palladium electrodes. We show that an atomic arrangement where the molecule is placed perpendicular to the electrodes is unstable for all distances between electrodes. In contrast, the configuration where the molecule bridges the electrodes is stable in a wide range of distances. In this last case the bonding state of the molecule does not hybridize with the leads and remains localized within the junction. As a result, this state does not transmit charge so that electronic transport is carried only through the antibonding state. This fact leads to conductances of 1G(0) at most, where G(0)=2e(2)/h. We indeed find that G is equal to 0.9 and 0.6G(0) for Pt and Pd contacts, respectively.

Using first principles simulations we perform a detailed study of the structural, electronic, and transport properties of monatomic platinum chains, sandwiched between platinum electrodes. First, we demonstrate that the most stable atomic configuration corresponds to a zigzag arrangement that gradually straightens as the chains are stretched. Second, we find that the averaged conductance shows slight parity oscillations with the number n of atoms in the chain. Additionally, the conductance of chains of fixed n oscillates as the end atoms are pulled apart, due to the gradual closing and opening of conductance channels as the chain straightens.

A critical analysis of point-contact Andreev reflection (PCAR) in ferromagnet/superconductor ballistic junctions is presented, based on tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. By accurately modeling the band structure of the bulk materials, we show that to reproduce the measured differential conductance of Cu/Pb and Co/Pb nanocontacts(1,2) one needs a detailed understanding of interface parameters which goes beyond simple averages of bulk matrix elements. As an example of how such a modification of surface parameters can affect spin-polarized transport, we explore the effect of an enhancement of the surface magnetization and show that this can significantly improve agreement with experiment. We also demonstrate that in general the spin polarization P of the junction may be different from the bulk polarization 15 of the magnetic metal used. For bulk Co we find (P) over bar = -0.4, whereas for the Co/Pb interface P = +0.4 and for the Co/Ir interface P = -0.01.

Using first principles simulations we perform a detailed study of the structural, electronic, and transport properties of monatomic platinum chains, sandwiched between platinum electrodes. First, we demonstrate that the most stable atomic configuration corresponds to a zigzag arrangement that gradually straightens as the chains are stretched. Second, we find that the averaged conductance shows slight parity oscillations with the number n of atoms in the chain. Additionally, the conductance of chains of fixed n oscillates as the end atoms are pulled apart, due to the gradual closing and opening of conductance channels as the chain straightens.

We study spin-dependent electronic transport across ferromagnet/superconductor ballistic junctions modeled using tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. The first result of this paper is that, by accurately modeling the band structure of the bulk materials, one can reproduce the measured differential conductance of Cu/Pb nanocontacts(1,2). In contrast the differential conductance of Co/Pb contacts can only be reproduced if an enhanced magnetic moment is present at the interface. The second result concerns the reliability of a method proposed in Refs. 1-3 for determining the degree of polarization of a ferromagnet. By fitting the material-specific differential conductance curves to curves calculated using a single-band model we show that this method does not yield reliable values for polarization and spin-dependent transmission.

We study the giant magnetoresistance (GMR) ratio in magnetic multilayers wit a single superconducting contact in the presence of spin-mixing processes. It has been recently shown [F. Taddei, S. Sanvito, J.H. Jefferson, and C. J. Lambert, Phys, Rev. Lett, 82, 4938 (1999)] that the GMR ratio of magnetic multilayers is strongly suppressed by the presence of a super conducting contact when spin flipping is not allowed. In this paper we demonstrate that the GMR ratio can be dramatically enhanced by spin-orbit interaction and/or noncollinear magnetic moments. The system is described using a tight-binding model with either s-p-d or s-d atomic orbitals per site.

We study spin-dependent electronic transport across ferromagnet/superconductor ballistic junctions modeled using tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. The first result of this paper is that, by accurately modeling the band structure of the bulk materials, one can reproduce the measured differential conductance of Cu/Pb nanocontacts(1,2). In contrast the differential conductance of Co/Pb contacts can only be reproduced if an enhanced magnetic moment is present at the interface. The second result concerns the reliability of a method proposed in Refs. 1-3 for determining the degree of polarization of a ferromagnet. By fitting the material-specific differential conductance curves to curves calculated using a single-band model we show that this method does not yield reliable values for polarization and spin-dependent transmission.

We study the giant magnetoresistance (GMR) ratio in magnetic multilayers wit a single superconducting contact in the presence of spin-mixing processes. It has been recently shown [F. Taddei, S. Sanvito, J.H. Jefferson, and C. J. Lambert, Phys, Rev. Lett, 82, 4938 (1999)] that the GMR ratio of magnetic multilayers is strongly suppressed by the presence of a super conducting contact when spin flipping is not allowed. In this paper we demonstrate that the GMR ratio can be dramatically enhanced by spin-orbit interaction and/or noncollinear magnetic moments. The system is described using a tight-binding model with either s-p-d or s-d atomic orbitals per site.

We examine transport properties of a magnetic superlattice with current perpendicular to the planes. In the limit that the phase-breaking and spin flip scattering lengths are greater than the system size, a multiple-scattering approach is used to calculate the 4-probe conductance. We show that by tuning the strength of tunnelling barriers placed between the current and voltage probes magnetoresistance ratios of arbitrary strength and sign are achievable.

We examine transport properties of a magnetic superlattice with current perpendicular to the planes. In the limit that the phase-breaking and spin flip scattering lengths are greater than the system size, a multiple-scattering approach is used to calculate the 4-probe conductance. We show that by tuning the strength of tunnelling barriers placed between the current and voltage probes magnetoresistance ratios of arbitrary strength and sign are achievable.

We study the effect on CPP GMR of changing the order of the layers in a multilayer. Using a tight-binding simple cubic two band model (s-d), magneto-transport properties are calculated in the zero-temperature, zero-bias limit, within the Landauer-Büttiker formalism. We demonstrate that for layers of different thicknesses formed from a single magnetic metal and multilayers formed from two magnetic metals, the GMR ratio and its dependence on disorder is sensitive to the order of the layers. This effect disappears in the limit of large disorder, where the results of the widely-used Boltzmann approach to transport are restored.

Using a scattering technique based on a parametrized linear combination of atomic orbitals Hamiltonian, we calculate the ballistic quantum conductance of multiwall carbon nanotubes. We find that interwall interactions not only block some of the quantum conductance channels, but also redistribute the current nonuniformly over individual tubes across the structure. Our results provide a natural explanation for the unexpected integer and noninteger conductance values reported for multiwall nanotubes by Stefan Frank et al. [Stefan Frank et al., Science 280, 1744 (1998)].

Using a scattering technique based on a parametrized linear combination of atomic orbitals Hamiltonian, we calculate the ballistic quantum conductance of multiwall carbon nanotubes. We find that interwall interactions not only block some of the quantum conductance channels, but also redistribute the current nonuniformly over individual tubes across the structure. Our results provide a natural explanation for the unexpected integer and noninteger conductance values reported for multiwall nanotubes by Stefan Frank et al. [Stefan Frank et al., Science 280, 1744 (1998)].

We study the effect on CPP GMR of changing the order of the layers in a multilayer. Using a tight-binding simple cubic two band model (s-d), magneto-transport properties are calculated in the zero-temperature, zero-bias limit, within the Landauer-Büttiker formalism. We demonstrate that for layers of different thicknesses formed from a single magnetic metal and multilayers formed from two magnetic metals, the GMR ratio and its dependence on disorder is sensitive to the order of the layers. This effect disappears in the limit of large disorder, where the results of the widely-used Boltzmann approach to transport are restored.

We analyze the interplay between disorder and band structure in current perpendicular to the planes giant magnetoresistance (GMR). We consider finite magnetic multilayers attached to pure crystalline leads, described by a tight-binding simple cubic two-band model (s-d). Several models of disorder are considered, including random on-site potentials, lattice distortions, impurities, vacancies, and cross-section fluctuations. Magnetotransport properties are calculated in the zero-temperature zero-bias limit, within the Landauer-Buttiker formalism. Using a very efficient numerical scattering technique, we are able to perform simulations, over large length scales, and to investigate spin transport in the ballistic, diffusive, and localized regimes, as well as the crossover between them. The competition between disorder-induced mean-free-path reduction and disorder-induced spin asymmetry enhancement of the conductance highlights several different regimes of GMR. [S0163-1829(99)01534-9].

A general Green's-function technique for elastic spin-dependent transport calculations is presented, which (i) scales linearly with system size and (ii) allows straightforward application to general tight-binding Hamiltonians (spd in the present work). The method is applied to studies of conductance and giant magnetoresistance (GMR) of magnetic multilayers in current perpendicular to planes geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various nonmagnetic materials from the 3d, 4d, and 5d transition metal series. Realistic tight-binding models for them have been constructed with the use of density functional calculations. We have identified three qualitatively different cases which depend on whether or not the bands (densities of states) of a nonmagnetic metal (i) form an almost perfect match with one of spin subbands of the magnetic metal las in Cu/Co spin valves), (ii) have almost pure sp character at the Fermi level (e.g.; Ag), and (iii) have almost pure d character at the Fermi energy (e.g., Pd, Pt). The key parameters which give rise to a large GMR ratio turn out to be (i) a strong spin polarization of the magnetic metal, (ii) a large energy offset between the conduction band of the nonmagnetic metal and one of spin subbands of the magnetic metal, and (iii) strong interband scattering in one of spin subbands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either nonmagnetic or magnetic layers, as observed experimentally.

We present an extensive numerical study of the ballistic current perpendicular to the plane (CPP) GMR. Using an original technique to calculate the spin conductance in the ballistic regime we studied A/B superlattices attached to pure crystalline semi-infinite leads, with A either Ni or Co, and B various 3d, 4d and 5d transition metals, with an FCC structure. From this analysis the Co/Ag and Co/Cu systems possess the larger GMR ratios, and this is mainly due to (i) a large off-set between the Cu (Ag) band and the minority spin sub-band of Co, and (ii) strong interband scattering at the interfaces in the minority spin sub-band.

The ballistic conductance and the CPP GMR in metallic magnetic multilayers have been studied as a function of the multilayer composition. The conductance is calculated with an original technique which combines semi-infinite crystalline leads with finite superlattices, described by s-p-d tight-binding Hamiltonians. The results are compared with an effective mass model, which identifies the main features giving rise to high GMR ratios.

We predict that current perpendicular to the plane giant magnetoresistance (GMR) in a phase-coherent magnetic multilayer is suppressed when one of the contacts is superconducting. This is a consequence of a superconductivity-induced magnetoresistive effect, whereby the conductance of the ferromagnetically aligned state is drastically reduced by superconductivity. To demonstrate this effect, we compute the GMR ratio of clean (Cu/Co)(n)Cu and (Cu/Co)(n)Pb multilayers, described by an ab initio spd tight-binding Hamiltonian. By analyzing a simpler model with two orbitals per site, we also show that the suppression survives in the presence of elastic scattering by impurities.

We analyze the interplay between disorder and band structure in current perpendicular to the planes giant magnetoresistance (GMR). We consider finite magnetic multilayers attached to pure crystalline leads, described by a tight-binding simple cubic two-band model (s-d). Several models of disorder are considered, including random on-site potentials, lattice distortions, impurities, vacancies, and cross-section fluctuations. Magnetotransport properties are calculated in the zero-temperature zero-bias limit, within the Landauer-Buttiker formalism. Using a very efficient numerical scattering technique, we are able to perform simulations, over large length scales, and to investigate spin transport in the ballistic, diffusive, and localized regimes, as well as the crossover between them. The competition between disorder-induced mean-free-path reduction and disorder-induced spin asymmetry enhancement of the conductance highlights several different regimes of GMR. [S0163-1829(99)01534-9].

A general Green's-function technique for elastic spin-dependent transport calculations is presented, which (i) scales linearly with system size and (ii) allows straightforward application to general tight-binding Hamiltonians (spd in the present work). The method is applied to studies of conductance and giant magnetoresistance (GMR) of magnetic multilayers in current perpendicular to planes geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various nonmagnetic materials from the 3d, 4d, and 5d transition metal series. Realistic tight-binding models for them have been constructed with the use of density functional calculations. We have identified three qualitatively different cases which depend on whether or not the bands (densities of states) of a nonmagnetic metal (i) form an almost perfect match with one of spin subbands of the magnetic metal las in Cu/Co spin valves), (ii) have almost pure sp character at the Fermi level (e.g.; Ag), and (iii) have almost pure d character at the Fermi energy (e.g., Pd, Pt). The key parameters which give rise to a large GMR ratio turn out to be (i) a strong spin polarization of the magnetic metal, (ii) a large energy offset between the conduction band of the nonmagnetic metal and one of spin subbands of the magnetic metal, and (iii) strong interband scattering in one of spin subbands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either nonmagnetic or magnetic layers, as observed experimentally.

The ballistic conductance and the CPP GMR in metallic magnetic multilayers have been studied as a function of the multilayer composition. The conductance is calculated with an original technique which combines semi-infinite crystalline leads with finite superlattices, described by s-p-d tight-binding Hamiltonians. The results are compared with an effective mass model, which identifies the main features giving rise to high GMR ratios.

We predict that current perpendicular to the plane giant magnetoresistance (GMR) in a phase-coherent magnetic multilayer is suppressed when one of the contacts is superconducting. This is a consequence of a superconductivity-induced magnetoresistive effect, whereby the conductance of the ferromagnetically aligned state is drastically reduced by superconductivity. To demonstrate this effect, we compute the GMR ratio of clean (Cu/Co)(n)Cu and (Cu/Co)(n)Pb multilayers, described by an ab initio spd tight-binding Hamiltonian. By analyzing a simpler model with two orbitals per site, we also show that the suppression survives in the presence of elastic scattering by impurities.

We present an extensive numerical study of the ballistic current perpendicular to the plane (CPP) GMR. Using an original technique to calculate the spin conductance in the ballistic regime we studied A/B superlattices attached to pure crystalline semi-infinite leads, with A either Ni or Co, and B various 3d, 4d and 5d transition metals, with an FCC structure. From this analysis the Co/Ag and Co/Cu systems possess the larger GMR ratios, and this is mainly due to (i) a large off-set between the Cu (Ag) band and the minority spin sub-band of Co, and (ii) strong interband scattering at the interfaces in the minority spin sub-band.

We present a numerical study of conductance oscillations of transition metal multilayers as a function of layer thickness. Using a realistic material-specific tight-binding model, we show that for disorder-free layers with random thicknesses but clean interfaces, long-period oscillations in the conductance can occur, which are reminiscent of those found in structures exhibiting GMR. Using a heuristic effective mass model, we argue that these oscillations arise from beating between the Fermi wavevector and a class of wavevectors characteristic of the superlattice structure.

We present a numerical study of conductance oscillations of transition metal multilayers as a function of layer thickness. Using a realistic material-specific tight-binding model, we show that for disorder-free layers with random thicknesses but clean interfaces, long-period oscillations in the conductance can occur, which are reminiscent of those found in structures exhibiting GMR. Using a heuristic effective mass model, we argue that these oscillations arise from beating between the Fermi wavevector and a class of wavevectors characteristic of the superlattice structure.