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The recent emergence of van der Waals (vdW) ferromagnets has opened new opportunities for designing spintronic devices. We theoretically investigate the coherent spin-dependent transport properties of the vdW ferromagnet Fe$_4$GeTe$_2$, by using density functional theory combined with the non-equilibrium Green's functions method. We find that the conductance in the direction perpendicular to the layers is half-metallic, namely it is entirely spin-polarized, as a result of the material's electronic structure. This characteristic persists from bulk to single layer, even under significant bias voltages, and it is little affected by spin-orbit coupling and electron correlation. Motivated by this observation, we then investigate the tunnel magnetoresistance (TMR) effect in an magnetic tunnel junction, which comprises two Fe$_4$GeTe$_2$ layers separated by the vdW gap acting as insulating barrier. We predict a TMR ratio of almost 500\%, which can be further boosted by increasing the number of Fe$_4$GeTe$_2$ layers in the junction., Comment: 16 pages, 15 figures and 4 tables

In this work, we study the cobalt radical interface obtained by depositing a monolayer of a derivative of the Blatter radical on polycrystalline cobalt. By examining the occupied and unoccupied states at the interface, using soft X-ray techniques, and combining them with ab initio calculations, we can completely determine the electronic structure at the interface, simultaneously, on both the molecular and ferromagnetic sides, and thus obtain a full understanding of the magnetic properties at the interface. We find that the molecule is strongly hybridized with the surface. Changes in the core level spectra reflect the modification of the molecule and cobalt electronic structure induced by the bonding across the interface. This causes a decrease in the magnetic moments of the cobalt atoms bonded to the molecules that lose their radical character. Our method allows screening, beforehand, the organic ferromagnetic interfaces in view of potential applications in spintronic.

We describe how the spin Hall effect (SHE) can be studied from ab-initio by combining density functional theory with the non-equilibrium Green's functions technique for quantum transport into the so-called DFT+NEGF method. After laying down our theoretical approach in particular discussing how to compute charge and spin bond currents, DFT+NEGF calculations are carried out for ideal clean systems. In these the transport is ballistic and the linear response limit is met. The SHE emerges in a central region attached to two leads when we apply a bias voltage so that electrons are accelerated by a uniform electric field. As a result, we obtain a finite spin-Hall current and, by performing a scaling analysis with respect to the system size, we estimate the ballistic spin Hall conductivity (SHC). We consider 5d metals with fcc and bcc crystal structures, finding that the SHC exhibits a rough qualitative dependence on the d-band filling, and comment on these results in relation to existing literature. Finally, within the same DFT+NEGF approach, we also predict the appearance of a current-induced spin dipole moment inside the materials' unit cell and estimate its magnitude., Comment: 16 pages, 8 figures

Te is a naturally p-doped semiconductor with a chiral structure, where an electrical current causes the conduction electrons to become spin polarized parallel to the transport direction. In this paper, we present a comprehensive theoretical study of this effect by employing density functional theory (DFT) combined with the non-equilibrium Green's functions (NEGF) technique for quantum transport. We suggest that the spin polarization can quantitatively be estimated in terms of two complementary quantities, namely the non-equilibrium magnetic moments and the spin current density. The calculated magnetic moments are directly compared with the values from previous theoretical studies obtaining overall consistent results. On the other hand, the inspection of the spin current density provides insights of the magnetotransport properties of the material. Specifically, we predict that the resistance along a Te wire changes when an external magnetic field is applied parallel or antiparallel to the charge current direction. The computed magnetoresistance is however quite small (~ 0.025%). Finally, we show that the description of the current-induced spin polarization in terms of the spin current establishes a straightforward connection with the phenomenon called chiral-induced spin selectivity, recently observed in several nano-junctions.

We present the results of first-principles calculations based on density functional theory estimating the magnitude of the current-induced spin polarization (CISP) at the surfaces of the $5d$ transition metals with fcc and bcc crystal structures. We predict that the largest surface CISP occurs for W and Ta, whereas CISP is considerably weaker for Pt and Au surfaces. We then discuss how CISP emerges over a length scale equal to few atomic layers as opposed to the spin accumulation characteristic of the SHE, which is related to the materials' spin diffusion length. Finally, using our estimates for the CISP magnitude, we suggest that the spin density appearing near W surfaces in experiments is mostly due to CISP, whereas that at Pt surfaces stems from the Hall effect.

We study the modification of the magnetocrystalline anisotropy (MCA) of Co slabs induced by several different conjugated molecular overlayers, i.e., benzene, cyclooctatetraene, naphthalene, pyrene and coronene. We perform first-principles calculations based on Density Functional Theory and the magnetic force theorem. Our results indicate that molecular adsorption tends to favour a perpendicular MCA at surfaces. A detailed analysis of various atom-resolved quantities, accompanied by an elementary model, demonstrates that the underlying physical mechanism is related to the metal-molecule interfacial hybridization and, in particular, to the chemical bonding between the molecular C $p_z$ and the out-of-plane Co $d_{z^2}$ orbitals. This effect can be estimated from the orbital magnetic moment of the surface Co atoms, a microscopic observable accessible to both theory and experiments. As such, we suggest a way to directly assess the MCA modifications at molecule-decorated surfaces, overcoming the limitations of experimental studies that rely on fits of magnetization hysteresis loops. Finally, we also study the interface between Co and both C$_{60}$ and Alq$_3$, two molecules that find widespread use in organic spintronics. We show that the modification of the surface Co MCA is similar upon adsorption of these two molecules, thereby confirming the results of recent experiments., Comment: 10 figures in main text and 3 in the SM, 20 pages

We present the combination of Density Functional Theory (DFT) and Dynamical Mean Field Theory (DMFT) for computing the electron transmission through two-terminals nanoscale devices. The method is then applied to metallic junctions presenting alternating Cu and Co layers, which exhibit spin-dependent charge transport and giant magnetoresistance (GMR) effect. The calculations show that the coherent transmission through the $3d$ states is greatly suppressed by electron correlations. This is mainly due to the finite lifetime induced by the electron-electron interaction and is directly related to the imaginary part of the computed many-body DMFT self-energy. At the Fermi energy, where in accordance with the Fermi-liquid behavior the imaginary part of the self-energy vanishes, the suppression of the transmission is entirely due to the shifts of the energy spectrum induced by electron correlations. Based our results, we finally suggest that the GMR measured in Cu/Co heterostructures for electrons with energies about 1 eV above the Fermi energy is a clear manifestation of dynamical correlation effects.

We present a computational approach for electronically correlated metallic surfaces and interfaces, which combines Density Functional and Dynamical Mean Field Theory using a multi-orbital perturbative solver for the many-body problem. Our implementation is designed to describe ferromagnetic metallic thin films on a substrate. The performances are assessed in detail for a Fe monolayer on a W(110) substrate, a prototypical nanoscale magnetic system. Comparing our results to photoemission data, we find qualitative and quantitative improvements in the calculated spectral function with respect to the results of Density Functional Theory within the local spin density approximation. In particular, the spin-splitting of the $d$ states is drastically reduced and, at the same time, their spectral width becomes narrower. The method is therefore able to account for the main correlation effects in the system.

The existence of spin-currents in absence of any driving external fields is commonly considered an exotic phenomenon appearing only in quantum materials, such as topological insulators. We demonstrate instead that equilibrium spin currents are a rather general property of materials with non negligible spin-orbit coupling (SOC). Equilibrium spin currents can be present at the surfaces of a slab. Yet, we also propose the existence of global equilibrium spin currents, which are net bulk spin-currents along specific crystallographic directions of materials. Equilibrium spin currents are allowed by symmetry in a very broad class of systems having gyrotropic point groups. The physics behind equilibrium spin currents is uncovered by making an analogy between electronic systems with SOC and non-Abelian gauge theories. The electron spin can be seen as the analogous of the color degree of freedom and equilibrium spin currents can then be identified with diamagnetic color currents appearing as the response to an effective non-Abelian magnetic field generated by SOC. Equilibrium spin currents are not associated with spin transport and accumulation, but they should nonetheless be carefully taken into account when computing transport spin currents. We provide quantitative estimates of equilibrium spin currents for several systems, specifically metallic surfaces presenting Rashba-like surface states, nitride semiconducting nanostructures and bulk materials, such as the prototypical gyrotropic medium tellurium. In doing so, we also point out the limitations of model approaches showing that first-principles calculations are needed to obtain reliable predictions. We therefore use Density Functional Theory computing the so-called bond currents, which represent a powerful tool to understand the relation between equilibrium currents, electronic structure and crystal point group.

The understanding of the Physics underlying the performances of organic spin-valve devices is still incomplete. According to some recent models, spin transport takes place in an impurity band inside the fundamental gap of organic semiconductors. This seems to be confirmed by recent experiments performed with La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/AlO$_x$/Co devices. The reported results suggest a possible correlation between the magnetoresistance and the variable oxygen doping in the Alq$_3$ spacer. In this paper we investigate by means of first-principles calculations the electronic and magnetic properties of O$_2$ molecules and ions in Alq$_3$ films to establish whether oxygen plays any important role for spin transport in La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/AlO$_x$/Co devices. The conclusion is that it does not. In fact, we show that O$_2$ molecules do not form an impurity band and there is no magnetic interaction between them. In contrast, we suggest that spin-transport may be enabled by the direct exchange coupling between Alq$_3^-$ ions., Comment: 6 pages, 2 figures

Organic paramagnetic and electroactive molecules are attracting interest as core components of molecular electronic and spintronic devices. Currently, further progress is hindered by the modest stability and reproducibility of the molecule/electrode contact. We report the synthesis of a persistent organic radical bearing one and two terminal alkyne groups to form Au-C {\sigma} bonds. The formation and stability of self-assembled monolayers and the electron transport through single-molecule junctions at room temperature have been studied. The combined analysis of both systems demonstrates that this linker forms a robust covalent bond with gold and a better-defined contact when compared to traditional sulfur-based linkers. Density functional theory and quantum transport calculations support the experimental observation highlighting a reduced variability of conductance values for the C-Au based junction. Our findings advance the quest for robustness and reproducibility of devices based on electroactive molecules.

We present a computational method to quantitatively describe the linear-response conductance of nanoscale devices in the Kondo regime. This method relies on a projection scheme to extract an Anderson impurity model from the results of density functional theory and non-equilibrium Green's functions calculations. The Anderson impurity model is then solved by continuous time quantum Monte Carlo. The developed formalism allows us to separate the different contributions to the transport, including coherent or non-coherent transport channels, and also the quantum interference between impurity and background transmission. We apply the method to a scanning tunneling microscope setup for the 1,3,5-triphenyl-6-oxoverdazyl (TOV) stable radical molecule adsorbed on gold. The TOV molecule has one unpaired electron, which when brought in contact with metal electrodes behaves like a prototypical single Anderson impurity. We evaluate the Kondo temperature, the finite temperature spectral function and transport properties, finding good agreement with published experimental results., Comment: in press, Phys. Rev. B (2017)

The ability to sense the magnetic state of individual magnetic nano-objects is a key capability for powerful applications ranging from readout of ultra-dense magnetic memory to the measurement of spins in complex structures with nanometer precision. Magnetic nano-objects require extremely sensitive sensors and detection methods. Here we create an atomic spin sensor consisting of three Fe atoms and show that it can detect nanoscale antiferromagnets through minute surface-mediated magnetic interaction. Coupling, even to an object with no net spin and having vanishing dipolar stray field, modifies the transition matrix element between two spin states of the Fe-atom-based spin sensor that changes the sensor's spin relaxation time. The sensor can detect nanoscale antiferromagnets at up to three nanometers distance and achieves an energy resolution of 10 micro-electronvolts surpassing the thermal limit of conventional scanning probe spectroscopy. This scheme permits simultaneous sensing of multiple antiferromagnets with a single spin sensor integrated onto the surface., Comment: 30 pages main text, 6 figures, Supplementary materials not inculded

We investigate the effect of potential barriers in the form of step edges on the scattering properties of Bi$_2$Se$_3$(111) topological surface states by means of large-scale ab-initio transport simulations. Our results demonstrate the suppression of perfect backscattering, while all other scattering processes, which do not entail a complete spin and momentum reversal, are allowed. Furthermore, we find that the spin of the surface state develops an out of plane component as it traverses the barrier. Our calculations reveal the existence of quasi-bound states in the vicinity of the surface barriers, which appear in the form of an enhanced density of states in the energy window corresponding to the topological state. For double barriers we demonstrate the formation of quantum well states. To complement our first-principles results we construct a two-dimensional low-energy effective model and show that band bending plays a significant role in the scattering process. Our findings are discussed in the context of a number of recent experimental works., Comment: Updated text, published version

Spin crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge as one has to describe the spin ground state of a transition metal as the legand field changes. The problem is dominated by the interplay between strong electron correlation at the transition metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory we then assess the performances of density functional theory. We find that the low spin state is always over-stabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about 50% of exact-exchange.

We demonstrate that the critical temperature for valence tautomeric interconversion in Cobalt dioxolene complexes can be significantly changed when a static electric field is applied to the molecule. This is achieved by effectively manipulating the redox potential of the metallic acceptor forming the molecule. Importantly our accurate density functional theory calculations demonstrate that already a field of 0.1 V/nm, achievable in Stark spectroscopy experiments, can produce a change in the critical temperature for the interconversion of 20 K. Our results indicate a new way for switching on and off the magnetism in a magnetic molecule. This offers the unique chance of controlling magnetism at the atomic scale by electrical means.

The formation of the magnetic moment in C- and N-doped MgO is the result of a delicate interplay between Hund's coupling, hybridization and Jahn-Teller distortion. The balance depends on a number of environmental variables including electron doping. We investigate such a dependence by self-interaction corrected density functional theory and we find that the moment formation is robust with respect to electron doping. In contrast, the local symmetry around the dopant is more fragile and two different geometries can be stabilized. Crucially the magnetic moment is always extremely localized, making any carrier mediated picture of magnetism in d^0 magnets unlikely.