arXiv subjects

Stefano Sanvito

Publications and source records attributed to Stefano Sanvito.

At least 127 records · Page 7Linked to original sources

arXiv2012

Electronic Transport Through EuO Spin Filter Tunnel Junctions

Epitaxial spin filter tunnel junctions based on the ferromagnetic semiconductor europium monoxide, EuO, are investigated by means of density functional theory. In particular, we focus on the spin transport properties of Cu(100)/EuO(100)/Cu(100) junctions. The dependence of the transmission coefficient and the current-voltage curves on the interface spacing and on the EuO thickness is explained in terms of the EuO density of states and the complex band structure. Furthermore we also discuss the relation between the spin transport properties and the Cu-EuO interface geometry. The level alignment of the junction is sensitively affected by the interface spacing, since this determines the charge transfer between EuO and the Cu electrodes. Our calculations indicate that EuO epitaxially grown on Cu can act as a perfect spin filter, with a spin polarization of the current close to 100%, and with both the Eu-5d conduction band and the Eu-4f valence band states contributing to the coherent transport. For epitaxial EuO on Cu a symmetry filtering is observed, with the Δ_1 states dominating the transmission. This leads to a transport gap larger than the fundamental EuO band gap. Importantly the high spin polarization of the current is preserved up to large bias voltages.

cond-mat.mes-hall↗

arXiv2012

The ground state of a spin-crossover molecule calculated by diffusion Monte Carlo

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.

cond-mat.mtrl-sci↗

arXiv2012

Bias asymmetry in the conductance profile of magnetic ions on surfaces probed by scanning tunneling microscopy

The conductance profiles of magnetic transition metal atoms, such as Fe, Co and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nano-magnets. In general the profiles are symmetric with respect to the applied bias. However a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries here we expand our previously developed perturbative approach to electron-spin scattering to the spin- polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip we demonstrate that the asymmetries are driven by the non-equilibrium occupation of the various atomic spin-levels, an effect that reminds closely that electron spin-transfer. In contrast when the tip is not spin-polarized such non-equilibrium population cannot be build up. In this circumstance we propose that the asymmetry simply originates from the transition metal ion density of state, which is included here as a non-vanishing real component to the spin-scattering self-energy.

cond-mat.mes-hall↗

arXiv2012

Magnetism and Antiferroelectricity in MgB$_6$

We report on a density functional theory study demonstrating the coexistence of weak ferromagnetism and antiferroelectricity in boron-deficient MgB6. A boron vacancy produces an almost one dimensional extended molecular orbital, which is responsible for the magnetic moment formation. Then, long-range magnetic order can emerge from the overlap of such orbitals above percolation threshold. Although there is a finite density of states at the Fermi level, the localized nature of the charge density causes an inefficient electron screening. We find that the Mg ions can displace from the center of their cubic cage, thus generating electrical dipoles. In the ground state these order in an antiferroelectric configuration. If proved experimentally, this will be the first material without d or f electrons displaying the coexistence of magnetic and electric order.

cond-mat.mtrl-sci↗

arXiv2012

Coexistance of giant tunneling electroresistance and magnetoresistance in an all-oxide magnetic tunnel junction

We demonstrate with first-principles electron transport calculations that large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist in an all-oxide device. The TMR originates from the symmetry-driven spin filtering provided by the insulating BaTiO3 barrier to the electrons injected from SrRuO3. In contrast the TER is possible only when a thin SrTiO3 layer is intercalated at one of the SrRuO3/BaTiO3 interfaces. As the complex band-structure of SrTiO3 has the same symmetry than that of BaTiO3, the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO3 layer. The SrTiO3 thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.

cond-mat.mtrl-sci↗

arXiv2011

Effects of edge chemistry doping on graphene nanoribbon mobility

Doping of semiconductor is necessary for various device applications. Exploiting chemistry at its reactive edges was shown to be an effective way to dope an atomically thin graphene nanoribbon (GNR) for realizing new devices in recent experiments. The carrier mobility limited by edge doping is studied as a function of the GNR width, doping density, and carrier density by using ab initio density functional and parameterized tight binding simulations combined with the non-equilibrium Green's function formalism for quantum transport. The results indicate that for GNRs wider than about 4nm, the mobility scales approximately linearly with the GNR width, inversely proportional to the edge doping concentration and decreases for an increasing carrier density. For narrower GNRs, dependence of the mobility on the GNR width and carrier density can be qualitatively different.

cond-mat.mes-hall↗

arXiv2011

Current-induced energy barrier suppression for electromigration from first principles

We present an efficient method for evaluating current-induced forces in nanoscale junctions, which naturally integrates into the non-equilibrium Green's function formalism implemented within density functional theory. This allows us to perform dynamical atomic relaxation in the presence of an electric current while also evaluating the current-voltage characteristics. The central idea consists in expressing the system energy density matrix in terms of Green's functions. In order to validate our implementation we perform a series of benchmark calculations, both at zero and finite bias. Firstly we evaluate the current-induced forces acting over an Al nanowire and compare them with previously published results for fixed geometries. Then we perform structural relaxation of the same wires under bias and determine the critical voltage at which they break. We find that, while a perfectly straight wire does not break at any of the voltages considered, a zigzag wire is more fragile and snaps at 1.4 V, with the Al atoms moving against the electron flow. Finally we demonstrate the capability of our scheme to tackle the electromigration problem by studying the current-induced motion of a single Si atom covalently attached to the sidewall of a (4,4) armchair single-walled carbon nanotube. Our calculations indicate that if Si is attached along the current path, then current-induced forces can induce migration. In contrast, if the bonding site is away from the current path, then the adatom will remain stable regardless of the voltage. An analysis based on decomposing the total force into a wind and an electrostatic component, as well as on a detailed evaluation of the bond currents, shows that this remarkable electromigration phenomenon is due solely to the position-dependent wind force.

cond-mat.mes-hall↗

arXiv2011

Electrical control of spin dynamics in finite one-dimensional systems

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 the s-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 V_g on the interconnecting channel and we map out the long-range dynamical spin transfer as a function of V_g. We identify regions of V_g giving rise to significant amplification of the spin transmission at low frequencies and relate this to the electronic structure of the channel.

cond-mat.mes-hall↗

arXiv2011

A pertubative approach to the Kondo effect in magnetic atoms on nonmagnetic substrates

Recent experimental advances in scanning tunneling microscopy make the measurement of the conductance spectra of isolated and magnetically coupled atoms on nonmagnetic substrates possible. Notably these spectra are characterized by a competition between the Kondo effect and spin-flip inelastic electron tunneling. In particular they include Kondo resonances and a logarithmic enhancement of the conductance at voltages corresponding to magnetic excitations, two features that cannot be captured by second order perturbation theory in the electron-spin coupling. We have now derived a third order analytic expression for the electron-spin self-energy, which can be readily used in combination with the non-equilibrium Green's function scheme for electron transport at finite bias. We demonstrate that our method is capable of quantitative description the competition between Kondo resonances and spin-flip inelastic electron tunneling at a computational cost significantly lower than that of other approaches. The examples of Co and Fe on CuN are discussed in detail.

cond-mat.mes-hall↗

arXiv2011

Spin-flip inelastic electron tunneling spectroscopy in atomic chains

We present a theoretical study of the spin transport properties of mono-atomic magnetic chains with a focus on the spectroscopical features of the I-V curve associated to spin-flip processes. Our calculations are based on the s-d model for magnetism with the electron transport treated at the level of the non-equilibrium Green's function formalism. Inelastic spin-flip scattering processes are introduced perturbatively via the first Born approximation and an expression for the associated self-energy is derived. The computational method is then applied to describe the I-V characteristics and its derivatives of one dimensional chains of Mn atoms and the results are then compared to available experimental data. We find a qualitative and quantitative agreement between the calculated and the experimental conductance spectra. Significantly we are able to describe the relative intensities of the spin excitation features in the I-V curve, by means of a careful analysis of the spin transition selection rules associated to the atomic chains.

cond-mat.mes-hall↗

arXiv2011

Electric field control of valence tautomeric interconversion in Cobalt dioxolene

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.

cond-mat.mes-hall↗

arXiv2010

Large bias-dependent magnetoresistance in all-oxide magnetic tunnel junctions with a ferroelectric barrier

All-oxide magnetic tunnel junctions (MTJs) incorporating functional materials as insulating barriers have the potential of becoming the founding technology for novel multi-functional devices. We investigate, by first-principles density functional theory, the bias-dependent transport properties of an all-oxide SrRuO3/BaTiO3/SrRuO3 MTJ. This incorporates a BaTiO3 barrier which can be found either in a non-ferroic or in a ferroelectric state. In such an MTJ not only can the tunneling magnetoresistance reach enormous values, but also, for certain voltages, its sign can be changed by altering the barrier electric state. These findings pave the way for a new generation of electrically-controlled magnetic sensors.

cond-mat.mtrl-sci↗

arXiv2010

Magnetic interaction of Co ions near the {10\bar{1}0} ZnO surface

Co-doped ZnO is the prototypical dilute magnetic oxide showing many of the characteristics of ferromagnetism. The microscopic origin of the long range order however remains elusive, since the conventional mechanisms for the magnetic interaction, such as super-exchange and double exchange, fail either at the fundamental or at a quantitative level. Intriguingly, there is a growing evidence that defects both in point-like or extended form play a fundamental role in driving the magnetic order. Here we explore one of such possibilities by performing {\it ab initio} density functional theory calculations for the magnetic interaction of Co ions at or near a ZnO \{10$\bar{1}$0\} surface. We find that extended surface states can hybridize with the $e$-levels of Co and efficiently mediate the magnetic order, although such a mechanism is effective only for ions placed in the first few atomic planes near the surface. We also find that the magnetic anisotropy changes at the surface from an hard-axis easy-plane to an easy axis, with an associated increase of its magnitude. We then conclude that clusters with high densities of surfacial Co ions may display blocking temperatures much higher than in the bulk.

cond-mat.mtrl-sci↗

arXiv2009

The magnetism of wurtzite CoO nanoclusters

The possibility that the apparent room temperature ferromagnetism, often measured in Co-doped ZnO, is due to uncompensated spins at the surface of wurtzite CoO nanoclusters is investigated by means of a combination of density functional theory and Monte Carlo simulations. We find that the critical temperature extracted from the specific heat systematically drops as the cluster size is reduced, regardless of the particular cluster shape. Furthermore the presence of defects, in the form of missing magnetic sites, further reduces $T_\mathrm{C}$. This suggests that even a spinodal decomposed phase is unlikely to sustain room temperature ferromagnetism in ZnO:Co.

cond-mat.mes-hall↗

arXiv2009

Electron doping and magnetic moment formation in N- and C-doped MgO

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.

cond-mat.mtrl-sci↗

arXiv2009

Electron transport across electrically switchable magnetic molecules

We investigate the electron transport properties of a model magnetic molecule formed by two magnetic centers whose exchange coupling can be altered with a longitudinal electric field. In general we find a negative differential conductance at low temperatures originating from the different scattering amplitudes of the singlet and triplet states. More interestingly, when the molecule is strongly coupled to the leads and the potential drop at the magnetic centers is only weakly dependent on the magnetic configuration, we find that there is a critical voltage V_C at which the current becomes independent of the temperature. This corresponds to a peak in the low temperature current noise. In such limit we demonstrate that the quadratic current fluctuations are proportional to the product between the conductance fluctuations and the temperature.

cond-mat.mes-hall↗

arXiv2008

Resonant electronic states and I-V curves of Fe/MgO/Fe(100) tunnel junctions

The bias dependence of the tunnel magnetoresistance (TMR) of Fe/MgO/Fe tunnel junctions is investigated theoretically with a fully self-consistent scheme that combines the non-equilibrium Green's functions method with density functional theory. At voltages smaller than 20 mVolt the I-V characteristics and the TMR are dominated by resonant transport through narrow interface states in the minority spin-band. In the parallel configuration this contribution is quenched by a voltage comparable to the energy width of the interface state, whereas it persists at all voltages in the anti-parallel configuration. At higher bias the transport is mainly determined by the relative positions of the $Δ_1$ band-edges in the two Fe electrodes, which causes a decrease of the TMR.

cond-mat.mes-hall↗

arXiv2008

Magnetism of CoO polymorphs

A microscopic explanation for the room temperature ferromagnetism in diluted ZnO:Co is at present rather elusive. Although standard secondary phases can usually be ruled out, it is less clear whether regions with high Co concentration coexist with undoped portions of the film, i.e. whether some form of CoO polymorph can be responsible for the magnetic signal. Since X-ray usually excludes the presence of the native rock-salt phase, the study of CoO polymorphs becomes particularly interesting. In this work we investigate theoretically the magnetism of CoO in both the wurtzite and zincblende phases. By using a combination of density functional theory with the LDA+$U$ approximation and Monte Carlo simulations we demonstrate that wurtzite and zincblende CoO have a complex frustrated anti-ferromagnetic ground state with no net magnetic moment in the bulk. Most importantly the estimated critical temperatures are well below room temperature for both cases. This suggests that bulk CoO polymorphs are not responsible for the room temperature magnetism observed for ZnO:Co, although the role of clusters with uncompensated spins or arranged in a spinodal decomposed structure still remains an open question.

cond-mat.other↗