Ana Maria Llois

Co-doped ceria: tendency towards ferromagnetism driven by oxygen vacancies

We perform an electronic structure study for cerium oxide homogeneously doped with cobalt impurities, focusing on the role played by oxygen vacancies and structural relaxation. By means of full-potential ab initio methods, we explore the possibility of ferromagnetism as observed in recent experiments. Our results indicate that oxygen vacancies seem to be crucial for the appearance of a ferromagnetic alignment among Co impurities, obtaining an increasing tendency towards ferromagnetism with growing vacancy concentration. However, the estimated couplings cannot explain the experimentally observed room-temperature ferromagnetism. In this systematic study, we draw relevant conclusions regarding the location of the oxygen vacancies and the magnetic couplings involved. In particular, we find that oxygen vacancies tend to nucleate in the neighborhood of Co impurities and we get a remarkably strong ferromagnetic coupling between Co atoms and the Ce(3+) neighboring ions. The calculated magnetic moments per cell depend on the degree of reduction, which could explain the wide spread in the magnetization values observed in the experiments.

Structural evolution of free Co cluster magnetism

Abstract We present a systematic study of the average magnetic moments of free Co N clusters having different geometries: hexahedral, octahedral and decahedral. The spin-polarized electronic structure is calculated with a parameterized Hubbard Hamiltonian with spd electrons within the unrestricted Hartree–Fock approximation, and spillover effects are considered. We compare our calculations with other theoretical results found in the literature and we comment on the existing experimental results.

I-V curves of Fe/MgO (001) single- and double-barrier tunnel junctions.

In this work, we calculate with ab initio methods the current-voltage characteristics for ideal single- and double-barrier Fe/MgO 001 magnetic tunnel junctions. The current is calculated in the phase-coherent limit by using the recently developed SMEAGOL code, combining the nonequilibrium Green function formalism with density-functional theory. In general we find that double-barrier junctions display a larger magnetoresistance, which decays with bias at a slower pace than their single-barrier counterparts. This is explained in terms of enhanced spin filtering from the middle Fe layer sandwiched in between the two MgO barriers. In addition, for double-barrier tunnel junctions, we find a well defined peak in the magnetoresistance at a voltage of V =0.1 V. This is the signature of resonant tunneling across a majority quantum well state. Our findings are discussed in relation to recent experiments.

Spin density wave instabilities in the NbS2 monolayer

In the present work, we study the magnetic properties of the NbS2 monolayer by first-principles calculations. The transition metal dichalcogenides (TMDC) are a family of laminar materials presenting exciting properties such as charge density waves (CDW), superconductivity and metal-insulating transitions among others. 2H-NbS2 is a particular case within the family, because it is the only one that is superconductor without exhibiting a CDW order. Although no long range magnetic order was experimentally observed in the TMDC, we show here that the single monolayer of NbS2 is on the verge of a spin density wave (SDW) phase. Our calculations indicate that a wave-like magnetic order is stabilized in the NbS2 monolayer in the presence of magnetic defects or within zig-zag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibilty and the corresponding nesting function of the clean NbS2 monolayer, showing that there are strong electronic instabilities at the same wavevector asociated with the calculated SDWs, also corresponding with one of the main nesting vectors of the Fermi surface. We conclude that the physical mechanism behind the spin-wave instabilities are the nesting properties, accentuated by the quasi 2D character of this system, and the rather strong Coulomb interactions of the 4d band of the Nb atom. We also estimate the amplitude of the spin-fluctuations and find that they are rather large, as expected for a system on the verge of a quantum critical transition.

Effects of vertex corrections on diagrammatic approximations applied to the study of transport through a quantum dot

In the present work, we calculate the conductance through a single quantum dot weakly coupled to metallic contacts. We use the spin-1/2 Anderson model to describe the quantum dot, while considering a finite Coulomb repulsion. We solve the interacting system using the non-crossing-approximation (NCA) and the one-crossing approximation (OCA). We obtain the linear response conductance as a function of temperature and energy position of the localized level. From the comparison of both approximations we extract the role of the vertex corrections, which are introduced in the OCA calculations and neglected in the NCA scheme. As a function of the energy position, we observe that the diagrams omitted within NCA are really important for appropriately describing transport phenomena in Kondo systems as well as in the mixed valence regime. On the other hand, as a function of temperature, the corrections introduced by OCA partly recover the universal scaling properties known from numerical approaches such as the Numerical Renormalization Group(NRG).

Combined impurity and band effects on the appearance of inverse giant magnetoresistance in Cu/Fe multilayers with Cr

We have studied the dependence of impurity vs band effects in the appearance of inverse giant magnetoresistance (IGMR) in Cu/Fe superlattices with Cr. Current in plane (CIP) and current perpendicular to the plane geometries are considered. For the calculation of the conductivities, we have used the linearized Boltzmann equation in the relaxation time approximation. Cr impurity effects are taken into account through the spin-dependent relaxation times and the band effects through the semiclassical velocities obtained from the local-density approximation calculated electronic structure. The larger the Cr/Fe hybridization strength, the bigger is the tendency towards IGMR. In particular, in CIP geometry roughness at these interfaces increases the IGMR range. The calculated giant magnetoresistance ratios have been compared with the experimental results. From this comparison we conclude that the experimental data can only be explained by taking into account Cr bands.

Calculation of the interface exchange coupling constants between Fe and FeF2-like fluorides

Abstract The interface exchange coupling between ferromagnetic (F) and antiferromagnetic (AF) materials is interesting in itself and has also attracted recent attention in relation to the exchange bias phenomenon. A major difficulty in developing a reliable exchange bias theory lies in the fact that both the F and AF interface characteristics (geometry and physical parameters) are hard to determine experimentally and complicated to estimate theoretically. We adopt in this paper two alternative interface configurations to obtain upper and lower bounds for the computed values of the exchange coupling across the interface between metallic Fe and insulating FeF 2 , derived on the basis of ab initio calculations implemented for a periodic supercell. Electronic structures and total energies were computed within density functional theory using the generalized gradient approximation for the exchange correlation potential. We expect the results obtained to be useful in model simulations with larger unit cells and non-collinear spins.

On the metallic behavior of Co clusters

The role of structure in the nonmetal–metal transition of Co clusters is investigated by performing calculations for different symmetries: hexahedral, octahedral and decahedral. This transition occurs when the density of states at the Fermi level exceeds 1/kBT and the discrete energy levels begin to form a quasi-continuous band. The electronic structure is calculated including spd orbitals and spillover effects in a Hubbard Hamiltonian solved within the unrestricted Hartree–Fock approximation. We find that in small clusters (N≤40) the metallic behavior is strongly related to the geometrical structure of the cluster. We compare our results with those coming out of a simple Friedels model.

Electronic and magnetic properties of Fe-Cu superlattices

The electronic and magnetic properties of Fe/Cu superlattices grown in the (111) direction, for which Fe takes the FCC structure, are studied with a parametrized tight-binding Hamiltonian. The splitting between majority and minority d bands is related to an effective exchange parameter J that we adjust to obtain within our model the bulk magnetization for BCC Fe. As the magnetization of bulk FCC Fe is very sensitive to this parameter we study the magnetic properties as a function of it. We find that the 2Fe/2Cu superlattice is possibly ferromagnetic and that 3Fe/3Cu has more than one stable state (paramagnetic, ferromagnetic or ferrimagnetic) with quite similar energies. As recent experimental results detect a small magnetic moment for this last superlattice, comparison with the calculations suggest that they are ferrimagnetic. Comparison with calculations for 2Fe and 3Fe FCC(111) free-standing slabs show that the 2Fe/2Cu and 3Fe/3Cu superlattices behave as low-dimensional systems, the effect of Cu being to slightly reduce the magnetic moment per atom of Fe with respect to a free-standing slab.

Kondo physics in a Ni impurity embedded in O-doped Au chains

By means of ab initio calculations we study the effect of O-doping of Au chains containing a nanocontact represented by a Ni atom as a magnetic impurity. In contrast to pure Au chains, we find that with a minimun O-doping the