O. Navarro

Effect of disorder on the electronic structure of the double perovskite Sr2FeMoO6

Recently, Sr2FeMoO6 has been established as a new colossal magnetoresistance material with substantial low-field magnetoresistance at room temperature and has attracted much attention in the double perovskite family. This material always appears with a certain degree of miss-site disorder where Fe and Mo interchange their positions. Using renormalized perturbation expansion, we calculate the density of states and determine the variation of the critical temperature in the low disorder regime.

Effect of cationic disorder on the spin polarization in FeMo double perovskites

Ordered Sr2FeMoO6 is expected to have complete spin polarization, however all samples usually present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. It can be inferred that disorder is detrimental to the half-metallicity. We present an electronic approach of this disorder using a model based on a correlated electron picture with localized Fe-spins and conduction electrons interacting with the local spins via a double-exchange–type mechanism. This succeeds in stabilizing a ferromagnetic ground state in the absence of disorder. Disorder is treated within the dynamical mean-field approach which is equivalent to the coherent potential approximation. We shall show how electron disorder affects the density of states and the ground-state properties with a particular emphasis on the spin polarization.

Electronic structure of the ferromagnetic superconductor UCoGe from first principles

The superconductor UCoGe is analysed with electronic structure calculations using a linearized augmented plane wave method based on density functional theory. Ferromagnetic and antiferromagnetic calculations with and without correlations (via LDA+UH) were done. In this compound the Fermi level is situated in a region where the main contribution to DOS comes from the U-5f orbital. The magnetic moment is mainly due to the Co-3d orbital with a small contribution from the U-5f orbital. The possibility of fully non-collinear magnetism in this compound seems to be ruled out. These results are compared with the isostructural compound URhGe; in this case the magnetism comes mostly from the U-5f orbital.

Magnetic polaron structures in the one-dimensional double and super-exchange model

Abstract An analytical and numerical study of the one-dimensional double and super-exchange model is presented. A phase separation between ferromagnetic and anti-ferromagnetic phases occurs at low super-exchange interaction energy. When the super-exchange interaction energy gets larger, the conduction electrons are self-trapped within separate small magnetic polarons. These magnetic polarons contain a single electron inside two or three sites depending on the conduction electron density and form a Wigner crystallization. A new phase separation is found between these small polarons and the anti-ferromagnetic phase. Our results could explain the spin-glass-like behavior observed in the nickelate one-dimensional compound Y 2−xCaxBaNiO5.

Formation of spin-polarons in the ferromagnetic Kondo lattice model away from half-filling

Even though realistic one-dimensional experiments in the field of half-metallic semiconductors are not at hand yet, we are interested in the underlying fundamental physics. In this regard we study a one-dimensional ferromagnetic Kondo lattice model, a model in which a conduction band is coupled ferromagnetically to a background of localized d moments with coupling constant J(H), and investigate the T = 0 phase diagram as a function of the antiferromagnetic interaction J between the localized moments and the band-filling n, since it has been observed that doping of the compounds has led to formation of magnetic domains. We explore the spin-polaron formation by looking at the nearest-neighbour correlation functions in the spin and charge regimes for which we use the density matrix renormalization group method, which is a highly efficient method to investigate quasi-one-dimensional strongly correlated systems.

Monte Carlo study of the double and super-exchange model with lattice distortion

In this work a magneto-elastic phase transition was obtained in a linear chain due to the interplay between magnetism and lattice distortion in a double and super-exchange model. It is considered a linear chain consisting of localized classical spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins tend to align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by anti-ferromagnetic super-exchange interactions between neighbor localized spins. Additionally, lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. Phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.

Solid solution formation in the new cuprate system (Sr, K)1−x(Ca, Na)xCuO2±z (0≤x≤0.5)

Abstract Synthesis and some experimental results of the new cuprate system (Sr,K) 1− x (Ca,Na) x CuO 2± z (0.0≤ x ≤1.0) are reported for the first time. We prepared a series of polycrystalline samples of this system with x -steps of 0.1, by solid-state reaction method in air at ambient pressure below melt temperature. It is observed the formation of a solid solution in the range 0≤ x ≤0.5 at 750°C. The characterization of the samples by X-ray powder diffraction indicates that the single phase is isostructural to the one for Cu 2 SrO 3 compound, reported in the JCPDS-International Center for Diffraction Data (ICDD) file No. 39-0250, which has an orthorhombic unit cell. The compositions in the range 0.6≤ x ≤1.0 have an unknown phase and CuO . Further characterization studies by scanning electron microscopy, thermogravimetric and differential thermal analysis are shown.

Non-perturbative ground-state of a generalized Hubbard model

A generalized Hubbard model with on-site interactionU, inter-site interactionV, and general correlated hopping is studied using a mapping method. An exact solution of the problem of one hole and one doublyoccupied site moving in a ferromagnetic spin background is obtained. The mapping method used is based on mapping the original many-body problem onto an equivalent tight-binding one with impurities in a higher dimensional space.

Theoretical calculations of valence states in Fe-Mo compounds

The half-metallic ferromagnetic double perovskite compound Sr2FeMoO6 is considered as an important material for spintronic applications. It appears to be fundamental to understand the role of electronic parameters controlling the half-metallic ground state. Fe-Mo double perovskites usually present some degree of Fe/Mo disorder which generally increases with doping. In this work, we study the valence states of Fe-Mo cations in the off-stoichiometric system Sr2Fe1+xMo1−xO6 (−1 ≤ x ≤ 1/3) with disorder. Our results for Fe and Mo valence states are obtained using the Green functions and the renormalization perturbation expansion method. The model is based on a correlated electron picture with localized Fe-spins and conduction Mo-electrons interacting with the local spins via a double-exchange-type mechanism.

SPIN-POLARONS IN AN EXCHANGE MODEL

Spin-polarons are obtained using an Ising-like exchange model consisting of double and super-exchange interactions in low dimensional systems. At zero temperature, a new phase separation between small magnetic polarons, one conduction electron self-trapped in a magnetic domain of two or three sites, and the anti-ferromagnetic phase was previously reported. On the other hand the important effect of temperature was missed. Temperature diminishes Boltzmann probability allowing excited states in the system. Static magnetic susceptibility and short-range spin-spin correlations at zero magnetic field were calculated to explore the spin-polaron formation. At high temperature Curie-Weiss behavior is obtained and compared with the Curie-like behavior observed in a nickelate one-dimensional compound.