Curie temperature, exchange integrals, and magneto-optical properties in off-stoichiometric bismuth iron garnet epitaxial films

in Physical Review b (2008), 78(9), 094429

We have studied the influence of the stoichiometry on the structural, magnetic, and magneto-optical properties of bismuth iron garnet (Bi3Fe5O12) thin films grown by pulsed laser deposition. Films with ... [more ▼]

We have studied the influence of the stoichiometry on the structural, magnetic, and magneto-optical properties of bismuth iron garnet (Bi3Fe5O12) thin films grown by pulsed laser deposition. Films with different stoichiometries have been obtained by varying the Bi/Fe ratio of the target and the oxygen pressure during deposition. Stoichiometry variations influence the Curie temperature T-C by tuning the (Fe)-O-[Fe] geometry: TC increases when the lattice parameter decreases, contrary to what happens in the case of stoichiometric rare-earth iron garnets. The thermal variation of the magnetization, the Faraday rotation, and the Faraday ellipticity have been analyzed in the frame of the Neel two-sublattice magnetization model giving energies of -48 K (4.1 meV), -29 K (2.5 meV), and 84 K (7.3 meV) for the three magnetic exchange integrals j(aa), j(dd), and j(ad), respectively. Magneto-optical spectroscopy linked to compositional analysis by Rutherford backscattering spectroscopy shows that Bi and/or Fe deficiencies also affect the spectral variation (between 1.77 and 3.1 eV). Our results suggest that bismuth deficiency has an effect on the magneto-optical response of the tetrahedral Fe sublattice, whereas small iron deficiencies affect predominantly the magneto-optical response of the octahedral sublattice. [less ▲]

A new hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides

in Physical Review b (2008), 77(16),

Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory (DFT) and hybrid exchange-correlation functionals for the prediction of the ... [more ▼]

Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory (DFT) and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. It is found that none of the available functionals is able to provide, at the same time, accurate electronic and structural properties of the cubic and tetragonal phases of BaTiO3 and PbTiO3. Some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrid functionals yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated with ferroelectricity, giving rise to an unacceptably large c/a ratio for the tetragonal phases of both compounds. This supertetragonality is found to be induced mainly by the exchange energy corresponding to the generalized gradient approximation (GGA) and, to a lesser extent, by the exact exchange term of the hybrid functional. We thus propose an alternative functional that mixes exact exchange with the recently proposed GGA of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves over the treatment of exchange of the most usual GGA's. The new functional renders an accurate description of both the structural and electronic properties of typical ferroelectric oxides. [less ▲]

Fe-57 Mossbauer spectral and muon spin relaxation study of the magnetodynamics of monodispersed gamma-Fe2O3 nanoparticles

in Physical Review b (2007), 76(17),

The Mossbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of ... [more ▼]

The Mossbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of the hyperfine Hamiltonian. In this model, the hyperfine field is assumed to relax between the six +/- x, +/- y, and +/- z directions in space with a distribution of relaxation rates that is temperature dependent. Muon spin relaxation measurements have been carried out on the 9 nm particles between 4.2 and 295 K. Both techniques reveal three regimes in the magnetic dynamics of these nanoparticles. In the low-temperature regime, between 4.2 and similar to 30 K, the nanoparticle magnetic moments are blocked and a spin-glass-like state is observed with nearly static hyperfine fields, as is indicated by the well resolved magnetic Mossbauer spectra and the slow exponential decay of the muon asymmetry functions. In the high-temperature regime, above similar to 125 K, the nanoparticle magnetic moments and, hence, the hyperfine fields, relax rapidly and a typical thermally activated superparamagnetic behavior is observed, as is indicated by the Mossbauer doublet line shape and the muon asymmetry functions that are unquestionably characteristic of monodispersed nanoparticles. In the intermediate regime between similar to 30 and 125 K, the Mossbauer spectra are the superposition of broad sextets and doublets and the muon asymmetry functions have been fitted with a sum of two terms, one relaxing term similar to that observed at and above 125 K and one term characteristic of static local fields. Hence, in this intermediate regime, the sample is magnetically inhomogeneous and composed of nanoparticles rapidly and slowly relaxing as a result of interparticle interactions. The magnetic anisotropy constants determined from both the Mossbauer spectral and magnetic susceptibility results decrease by a factor similar to 4 with increasing diameter from 4 to 22 nm and increase linearly with the percentage of iron(III) ions present at the surface of the nanoparticles. The interparticle interaction energy is estimated to be between 89 and 212 K from the temperature dependence of the magnetic hyperfine field measured on the 9 nm nanoparticles. [less ▲]

Weak ferromagnetism in Fe1-xCoxSb2

in Physical Review b (2007), 76(22),

Weak ferromagnetism in Fe1-xCoxSb2 is studied by magnetization and Mossbauer measurements. A small spontaneous magnetic moment of the order of similar to 10(-3)mu(B) appears along the b axis for 0.2 <= x ... [more ▼]

Weak ferromagnetism in Fe1-xCoxSb2 is studied by magnetization and Mossbauer measurements. A small spontaneous magnetic moment of the order of similar to 10(-3)mu(B) appears along the b axis for 0.2 <= x <= 0.4. Based on a structural analysis, we argue against extrinsic sources of weak ferromagnetism. We discuss our results in the framework of the nearly magnetic electronic structure of the parent compound FeSb2. [less ▲]

Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films

in Physical Review b (2006), 73(9),

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO3 films. Our measurements ... [more ▼]

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab initio calculations, unambiguously demonstrate noncentrosymmetry in films a few unit cells thick, imply that films as thin as three unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface. [less ▲]

Neutron and nuclear inelastic scattering study of the Einstein oscillators in Ba-, Sr-, and Eu-filled germanium clathrates

in Physical Review b (2005), 72(17),

Inelastic neutron scattering measurements on Ba8Ga16Ge30 and Sr8Ga16Ge30 indicate the presence of soft local modes with Einstein oscillator energies of 4.9 +/- 0.1 and 4.4 +/- 0.1 meV, modes which are ... [more ▼]

Inelastic neutron scattering measurements on Ba8Ga16Ge30 and Sr8Ga16Ge30 indicate the presence of soft local modes with Einstein oscillator energies of 4.9 +/- 0.1 and 4.4 +/- 0.1 meV, modes which are associated with the lowest "rattling" mode of the Ba(B) and Sr(B) guests, respectively. Nuclear inelastic scattering measurements on Eu8Ga16Ge30 yield Einstein oscillator energies of between 3.0 +/- 0.3 and 7.5 +/- 1.2 meV for the different modes of the Eu guests. Further, the resonant character of the measurements on Eu8Ga16Ge30 reveals, without question, that neither of the Eu guests exhibit any vibrational modes above 9 meV. [less ▲]

Temperature dependence of the electro-optic tensor and refractive indices of BaTiO3 from first principles

in Physical Review b (2005), 71(13),

We present a method to compute the temperature dependence of the refractive indices and electro-optic coefficients of ferroelectrics from a first-principles effective Hamiltonian and apply it to BaTiO3 in ... [more ▼]

We present a method to compute the temperature dependence of the refractive indices and electro-optic coefficients of ferroelectrics from a first-principles effective Hamiltonian and apply it to BaTiO3 in its tetragonal phase. We show that the effective Hamiltonian is a valid approach to study optical properties. We compare our formalism with the model of DiDomenico and Wemple and justify why the latter was successful, although its basic assumption is not met in practice. [less ▲]

Mossbauer spectral study of the magnetocaloric FeMnP1-xAsx compounds

in Physical Review b (2004), 70(21),

The magnetic phase transitions in the FeMnP1-xAsx compounds with x= 0.25, 0.35, 0.45, 0.50, and 0.55, have been studied by iron-57 Mossbauer spectroscopy. The ferromagnetic and antiferromagnetic spectra ... [more ▼]

The magnetic phase transitions in the FeMnP1-xAsx compounds with x= 0.25, 0.35, 0.45, 0.50, and 0.55, have been studied by iron-57 Mossbauer spectroscopy. The ferromagnetic and antiferromagnetic spectra have been analyzed with a model that takes into account the random distribution of the P and As near-neighbor anions of a given iron site. This distribution is a binomial distribution of the contributions to the spectra of each iron with n As near neighbors. A magnetostriction model has been used to characterize the temperature induced paramagnetic to ferromagnetic first-order phase transition and order parameters, eta(F)=2.6, 2.3, 2.0, 1.57, and 1.43 have been obtained for x=0.25, 0.35, 0.45, 0.50, and 0.55, respectively. A detailed phase diagram has been derived from the Mossbauer spectral analysis and reveals a magnetic triple point at x=similar to0.35 and similar to210 K. A model that takes into account the random binomial P and As distribution and the contribution from the iron and manganese magnetic sublattices yields excellent fits of the spectral components assigned to the ferromagnetic and incommensurate antiferromagnetic components for the x=0.25 and 0.35 compounds at all temperatures. [less ▲]

Magnon-polaron and spin-polaron signatures in the specific heat and electrical resistivity of La0.6Y0.1Ca0.3MnO3 in zero magnetic field and the effect of Mn - O - Mn bond environment

in Physical Review B (2002), 66(17),

La0.6Y0.1Ca0.3MnO3, an ABO(3) perovskite manganite oxide, exhibits a nontrivial behavior in the vicinity of the sharp peak found in the resistivity rho as a function of temperature T in zero magnetic ... [more ▼]

La0.6Y0.1Ca0.3MnO3, an ABO(3) perovskite manganite oxide, exhibits a nontrivial behavior in the vicinity of the sharp peak found in the resistivity rho as a function of temperature T in zero magnetic field. The various features seen on drho/dT are discussed in terms of competing phase transitions. They are related to the Mn-O-Mn bond environment depending on the content of the A crystallographic site. A Ginzburg-Landau type theory is presented for incorporating concurrent phase transitions. The specific heat C of such a compound is also examined from 50 to 200 K. A log-log analysis indicates different regimes. In the low temperature conducting ferromagnetic phase, a collective magnon signature (Csimilar or equal toT(3/2)) is found as for what are called magnon-polaron excitations. A Csimilar or equal toT(2/3) law is found at high temperature and discussed in terms of the fractal dimension of the conducting network of the weakly conducting (so-called insulating) phase and an Orbach estimate of the excitation spectral behaviors. The need of considering both independent spin scattering and collective spin scattering is thus emphasized. The report indicates a remarkable agreement for the Fisher-Langer formula, i.e., Csimilar todrho/dT at second order phase transitions. Within the Attfield model, we find an inverse square root relationship between the critical temperature(s) and the total local Mn-O-Mn strain. [less ▲]

Local structure of liquid GeTe via neutron scattering and ab initio simulations

in Physical Review B (2002), 65(11),

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare ... [more ▼]

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare these results with ab initio molecular dynamics simulations. The simulations were based on interatomic forces derived from pseudopotentials constructed within density functional theory. At the melting temperature, the Peierls distortion responsible for the lower-temperature crystal phase is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder in the Ge environment determines the eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. [less ▲]