OSMAR FLAVIO DA SILVEIRA LEITE NETO

OSMAR FLAVIO DA SILVEIRA LEITE NETO

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Cd impurities in Vanadium oxides
2021 - BURIMOVA, ANASTASIA; LEITE NETO, OSMAR F.; BOSCH-SANTOS, BRIANNA; CORREA, EDUARDO L.; PEREIRA, LUCIANO F.D.; SALES, TATIANE S.N.; RIBEIRO JUNIOR, IBERE S.; COSTA, MESSIAS S.; COSTA, CLEIDILANE; DANG, THANH T.; ZYABKIN, DMITRY; VAN STIPHOUT, KOEN K.F.; GERAMI, ADELEH M.; CORREIA, JOAO M.; SCHELL, JULIANA; CARBONARI, ARTUR W.
The number of metastable phases, the capriciousness at changing external conditions, and lack of accurate description of local behavior already resulted in severe misinterpretation of experimental outcomes for vanadium oxides[1–3]. The scope of already implemented and potential applications of vanadium oxides is, indeed, impressive, particularly as a battery cathode for energy storage[ 4]. Doping is widely exploited as a means of application-oriented tuning of the material properties. The properties of each particular phase may be tuned by doping to satisfy specific requirements and/or improve the functional performance. In the work here reported, electric quadrupole interaction on 111mCd nuclei implanted in vanadium pentaoxide doped with different concentrations of Cd were measured with time-differential perturbed angular correlations (TDPAC). Pure V2O5 as well as doped with 1%, 5%, and 10% of Cd were measured at different temperatures. To correlate the results with the possible formation of different phases and compounds, samples of VO2, CdV2O6, and Cd2V2O7 were also measured. The intention is to provide a comprehensive description, at an atomic level, of the doping effects on the local crystal structure and the electronic structure around the impurity and the consequences on the properties of the host oxides. Preliminary results show that the probability of formation of cadmium vanadates is low but the temperature and atmosphere of measurements have an important effect on the local scale.
Synthesis and TDPAC characterization of multiferroics
2021 - LIMA, N.P.; BURIMOVA, A.; SOUZA, A.P.S.; LEITE NETO, OSMAR F.S.; CARBONARI, A.W.
Locally symmetric oxygen vacancy around Cd impurities in CeO2
2021 - FERREIRA, W.L.; PEREIRA, L.F.D.; LEITE NETO, O.F.S.; MACIEL, L.S.; GONÇALVES, V.C.; SAXENA, R.N.; CARBONARI, A.W.; COSTA, M.S.; CABRERA-PASCA, G.A.
A detailed investigation of the electronic structure in the neighborhood of Cd impurities in CeO2 has been performed by ab initio calculations to elucidate the interplay between the oxygen vacancies and electric quadrupole interactions. The quadrupole frequency related to the major component of the electric-field gradient (EFG) at impurities sites from its neighboring charge density as well as its symmetry were calculated by simulating oxygen vacancies at oxygen nearest neighbor of Cd. Results show a very good agreement with experimental hyperfine interactions measurements at the 111Cd nucleus replacing Ce at CeO2. A systematic mapping of oxygen vacancies in CeO2 supercells was proposed within the framework of density-functional theory using the WIEN2K code focusing on the electronic distribution in the vicinity of Cd impurities. Results show that the calculated values of EFG crucially depend on impurity-vacancy complex position and the striking axial symmetry observed when an oxygen monovancy is at the nearest neighborhood of Cd is explained by a rearrangement of its p orbitals.
Study of hyperfine interactions in Perovskite structure CaTiO3 with perturbed angular correlation spectroscopy
2019 - LEITE NETO, O.F.S.; SANTOS, B.S.; NARESSI, A.L.E.; SALES, T.S.N.; SAXENA, R.N.; CARBONARI, A.W.
Ceramic materials of the perovkista were identified as candidates for immobilization of medium and high levels of radioactive waste due to the fact that they are thermodynamically chemically and physically stable. Because it is a nuclear technique, PAC spectroscopy has great precision and efficiency in the measurement of local hyperfine fields on an atomic scale, constituting an ideal tool for the investigation of the atomic origin of structural phenomena, such as the influence of defects in the macroscopic properties of many materials or the local neighborhood of metallic atoms in the oxide structure. The samples has been prepared by sol-gel method and characterized by X-ray diffraction. In this study the time differential perturbed angular correlation (PAC) spectroscopy was used to study the structure of CaTiO3 and the possible defects made by radiation exposure. The probe nucleus used in this research was 111Cd. The PAC method is based on the hyperfine interaction of nuclear moments of the probe with extra nuclear magnetic fields or electric field gradients (EFGs). In the case of quadrupolar electric interaction, the experimental measurement gives the quadrupolar frequency νQ with respective distribution δ as well as the asymmetry parameter η of EFG. The γ-γ PAC measurements were carried out using a standard set up with four conical BaF2 detector scintillators with a time resolution of 0.6 ns (FWHM).
RE-doped Fe3O4 (RE = Eu, Gd, Er) nanoparticles for nanothermometry
2019 - CORREA, E.; BOSCH-SANTOS, B.; SALES, T.; CABRERA-PASCA, G.; CORREA, B.S.; NETO, O.F.; CARBONARI, A.W.; OLESHKO, V.; DENNIS, C.
Temperature affects every physical system, chemical reaction, and biological process. A new method, magnetic nanothermometry, is being explored to measure temperature throughout a volume. This method uses large changes in magnetization as function of temperature, which cannot be obtained with current magnetic nano-objects (MNO). To get a large magnetization change we have examined the magnetic properties of RE-doped Fe3O4 (RE = Eu, Gd, Er) MNO. Samples were synthesized by co-precipitation. For the doped material, RE+3 were chosen in order to replace the Fe+3 in the (Fe+2)(Fe+3)2O4 structure. Structural characterization was performed by X-ray diffraction and transmission electron microscopy. Hyperfine interaction parameters as a function of temperature (300 K to 873 K) were obtained by perturbed angular g-g correlation (PAC) spectroscopy using 111In(111Cd) as probe nuclei. To fit the PAC spectra, the 111Cd probes were considered to occupy three sites: tetrahedral, octahedral, and a third site where the probes are located at the nanoparticle surface [1]. The hyperfine magnetic field Bhf was calculated using the Larmor equation, and its behavior as a function of temperature follows a Brillouin-type transition. For example, the Curie temperature (TC) obtained for 5% Er-doped was approx. 846 K (FIG. 1), which is higher than the expected TC for pure Fe3O4 (approx. 722 K) [2]. Magnetization as a function of temperature shows a 70 % change in magnetization around 100 K for Er-doped Fe3O4 (FIG.2), which is an improvement in temperature on pure Fe3O4 (below 50 K) [3]. Current work is focused on correlating the temperature range in which the magnetization change occurs and determining if it depends on the dopant element.
Magnetic hyperfine field at 119Sn and 111Cd probes in Gd5Ge4 studied by Mössbauer and PAC spectroscopy
2019 - KRYLOV, V.I.; BOSCH-SANTOS, B.; CABRERA-PASCA, G.A.; CARBONARI, A.W.; SAXENA, R.N.; LEITE NETO, O.F.S.
The magnetic structure of Gd5Ge4 belonging to the family of giant magnetocaloric Gd5(Si1-xGex)4 alloys [1] has been examined by magnetization measurements in single crystal [2, 3], X-ray resonant magnetic scattering (XRMS) [4], and neutron powder diffraction (NPD [5]. Gd5Ge4 crystallizes in the orthorhombic structure (space group Pnma) having three non-equivalent Ge-sites. At low temperatures, the Gd magnetic moments are ferromagnetically (FM) aligned within the slabs along the c-direction, while their stacking in the b-direction is antiferromagnetic (AFM) below 127 K [2, 4]. In this work, the temperature dependence of the magnetic hyperfine field (HF) and electric quadrupole interaction on 119Sn and 111Cd probe nuclei in Gd5Ge4 compound have been investigated by Mössbauer and perturbed angular correlation (PAC) spectroscopy. At the saturation, the HF magnitudes reach of Bhf =28.0(5) T and Bhf= 12.6(5) T for 119Sn and 111Cd, respectively, which correspond to the FM local magnetic environment for these probes. It was found that 119Sn probes are placed in two Ge positions, whereas 111Cd probes are located in only one of the three Ge positions in Gd5Ge4. Temperature dependence of Bhf for both 119Sn and 111Cd probes in Gd5Ge4 present anomalous behavior. In the range from 20 K to 120K, the HF decreases almost linearly when temperature increases. These anomalous behavior of Bhf(T) for 119Sn and 111Cd probes are in agreement with the temperature variation of the magnetic (070) and (010) peak intensities that have been found in XRMS [3] and NPD [4] studies. The AFM ordering temperature of Gd5Ge4, TN=129(1) K, found from the Bhf (T) is in agreement with previous results of [2 - 5]. Changes of the values and sign of the quadrupole shift of Mössbauer spectra for 119Sn atoms with the increase of temperature from 30 to 50 K confirm the spin-flop transition in Gd5Ge4 [2, 4].
Influence of oxygen atmosphere in the annealing of HfO2 thin films studied by perturbed angular correlation spectroscopy
2016 - CAVALCANTE, FABIO H. de M.; SALES, TATIANE S.N.; DAY, TRAVIS; LEITE NETO, OSMAR F.S.; PEREIRA, LUCIANO F.D.; SAXENA, RAJENDRA N.; CARBONARI, ARTUR W.; MENONI, CARMEN
The thin films of HfO2 have been grown by dual ion beam sputtering (DIBS) on transparent quartz substrates (grade silica) and silicon wafers. The major benefits of the DIBS process are the increased packing density of the deposited films which makes them more bulk-like, the improved adhesion resulting from the mixing of the materials at the interfaces between each layer, and the reduction of the high tensile stress in the layers. Deposition conditions were adjusted to obtain polycrystalline as well as amorphous films. In this study the time differential perturbed angular correlation (PAC) spectroscopy was used to study the influence of oxygen atmosphere in the annealing of the HfO2 thin film. The PAC method is based on the hyperfine interaction of nuclear moments of the probe with extra nuclear magnetic fields or electric field gradients (EFGs). In the case of quadrupolar interaction, the experimental measurement gives the quadrupolar frequency νQ with respective distribution δ as well as the asymmetry parameter η of EFG. The presence of the 180Hf isotope in natural hafnium allows the possibility of using 181Ta as a probe nucleus formed by irradiating the samples with neutrons via 180Hf(n,γ)181Hf reaction. After irradiation, samples were annealed in oxygen atmosphere at different temperatures. The γ-γ PAC measurements were then carried out using a standard set up with four conical BaF2 detector scintillators with a time resolution of 0.6 ns (FWHM). Results show a structural transition from monoclinic to tetragonal in the annealed samples.
First-principles calculations of the magnetic hyperfine field at Cd sites in RECd (RE = rare-earth element)
2016 - PEREIRA, LUCIANO F.; MESTNIK-FILHO, JOSE; CARBONARI, ARTUR; CAVALCANTE, FABIO; LEITE-NETO, OSMAR
Binary magnetic compounds based on rare-earth (RE) elements of the type REX, where X is a transition metal, with the simplest stoichiometry and crystalline cubic structure, are isostructural systems with different RE constituents, which differ only slightly in the crystallographic properties, but strongly in the magnetic properties. They thus offer excellent conditions for the separation of the magnetic from other solid-state parameters and permit the magnetism studies. However, in most RE compounds and, particularly when X=Cd, details of the magnetic structure at an atomic scale are not available mainly because is not possible to use neutron diffraction to investigate them. In this paper, the magnetic hyperfine field (mhf) at Cd sites in RECd compounds was studied by first-principles calculation. These compounds crystalizes in the CsCl prototype cubic structure (Pm-3m space-group) and order ferromagnetically below the Curie temperature with exception of PrCd, which presents antiferromagnetism. Recently, it was observed an unexpected transition in the mhf at 111Cd in DyCd below 40 K, when mhf vanishes [1]. We have used the density functional theory framework with full potential Augmented Planes Waves plus local orbitals (APW+lo) method embodied in the WIEN2k code to simulate electronic structures of the ferromagnetic and antiferromagnetic crystal cells in these compounds. For the exchange-correlation effects we chose local density approximation. The mhf at Cd sites and the formation energy were calculated for each compound as well as the density of states were obtained. Results of mhf are in very good agreement (less than 10%) with the experimental values previously reported [1]. Interestingly, for DyCd the energy for the formation of the antiferromagnetic ordering with (π, π, 0) structure is smaller than that for the ferromagnetic and others possible antiferromagnetic phases. As far as we know, it is the first time that such an excellent agreement between the calculated and experimental mhf at Cd sites is observed. Moreover, this results is even more important because the calculations were fully variational without the use of the Hubbard model which takes into account the on-site correlation of 4f electrons.