Made available in DSpace on 2016-08-18T18:44:20Z (GMT). No. of bitstreams: 1 Tese-RosivaldoXavierSilva.pdf: 12741226 bytes, checksum: 7bc7743bfa3e2f958b913f2d67969531 (MD5) Previous issue date: 2015-12-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The RE2CoMnO6 (RE = La e Y) and Gd(Co0.5Mn0.5)O3 samples were obtained via modified Pechini method (MPM). Raman spectroscopy (RS), Fourier transform infrared (FTIR) spectroscopy, X ray powder diffraction, X ray photoelectron spectroscopy (XPS), SQUID and scanning electron microscopy (SEM) were used to characterize the samples, investigate structural and microstructural evolution, as well as evaluate their vibrational, magnetic and intrinsic dielectric properties. The impact of synthesis conditions on the structural ordering was investigated. We obtained a good control of structural order depending on the annealing temperature for LCMO. We observed an increase in the saturation magnetization, lifetime of the phonons and dielectric constant while suppression of anti-site defects and reduction of dielectric loss. Our investigations on polar phonons by FTIR revealed the extrinsic character of CDC effect on LCMO and clarified the contributions of phonons for dielectric constant in these systems. The temperature dependence Raman spectra of GCMO was investigated between 40 and 300 K and revealed an intriguing spin-phonon coupling, characterized by an increase in the energy of the most intense stretching mode near the magnetic transítion. The correlation between the Raman data and the magnetization suggests that the structure influences the magnitude of the spin-phonon coupling. Correlated analysis of LCMO, GCMO and YCMO systems show that all samples have vibrational properties quite similar. The intrinsic dielectric constants were obtained from the polar phonons dispersive parameters ε intr ̴ 15.8, 17.9 and 16.0, making the contributions to this value explicit, as well as the quality factors, reciprocal of dielectric losses , 𝑄𝑢×𝑓 ≈ 124,74 e 83 THz, extrapolated to microwave region at 10 GHz, to LCMO, GCMO and YCMO, respectively. XPS measures showed that oxidation state for Co and Mn ions are similar each another, being mostly Co2 + and Mn4 + for all investigated systems, however to GCMO, the spin-phonon coupling behavior and losses relatively large indicate that this compound has a high level of structural disordering. / Amostras de RE2CoMnO6 (RE = La e Y) e Gd(Co0.5Mn0.5)O3 foram obtidas pelo método Pechini modificado (MPM). Espectroscopia Raman (RS), espectroscopia no infravermelho por transformada de Fourier (FTIR), difração de raios X (DRX), espectroscopia de fotoelétrons excitados por raios X (XPS), magnetometria e microscopia eletrônica de varredura (MEV) foram utilizados para caracterizar as amostras, investigar evolução estrutural e microestrutural bem como avaliar suas propriedades vibracionais, magnéticas e dielétricas intrínsecas. O efeito da temperatura de tratamento térmico sobre o ordenamento estrutural foi investigado. Um bom controle do ordenamento estrutural em função da temperatura de tratamento térmico para o La2CoMnO6 (LCMO) foi obtido. Observa-se um incremento da magnetização de saturação, aumento do tempo de vida dos fônons, supressão de defeitos de anti-sítio, redução de perdas dielétricas e incremento da constante dielétrica. A investigação dos fônons polares via FTIR revelaram o caráter extrínseco da constante dielétrica colossal (CDC) no LCMO e explicitou as contribuições dos fônons para a constante dielétrica nesses sistemas. A dependência dos espectros Raman com a temperatura do Gd(Co1/2Mn1/2)O3 (GCMO) foi investigada entre 40 e 300 K revelando um acoplamento spin-fônon, caracterizado pelo incremento na energia do modo de estiramento mais intenso próximo à transição magnética. A correlação entre os resultados obtidos pela espectroscopia Raman e a magnetização sugere que a ordem estrutural influencia a magnitude do acoplamento spin-fônon. Análise correlacionada dos sistemas LCMO, GCMO e YCMO, mostram que todas as amostras apresentam características vibracionais bastante semelhantes. A partir dos parâmetros de dispersão dos fônons polares foram obtidas as constantes dielétricas intrínsecas ε intr ̴ 15,8, 17,9 e 16,0, explicitando as contribuições dos fônons para esses valores, e o fator de qualidade, recíproco da perdas dielétricas, 𝑄𝑢×𝑓 ≈ 124,74 e 83 THz, extrapolado para a região de micro-ondas em 10 GHz, para o LCMO, GCMO e YCMO, respectivamente. Medidas de XPS mostraram que os estados de oxidação dos íons de Co e Mn são semelhantes entre si, e principalmente do tipo Co2+/ Mn4+ para todos os sistemas estudados, sendo que para o GCMO, o acoplamento spin-fônon e as perdas dielétricas relativamente maiores indicaram que esse sistema possui elevada desordem estrutural.

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The aim of the present work was to develop a detailed picture of the electronic and magnetic properties of a number of interesting transition metal compounds. A number of complementary experimental and theoretical techniques have been applied, special emphasis was given to x-ray spectroscopies. The studies led to a number of results, and the following conclusions can be drawn: The influence of the magnetic ground state (high-spin (FeO) vs. low-spin (FeS2)) with respect to the recorded x-ray spectra was investigated. In particular, by performing RXES on the Fe L edge of the two compounds, very different ratios of La / Lβ integrated intensity for excitation energies close to the L2 edge have been observed. This effect has been explained in terms of the magnetic structure of FeO (high spin), which inhibits Coster-Kronig processes. Special attention has been given to the direct investigation of orbital ordering in a three dimensional CMR manganite, namely La7/8Sr1/8MnO3, by means of x-ray linear dichroism (XLD). We obtained, for the first time, rather strong indications that the coherently distorted Jahn-Teller phase in La7/8Sr1/8MnO3 is accompanied by a predominantly cross type (x2-z2) / (y2-z2) orbital ordering. In addition to manganites the double perovskite Sr2FeMoO6 the combined study by means of x-ray spectroscopies, magnetic measurements and theoretical band structure calculations could resolve some points discussed controversially in the literature. Both, paramagnetic measurements as well as core level spectroscopy of the Fe 2p, Fe 3s and the Mo 3d states suggest a mixed iron valence state involving around 30% Fe3+- Mo5+ and 70% Fe2+ - Mo6+ states in highly ordered Sr2FeMoO6. XPS valence band studies reveal that the Fe 3d states are not extremely localized, and we find evidence that charge transfer between Fe 3d and O 2p states plays an essential role.

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