Abstract Recently, the large magneto-resistance effects in epitaxial manganite thin films has interested in the doped manganite perovskite materials for magnetic random access memory (MRAM) and read-head application. The relation between the magneto-resistance and microstructure of the colossal magneto-resistance materials has been evaluated in this study. Different thickness of La0.67Ca0.33Mn03 (LCMO) thin films were grown on (001) MgO and (001) SrTi03 (STO) substrates at growth temperature 750 degree C with RF magnetron sputtering technique, respectively. These substrates provide two different lattice-mismatch conditions for the LCMO films (+9% for MgO and +1% for STO). The crystal structure of LCMO films were characterized with X-ray diffraction (XRD), the surface morphology of LCMO films were observed by scanning electron microscope (SEM), the interface of microstructure between LCMO films and substrate were studied by transmission electron microscope (TEM), the thickness and chemical composition of LCMO films were determined by Rutherford backscattering spectrometer (RBS), and finally the resistance and I-M transition temperature were evaluated at temperature range from 77K to 300K. The results show that the epitaxial LCMO films with a superlattice structure were obtained on STO substrate and polycrystal structure of LCMO films were on MgO substrate due to larger lattice mismatch.. The transition temperature of magneto-resistance of LCMO thin film is quite sensitive with film thickness. The transition temperature increases with film thickness increased. When the film with thickness excess of 2000A, the transition temperature is nearly same as that of LCMO bulk material.. The existed strain and the microstructure of LCMO films are two important factors related with magnetic resistance and electrical properties of LCMO films.

Abstract Recent progress in oxide perovskite thin-film technology has led to the discovery of a large negative magnetoresistance in doped manganate perovskite thin films. These films may have potentials for magnetic random access memory (MRAM) and magnetic sensors. Therefore, the research of magnetoresistance has been attracted a lot of attentions. The magnetoresistance is directly related to the microstructure. In an application point of view, the ulta-thin film may be more appropriate compares with those utilizing with thicker films. In this paper, we report the detail results of electrical property of La0.67Ca0.33MnO3 (LCMO) films related with their microstructure. The La0.67Ca0.33MnO3 (LCMO) films were deposited on (001) STO substrate with RF sputtering technique. The working pressure was maintained at 100m torr and the growth temperature was kept at 750¢J. After growth the films was annealed at 850¢J for 1 hour in a 500 torr O2 annealing environment. The growth time was 3mins.¡B6mins. and 12mins. respectively. The crystal structure of LCMO films were characterized with X-ray diffraction (XRD). The surface morphology of LCMO films were observed by scanning electron microscope (SEM) and the interface of microstructure between LCMO films and STO substrate were investigated by transmission electron microscope (TEM). Finally the M-I transition temperature were evaluated with 4-point probe at the temperature range from 300K down to 77K. The results show that the LCMO films were amorphous when the growth time was in 3 mins. The microstructure of the film gradually became poly-crystal and had a (001) prefer orientation after the growth time increasing to 6 mins. The grain size of the 12 mins growth film was at 40-50 nm scale. The Curie temperature and magnetoresistance change of these films were increased as the degree of crystallization of these films became better.

A ênfase deste trabalho foi a aplicação da técnica de RMN em campo zero para a obtenção do acoplamento quadrupolar e dos espectros de RMN em banda larga dos núcleos de 27Al (amostra GdAl2), 139La e 55Mn (amostras de Manganita), à temperatura de 4,2 K. O objetivo do trabalho foi obter resultados confiáveis tanto em relação aos espectros quanto aos valores dos acoplamentos quadrupolares das amostras de GdAl2 e de Manganitas, e utilizar estes resultados para auxiliar a entender e a determinar algumas das propriedades físicas destes materiais (no caso do GdAl2 o comportamento tipo Spin-Glass e o efeito Magnetocalórico, no caso das Manganitas a Magnetoresistência Colossal). Para se determinar o valor do acoplamento quadrupolar utilizou-se a técnica de medida das oscilações quadrupolares. Os espectros de RMN, em banda larga, foram obtidos utilizando-se a técnica de eco de Hahn variando-se a frequência de excitação em uma faixa de 5 a 500 MHz. A maior sensibilidade do equipamento montado em nosso laboratório permitiu que se observassem os múltiplos ecos devidos à interação quadrupolar. Consequentemente, estes ecos permitiram a obtenção de espectros de RMNz de 27Al com alta resolução e a medida do valor do acoplamento quadrupolar diretamente do espectro. Também foi mostrado que os múltiplos ecos do 27Al são originados em regiões diferentes da amostra. Os resultados dos experimentos de oscilação quadrupolar em amostras de Manganitas permitiram que se observassem vários valores de acoplamento quadrupolar, que podem ser devido à desordem da estrutura cristalina dos octaedros de MnO6 ou à variação angular do ângulo entre o campo magnético hiperfino e a o eixo de fácil magnetização. Também foi mostrado que os valores de acoplamento quadrupolar mudam em função do elemento dopante utilizado. A aplicação do campo externo diminui a distribuição de acoplamento quadrupolar, mostrando que a aplicação do campo magnético externo causa mudanças ou na rede cristalina e/ou na orientação do campo magnético hiperfino e, consequentemente, sobre as propriedades físicas das Manganitas. A comparação entre os espectros de 139La e 55Mn das diferentes amostras de Manganita mostrou a coexistência de fases magnéticas. Estes resultados mostram que a técnica de RMNz é uma excelente ferramenta para se determinar o estado magnético fundamental dos materiais magnéticos e as mudanças na estrutura cristalina através da observação do acoplamento quadrupolar. / The emphasis of this work was the application of zero-field NMR technique to obtain the quadrupole coupling and NMR broadband spectra of the nuclei 27Al (GdAl2 sample), 139La and 55Mn (Manganites samples), at 4.2 K. The objective was to obtain reliable results for both spectra and quadrupole coupling values of samples GdAl2 and Manganites, and use these results to help understand and determine some of the physical properties of these materials (for GdAl2 the Spin-Glass type behavior and the Magnetocaloric effect and for Manganites the Colossal Magnetoresistance effect). To determine the value of the quadrupole coupling it was used the quadrupole oscillations technique. The NMR broadband spectra were obtained using the Hahn echo technique, in the frequency excitation range 5 to 500 MHz. The greater sensitivity of the equipment allowed observing multiples echoes due to quadrupole interaction, these echoes allowed to obtain spectra of zero field RMN of 27Al with high resolution and measure the value of the quadrupole coupling directly from the spectrum. It was also shown that the 27Al multiple echoes are generated in different regions of the sample. The quadrupole oscillation experiments results on Manganite samples have obtaned several values of quadrupole coupling, indicating that the Manganites Ferromagnetic Metallic Phase present high degree of disorder or, in the crystalline structures or in the hyperfine magnetic field direction. It was shown that the change in the value of the quadrupole coupling depends on the dopant used in the Manganites composition. The results of the quadrupole oscillations with application of an external magnetic field showed that the degree of disorder decreases due to the external magnetic field, showing the influence of the external magnetic field on the crystal network and/or in the hyperfine magnetic field direction, and consequently of some Manganites physical properties. The comparison between the spectra of 139La and 55Mn of the different Manganite samples shows the coexistence of different magnetic phases. These results showing that the technique of zero-field NMR is excellent to determine the fundamental magnetic state of the magnetic materials and observe changes in the crystal structure by the observation of the quadrupole coupling.

Amostras policristalinas de La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; foram produzidas pelo método da mistura estequiométrica de óxidos e tratadas termicamente ao ar a 1400 oC. As amostras foram caracterizadas através de medidas de difração de raios-X, resistividade elétrica rho(T), susceptibilidade magnética chi(T) e magnetorresistividade rho(T, H = 50 kOe). Os resultados das análises dos diagramas de raios-X indicaram que os materiais são de fase única e que houve uma substituição efetiva de La por Pr no sítio A ao longo da série. Medidas de rho(T) e chi(T) revelaram que a temperatura de transição de fase metal-isolante TMI e temperatura de Curie TC decrescem com o aumento da concentração y e que a resistividade residual rho0 (rho(T = 10 K)) é consideravelmente alta em amostras com y 0.35. Ainda, com o decréscimo de T, as amostras com y 0.35 transitam para uma fase de ordenamento de carga em TOC ~ 194 K e, em seguida, para uma fase metálica em TMI. Essas medidas também sugerem a coexistência de fases ferromagnética-metálica FMM e de ordenamento de carga isolante OCI nesses materiais. Nas propriedades físicas macroscópicas, a fase FMM mostrou ser a dominante para os compostos com pequenas concentrações de Pr (y 0.25) e a fase OCI dominante para os compostos com altas concentrações de Pr (y 0.40). As medidas de rho(T, H = 50 kOe) mostram que a magnitude da resistividade elétrica decresce drasticamente nas vizinhanças de TMI sob a aplicação de um campo magnético externo. A magnitude de MR (MR = (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe)) entre os extremos da série (y = 0 e 0.625) varia até sete ordens de grandeza, sendo que o máximo valor de MR para amostras com y = 0 é de ~ 0.75 e naquelas com y = 0.625 é ~ 3.4x106 . O diagrama de fases deste composto evidencia uma região crítica (0.30 y 0.40) onde os valores de TMI, TC, MR e 0 variam abruptamente como função de y, sendo que em outras regiões tal variação é mais suave. A variação significativa desses quatro valores indica uma competição mais forte entre as fases coexistentes ocorre na região crítica. Algumas características marcantes podem ser observadas nas amostras da região crítica tais como: a presença de um segundo pico, abaixo de TMI, em ~ 90 K e ~ 72 K na curva de rho(T) de amostras com y = 0.30 e 0.35, histerese térmica mais pronunciada em rho(T) e chi(T), MR torna-se colossal, relaxação significativa da resistividade elétrica com o tempo, entre outras. Assim, as propriedades de transporte e magnéticas nessa região crítica são dominadas pela forte competição entre as fases coexistentes. / Polycrystalline samples of La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; were synthesized by the solid-state reaction method and sintered in air at 1400 oC. These compounds were studied by measurements of X-ray powder diffraction, magnetic susceptibility chi(T), and electrical resistivity rho(T, H). X-ray powder diffraction measurements indicated single phase materials and an effective substitution of La by Pr. Results from rho(T) and chi(T) revealed that increasing y in this series results in a rapid reduction of both the insulator to metal transition temperature TMI and the Curie temperature TC. Such a rapid decrease in TMI with increasing y is also accompanied by the occurrence of a new transition temperature, termed TCO, which is related to the transition to the charge ordered CO state. Such a temperature, which is essentially independent of y, occurs at TCO ~ 194 K and is mainly observed in samples with y 0.35. The other feature is the presence of a large residual resistivity electrical rho(0 = (10 K)) for large y (y 0.35) at low-T even though rho(T) suggests a metallic behavior below TMI. The temperature for the maximum magnetoresistance effect occurs near TMI, that shifts to higher T with increasing field. The MR is defined here as (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe) and is enhanced by seven orders of magnitude from ~ 0.75 up to ~ 3.4x106 in samples with y = 0 and y = 0.625, respectively. Some features like the thermal hysteresis observed in both rho(T) and chi(T) curves indicate the coexistence of different phases in a range of y concentration, i. e., the ferromagnetic-metallic FMM and the charge ordered-insulating COI domains. The FMM is stable for y 0.25, but the COI state becomes dominant for y 0.40. There is a critical region in the phase diagram, ranging from y = 0.30 to 0.40, where the magnitude of the TMI, TC, MR, and 0 were found to display abrupt changes with increasing y. Some anomalous features like a second peak in rho(T) below TMI, a two-step increasing in chi(T), a colossal MR effect and others are observed for compositions belonging to this critical region. Our combined data suggest that the general physical properties of these compounds in such a critical region are dominated by the strong competition between coexisting ferromagnetic-metallic and charge ordered-insulating phases.

Amostras policristalinas de La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; foram produzidas pelo método da mistura estequiométrica de óxidos e tratadas termicamente ao ar a 1400 oC. As amostras foram caracterizadas através de medidas de difração de raios-X, resistividade elétrica rho(T), susceptibilidade magnética chi(T) e magnetorresistividade rho(T, H = 50 kOe). Os resultados das análises dos diagramas de raios-X indicaram que os materiais são de fase única e que houve uma substituição efetiva de La por Pr no sítio A ao longo da série. Medidas de rho(T) e chi(T) revelaram que a temperatura de transição de fase metal-isolante TMI e temperatura de Curie TC decrescem com o aumento da concentração y e que a resistividade residual rho0 (rho(T = 10 K)) é consideravelmente alta em amostras com y 0.35. Ainda, com o decréscimo de T, as amostras com y 0.35 transitam para uma fase de ordenamento de carga em TOC ~ 194 K e, em seguida, para uma fase metálica em TMI. Essas medidas também sugerem a coexistência de fases ferromagnética-metálica FMM e de ordenamento de carga isolante OCI nesses materiais. Nas propriedades físicas macroscópicas, a fase FMM mostrou ser a dominante para os compostos com pequenas concentrações de Pr (y 0.25) e a fase OCI dominante para os compostos com altas concentrações de Pr (y 0.40). As medidas de rho(T, H = 50 kOe) mostram que a magnitude da resistividade elétrica decresce drasticamente nas vizinhanças de TMI sob a aplicação de um campo magnético externo. A magnitude de MR (MR = (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe)) entre os extremos da série (y = 0 e 0.625) varia até sete ordens de grandeza, sendo que o máximo valor de MR para amostras com y = 0 é de ~ 0.75 e naquelas com y = 0.625 é ~ 3.4x106 . O diagrama de fases deste composto evidencia uma região crítica (0.30 y 0.40) onde os valores de TMI, TC, MR e 0 variam abruptamente como função de y, sendo que em outras regiões tal variação é mais suave. A variação significativa desses quatro valores indica uma competição mais forte entre as fases coexistentes ocorre na região crítica. Algumas características marcantes podem ser observadas nas amostras da região crítica tais como: a presença de um segundo pico, abaixo de TMI, em ~ 90 K e ~ 72 K na curva de rho(T) de amostras com y = 0.30 e 0.35, histerese térmica mais pronunciada em rho(T) e chi(T), MR torna-se colossal, relaxação significativa da resistividade elétrica com o tempo, entre outras. Assim, as propriedades de transporte e magnéticas nessa região crítica são dominadas pela forte competição entre as fases coexistentes. / Polycrystalline samples of La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; were synthesized by the solid-state reaction method and sintered in air at 1400 oC. These compounds were studied by measurements of X-ray powder diffraction, magnetic susceptibility chi(T), and electrical resistivity rho(T, H). X-ray powder diffraction measurements indicated single phase materials and an effective substitution of La by Pr. Results from rho(T) and chi(T) revealed that increasing y in this series results in a rapid reduction of both the insulator to metal transition temperature TMI and the Curie temperature TC. Such a rapid decrease in TMI with increasing y is also accompanied by the occurrence of a new transition temperature, termed TCO, which is related to the transition to the charge ordered CO state. Such a temperature, which is essentially independent of y, occurs at TCO ~ 194 K and is mainly observed in samples with y 0.35. The other feature is the presence of a large residual resistivity electrical rho(0 = (10 K)) for large y (y 0.35) at low-T even though rho(T) suggests a metallic behavior below TMI. The temperature for the maximum magnetoresistance effect occurs near TMI, that shifts to higher T with increasing field. The MR is defined here as (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe) and is enhanced by seven orders of magnitude from ~ 0.75 up to ~ 3.4x106 in samples with y = 0 and y = 0.625, respectively. Some features like the thermal hysteresis observed in both rho(T) and chi(T) curves indicate the coexistence of different phases in a range of y concentration, i. e., the ferromagnetic-metallic FMM and the charge ordered-insulating COI domains. The FMM is stable for y 0.25, but the COI state becomes dominant for y 0.40. There is a critical region in the phase diagram, ranging from y = 0.30 to 0.40, where the magnitude of the TMI, TC, MR, and 0 were found to display abrupt changes with increasing y. Some anomalous features like a second peak in rho(T) below TMI, a two-step increasing in chi(T), a colossal MR effect and others are observed for compositions belonging to this critical region. Our combined data suggest that the general physical properties of these compounds in such a critical region are dominated by the strong competition between coexisting ferromagnetic-metallic and charge ordered-insulating phases.

A ênfase deste trabalho foi a aplicação da técnica de RMN em campo zero para a obtenção do acoplamento quadrupolar e dos espectros de RMN em banda larga dos núcleos de 27Al (amostra GdAl2), 139La e 55Mn (amostras de Manganita), à temperatura de 4,2 K. O objetivo do trabalho foi obter resultados confiáveis tanto em relação aos espectros quanto aos valores dos acoplamentos quadrupolares das amostras de GdAl2 e de Manganitas, e utilizar estes resultados para auxiliar a entender e a determinar algumas das propriedades físicas destes materiais (no caso do GdAl2 o comportamento tipo Spin-Glass e o efeito Magnetocalórico, no caso das Manganitas a Magnetoresistência Colossal). Para se determinar o valor do acoplamento quadrupolar utilizou-se a técnica de medida das oscilações quadrupolares. Os espectros de RMN, em banda larga, foram obtidos utilizando-se a técnica de eco de Hahn variando-se a frequência de excitação em uma faixa de 5 a 500 MHz. A maior sensibilidade do equipamento montado em nosso laboratório permitiu que se observassem os múltiplos ecos devidos à interação quadrupolar. Consequentemente, estes ecos permitiram a obtenção de espectros de RMNz de 27Al com alta resolução e a medida do valor do acoplamento quadrupolar diretamente do espectro. Também foi mostrado que os múltiplos ecos do 27Al são originados em regiões diferentes da amostra. Os resultados dos experimentos de oscilação quadrupolar em amostras de Manganitas permitiram que se observassem vários valores de acoplamento quadrupolar, que podem ser devido à desordem da estrutura cristalina dos octaedros de MnO6 ou à variação angular do ângulo entre o campo magnético hiperfino e a o eixo de fácil magnetização. Também foi mostrado que os valores de acoplamento quadrupolar mudam em função do elemento dopante utilizado. A aplicação do campo externo diminui a distribuição de acoplamento quadrupolar, mostrando que a aplicação do campo magnético externo causa mudanças ou na rede cristalina e/ou na orientação do campo magnético hiperfino e, consequentemente, sobre as propriedades físicas das Manganitas. A comparação entre os espectros de 139La e 55Mn das diferentes amostras de Manganita mostrou a coexistência de fases magnéticas. Estes resultados mostram que a técnica de RMNz é uma excelente ferramenta para se determinar o estado magnético fundamental dos materiais magnéticos e as mudanças na estrutura cristalina através da observação do acoplamento quadrupolar. / The emphasis of this work was the application of zero-field NMR technique to obtain the quadrupole coupling and NMR broadband spectra of the nuclei 27Al (GdAl2 sample), 139La and 55Mn (Manganites samples), at 4.2 K. The objective was to obtain reliable results for both spectra and quadrupole coupling values of samples GdAl2 and Manganites, and use these results to help understand and determine some of the physical properties of these materials (for GdAl2 the Spin-Glass type behavior and the Magnetocaloric effect and for Manganites the Colossal Magnetoresistance effect). To determine the value of the quadrupole coupling it was used the quadrupole oscillations technique. The NMR broadband spectra were obtained using the Hahn echo technique, in the frequency excitation range 5 to 500 MHz. The greater sensitivity of the equipment allowed observing multiples echoes due to quadrupole interaction, these echoes allowed to obtain spectra of zero field RMN of 27Al with high resolution and measure the value of the quadrupole coupling directly from the spectrum. It was also shown that the 27Al multiple echoes are generated in different regions of the sample. The quadrupole oscillation experiments results on Manganite samples have obtaned several values of quadrupole coupling, indicating that the Manganites Ferromagnetic Metallic Phase present high degree of disorder or, in the crystalline structures or in the hyperfine magnetic field direction. It was shown that the change in the value of the quadrupole coupling depends on the dopant used in the Manganites composition. The results of the quadrupole oscillations with application of an external magnetic field showed that the degree of disorder decreases due to the external magnetic field, showing the influence of the external magnetic field on the crystal network and/or in the hyperfine magnetic field direction, and consequently of some Manganites physical properties. The comparison between the spectra of 139La and 55Mn of the different Manganite samples shows the coexistence of different magnetic phases. These results showing that the technique of zero-field NMR is excellent to determine the fundamental magnetic state of the magnetic materials and observe changes in the crystal structure by the observation of the quadrupole coupling.

In the 4d transition metal oxides, the extension of the 4d orbitals leads to comparable and thus competitive kinetic and coulomb energies. As a result, small perturbations can induce significant changes in their physical properties, giving rise to a class of exotic phenomena that are rarely found in other materials. The ruthenates materials with readily tunable parameters open an avenue to study the strong electronic correlation in the rarely explored territory: the 4d transition metal oxides. The bilayered system, Ca3Ru2O7, belongs to the Ruddlesden-Popper series in which the physical properties are intimately linked to the lattice degrees of freedom. Ca3Ru2O7, with its quasi-2D and severe structure distortion, is believed to be placed in a unique position at which the role of orbital degrees of freedom is highlighted. The system displays strikingly different behaviors when the field is applied along different crystalline axes. A ferromagnetic (FM) state with full spin polarization is achieved for B||a-axis, but colossal magnetoresistance is realized only for B||b-axis by avoiding the ferromagnetic state. In addition, for B rotating within the ac-plane, slow and strong SdH oscillations periodic in 1/B are observed for T.1.5 K in the presence of metamagnetism. For B|| [110], oscillations are also observed but periodic in B (rather than 1/B) and persist up to 15 K. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom. Complex thermodynamic properties are also observed in the other ruthenates system such as Sr4Ru3O10 and Pr3RuO7. The Sr4Ru3O10 is a triple-layered system that shows a dedicate balance between fluctuations and order. Besides the anomaly at TC=102K, anomalous behavior at low temperatures are also observed in the thermal study, indicative of an unusual magnetic order in this material. The Pr3RuO7 shows one-dimensional structure with zig-zag chain of corner sharing RuO6 octahedra running in parallel with the rows of edge-shared PrO8 pseudo-cubes. Magnetic and thermal properties studies on its single crystals indicate that the exchange interaction is strongly anisotropic. A Schottky-type anomaly at low temperature suggests that the gorderedh chain Pr ions are still sensitive to a crystal field.

No description available.

Hole-doped maganite with middle to narrow bandwidth La1-xCaxMnO3 was extensively studied because of its colossal magnetoresistance (CMR) characteristic under a magnetic field. These kind of materials show un- common magnetic and electric properties. The charge order phase only happens to the region x> 0.5, and along with decreasing temperature, its phase goes from para-insulator to charge-ordered then to antiferromagne- tism. In our studies, we apply correlation function of Green¡¦s function to LCMO and get susceptibility of charge and spin. Then we can get the cri- tical value of Coulomb repulsion inside the material by substituting the experimental values of phase transition temperature. This critical values is the key point of charge-ordered. Then we can also get the size of char- ge gap which decides the stability of charge-ordered phase. After know- ing the Coulomb repulsion and charge gap, we can picture the relation of inside and on-site Coulomb repulsion qualitatively while the transition happens. Here the on-site Coulomb repulsion means to the Hund¡¦s coupl- ing between d electrons. And by this we¡¦ll understand the physics inside CMR materials.

No description available.