Buitrago Piñeros, Ivón R. (2018) Propiedades magnéticas y electrónicas de perovskitas con electrones fuertemente correlacionados. / Magnetic and electronic propierties of perovskites with strongly correlated electrons. Tesis Doctoral en Física, Universidad Nacional de Cuyo, Instituto Balseiro.
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Resumen en español
En esta tesis hemos investigado las propiedades magnéticas y electrónicas de diversos óxidos de metales de tierras raras y metales de transición con estructura cristalina tipo perovskita, de interés por sus posibles aplicaciones tecnológicas. Los compuestos estudiados tienen en común que presentan diagramas de fases que exhiben varias fases con ordenamientos complejos de carga, orbital y de espın acoplados, dificultándose su estudio experimental por las correlaciones entre los distintos grados de libertad involucrados. Más aun, todos ellos presentan distintos grados de disproporcionación de carga y no está claramente identificado el ordenamiento magnético en el estado fundamental. En esta tesis hemos propuesto modelos simplificados para la descripción de varios de estos materiales y empleamos técnicas apropiadas para la investigación de sus propiedades. La tesis consta de dos partes. En la primera, estudiamos la dinámica de espines para investigar indirectamente la naturaleza del estado fundamental de carga y magnético para diversos materiales. En particular estudiamos los óxidos de manganeso laminares R_1−xD_1+xMnO_4 y bilaminares R_2−2xD_1+2xMn_2O_7 (R=tierras raras, D=alcalinotérreos), y los óxidos de níquel RNiO_3 (R= tierras raras). En la segunda parte de la tesis, proponemos el primer modelo que permite explicar la transición de fase inducida por temperatura en la perovskita doble CaCu_3Fe_4O_12, a través de un mecanismo que involucra las correlaciones electrónicas entre los sitios de Fe. Concretamente, para las manganitas laminares del tipo La_0.5Sr_1.5MnO_4 [Senff et al., Phys. Rev. Lett. 96, 257201 (2006)] y bilaminares Pr(Ca_0.9Sr_0.1)_2Mn_2O_7 [Johnstone et al., Phys. Rev. Lett. 109, 237202 (2012)] existen resultados experimentales de dispersión inelástica de neutrones (INS), en que se han determinado algunas bandas de mangones para estos compuestos. Los autores de estos estudios han remarcado que sus resultados para estas manganitas semidopadas son consistentes con un estado fundamental con el ordenamiento magnético característico de una fase antiferromagnética de tipo CE generalizada, habiendo sido la fase CE original propuesta por J. Goodenough para las manganitas de lantano semidopadas La_0.5D_0.5MnO_3 [Phys. Rev. 100, 564 (1955)]. Asimismo, estos autores afirman que sus resultados permiten excluir la fase de dímeros o de polarones Zener (ZP) propuesta por Daoud-Aladine, para explicar sus datos de difracción de neutrones en Pr_0.6Ca_0.4MnO_3 [Phys. Rev. Lett. 89, 097205 (2002)]. Mas alla de estos dos escenarios, Efremov, Van den Brink y Khomskii propusieron una nueva fase, que denominaron fase intermedia [Nat. Mater. 3, 853 (2004)], la cual indicaron podría ser relevante para reconciliar diversos aspectos de las fases CE y de polarones ZP, e interpolar entre dichas fases. Nuestro trabajo de investigación es el primero en el que se estudia la dinámica de espines de la fase intermedia e indirectamente se la usa para investigar el posible estado fundamental de las manganitas semidopadas. La fase intermedia consta de dímeros de espın ubicados a lo largo de las cadenas zig-zag presentes en los planos de las manganitas semidopadas. Los dímeros pueden constar de dos espines de distinta magnitud, como una forma de representar la posible disproporcionación de carga de los Mn. Además se propone que los dímeros consecutivos en las cadenas puedan no ser paralelos entre sí. En nuestro estudio para las manganitas laminares y bilaminares, propusimos un modelo de espines localizados e interactuantes, con el cual encontramos que la fase intermedia es estable para espines clásicos. En el caso cuántico, calculamos las excitaciones magnéticas del modelo para la fase intermedia. Realizamos un estudio exhaustivo del efecto que tienen sobre las bandas de magnones varios de los parámetros de acoplamiento magnético incluidos en el modelo, en particular los nuevos que proponemos para mejorar los ajustes previos de los experimentos de INS para La_0.5Sr_1.5MnO_4, Pr(Ca_0.9Sr_0.1)_2Mn_2O_7 y Nd_0.5Sr_0.5MnO_3 [Ulbrich et al., Phys. Rev. B. 84, 094453 (2011)]. Además, al analizar la estabilidad de la fase intermedia cuántica con el modelo propuesto, encontramos que la fase intermedia ortogonal, con ángulo π/2 tal como fue propuesta por Efremov et al. no resulta estable. Para la manganita laminar La_0.5Sr_1.5MnO_4, para la cual se midieron solo las excitaciones magnéticas hasta 40 meV, pudimos ajustar los magnones medidos tanto en base a fases CE generalizadas como mediante una fase de dímeros [Buitrago y Ventura, J. Supercond. Nov. Magn. 26 (6), 2303 (2012)]. En nuestro trabajo para manganitas bilaminares [Buitrago I. et al., enviado a publicar en J. Magn. Magn. Mater (enero 2018)] también encontramos que aparecen diferencias entre las excitaciones de las diversas fases bajo estudio, principalmente en las bandas de magnones por encima de la brecha de energía en el espectro de excitaciones (por encima de 40 meV). Por esto, consideramos que con los resultados experimentales disponibles para manganitas laminares aún no puede excluirse ninguna de las propuestas para el estado fundamental, siendo esencial contar con mediciones de magnones por encima de la brecha de energía. Además, analizando ajustes para los mangones medidos alrededor de la brecha de energía para la manganita iluminar Pr(Ca_0.9Sr_0.1)_2Mn_2O_7 hemos encontrado en nuestro trabajo que es posible obtener un mejor ajuste de las excitaciones magnéticas reportadas. En particular, introduciendo un nuevo acoplamiento magnético a segundos vecinos entre cadenas, no tenido en cuenta antes, podemos describir la curvatura de la dispersión que presentan las excitaciones en distintos caminos de la ZB, no descriptos los ajustes previos de sus experimentos por Johnstone et al.. Nuestros estudios para la fase intermedia nos permiten concluir que la descripción óptima de la dinámica de espines medida experimentalmente en manganitas semidopadas se obtiene suponiendo que los espines de los dímeros ubicados a lo largo de las cadenas zig-zag son paralelos. Para los niquelatos de tierras raras RNiO_3 con estructura de perovskita, estudiados como posibles compuestos multiferroicos, existen también diversas fases magnéticas propuestas para el estado fundamental a bajas temperaturas. En este caso por un lado, el vector de onda magnético característico da origen a una estructura magnética inusual, en la que cada sitio de Ni se acopla ferromagnéticamente con tres de sus primeros vecinos y antiferromagnéticamente con los tres restantes [García et al., Europhys. Lett., 20 (3), 241 (1992), Muñoz et al., J. Salid Statu Chem. 182, 1982 (2009)]. Por otra parte, se ha confirmado que existe un cierto grado de disproporcionación de carga en los iones de Ni en la serie RNiO_3 (R= Ho, Y, Er, Tm, Yb, Lu) [Alonso et al., Phys. Rev. Lett. 82, 3871 (1999)] donde en lugar de la valencia nominal Ni"3+ estos pueden tener el estado de valencia mixta Ni"(3-δ)+ y Ni(3+δ)+, sin que se haya alcanzado un acuerdo sobre el valor preciso de δ. Al considerar el posible ordenamiento tipo NaCl de los iones Ni"(3-δ)+ y Ni"(3+δ)+ y el vector de onda característico de la estructura magnética, existen al menos tres fases compatibles con estos hallazgos experimentales. Se trata de la fase colonial S propuesta a partir de experimentos de difracción de rayos X y de neutrones para PrNiO_3 y NdNiO_3 por García et al., así como para HoNiO_3 [Medarde et al., Phys. Rev. B 64,144417 (2001)]; la fase colineal T propuesta a partir de cálculos DFT [Giovannetti et al., Phys. Rev. Lett. 103, 156401 (2009)]; y la fase no-colineal N motivada por experimentos dispersión de rayos X suaves resonantes en los bordes L_2,3 del Ni y el Nd para NdNiO_3 [Scagnoli et al., Phys. Rev. B 73, 100409 (2006)]. Dado que estas fases son difíciles de distinguir a partir de experimentos de difracción, es útil contar con predicciones de las excitaciones magnéticas para ellas. En tal sentido, hemos calculado las excitaciones magnéticas de una cadena de espines unidimensional (1D), que resultarían para las fases colineal y no-colineal propuestas para estos materiales. A partir de un modelo simplificado de momentos localizados interactuantes, encontramos que, aun sin considerar la estructura tridimensional, existen diferencias notables en las bandas de magnones obtenidas para cada fase. En particular, los espectros de magnones para las fases colineal y no colineal exhiben diferencias en el número de bandas de magnones, y además la disproporcionación de carga en los sitios de Ni también afecta de diferente forma las excitaciones de cada fase. Estas diferencias predichas en nuestro trabajo [Buitrago y Ventura, J. Magn. Magn. Mater., 394, 148 (2015)] posibilitarían identificar con experimentos de dispersión inelástica de neutrones, hasta ahora inexistentes, cuál es fase presente entre las diversas propuestas bajo análisis. En la segunda parte de la tesis pasamos a estudiar las perovskitas dobles ACu_3Fe_4O_12 (A=Ca, Sr, Y, Ce y lantánidos), para las cuales experimentalmente se han encontrado diferentes tipos de transiciones de fase inducidas por temperatura, asociadas a distintos mecanismos de relajación del estado de oxidación inusualmente alto del Fe en estos compuestos. Para la perovskita doble CaCu_3Fe_4O_12 el estado Fe"4+ en la fase paramagneticametálica de alta temperatura se relaja desproporcionando la carga en los sitios de Fe, de forma que por debajo de ∼210 K se encuentra Fe"3+ y Fe"5+ en la misma proporción en una fase ferrimagnética-aislante [Yamada et al., Angew. Chem. Int. Ed. 47, 7032 (2008)]. En contraste, en la perovskita LaCu_3Fe_4O_12 el estado Fe"3.75+, presente en la fase paramagnética-metálica de altas temperaturas, se transforma en Fe"3+ en la fase antiferromagnetica-aislante por debajo 393 K. Con lo cual, en lugar de una disproporcionación de carga en los Fe ocurre una transferencia de carga del Cu al Fe [Long et al., Nature 458, 60 (2009)]. En el caso de las soluciones solidas La_xCa_1−xCu_3Fe_4O_12 con x = 0.5, 0.75, 1 el problema es más complejo, habiendo indicios de una separación de fases en las muestras, presentándose al bajar la temperatura transferencia de carga Cu-Fe en una parte de la muestra y a menor temperatura disproporcionación de carga en los Fe en el resto de la muestra [Chen et al., Sci. Rep. 2, 449 (2012)]. Para el LaCu_3Fe_4O_12, existía un modelo microscópico que reproduce las principales características de las transiciones de fase inducidas por temperatura o presión [Allub y Alascio, J. Phys.: Condens. Matter. 24, 495601 (2012)]. Además, existen varios trabajos basados en cálculos de primeros principios usando la teoría de la funcional densidad (DFT) [como por ej. Alippi et al., Eur. Phys. J. B 85, 82 (2012)] que han discutido las propiedades electrónicas y magnéticas de este compuesto. En comparación, para el CaCu_3Fe_4O_12, el mas simple de los compuestos con disproporcionación de carga en los Fe que ha sido reportado, sólo existen unos pocos cálculos DFT [como por ej. Hao et al., Phys. Rev. B 79, 113101 (2009)] que han investigado las propiedades estructurales, electrónicas y magnéticas del CaCu_3Fe_4O_12, y en particular la valencia y el estado de espın de los Fe en este compuesto. En esta tesis abordamos el estudio del CaCu_3Fe_4O_12, proponiendo el primer modelo microscópico efectivo para su descripción, basado en orbitales 3d efectivos para los sitios de Fe. Por un lado tenemos en cuenta orbitales t_2g efectivos localizados, representados por un espın S = 3/2 con acoplamientos magnéticos entre ellos. Además, incluimos dos orbitales itinerantes efectivos degenerados eg, con correlaciones electrónicas locales y a primeros vecinos, y una integral de salto (hopping) efectiva entre orbitales eg de la misma simetría, por simplicidad. Mediante un cálculo analítico basado en funciones de Green para describir las bandas asociadas a los orbitales itinerantes, y una serie de aproximaciones apropiadas para tratar las diversas correlaciones fuertes e intermedias incluidas, se determinó la energía libre del sistema. A partir de su minimización numérica, se obtuvo el diagrama de fases en función de la temperatura y el hopping efectivo entre los orbitales itinerantes de Fe efectivos, para diferentes parámetros del modelo. El mismo incluye una fase desproporcionada en carga y paramagnética (D-PM), una fase homogénea en carga y ferrimagnética (H -FiM), además de las dos fases experimentalmente observadas por Yamada et al. en 2008: una desproporcionada en carga y ferrimagnética (D-FiM) a bajas temperaturas y una fase homogénea en carga y paramagnética (H-PM) por encima de 210 K. En nuestro estudio pudimos identificar los parámetros óptimos del modelo con los cuales se explica la transición de fase experimentalmente observada [Buitrago et al., enviado para publicación en J. Appl. Phys. (abril 2018)]. En particular fue posible describir la dependencia con temperatura de la magnetización, y también la disproporcionación de carga entre Fe, que está de acuerdo con los resultados Experimentales de corrimiento isométrico. Adicionalmente, en otro rango de parámetros del modelo, nuestros resultados predicen nuevas fases que exhiben selectividad orbital, es decir, asimetría en la ocupación de los dos orbitales itinerantes en cada sitio de Fe.
Resumen en inglés
In this Thesis we have investigated the magnetic and electronic properties of a series of rare-earth metal and transition metal oxides with perovskite-like crystalline structure, of interest for their possible technological applications. Common to the studied compounds is the fact that they exhibit phase diagrams, involving complex coupled charge, orbital and spin orderings, difficulting their experimental study the correlations present between the different relevant degrees of freedom. Furthermore, all the compounds studied exhibit different degrees of charge disproportionation and in many cases also the magnetic ordering of the ground state is not clearly identified. In this Thesis we proposed simplified models for the description of many of these compounds, and have employed appropriate techniques to investigate their properties. The thesis consists of two parts. In the first one, we study the spin dynamics of different materials in order to indirectly investigate the nature of the charge and magnetic ground state present. We study in particular the following manganese oxides: layered R_1−xD_1+xMnO_4 and bilayer R_2−2xD_1+2xMn_2O_7 (R = rare earth, D = alkaline earths) , as well as nickel oxides: RNiO3 (R =rare earth). In the second part of the Thesis, we propose the first microscopic model which allows to describe the temperature-induced phase transition in the A−site-ordered double perovskite CaCu3Fe4O12, through a mechanism which involves the electronic correlations between the Fe sites. Concretely, for layered manganites like La_0.5Sr_1.5MnO_4 [Senff et al., Phys. Rev. Lett. 96, 257-201 (2006)] and bilayer Pr(Ca_0.9Sr_0.1)_2Mn_2O_7) [Johnstone et al., Phys. Rev. Lett. 109, 237202 (2012)] there are experimental results of inelastic neutron scattering experiments (INS), through which some magnon bands of these compounds have been determined. The authors of these studies have remarked that their results for these half-doped manganites are consistent with the presence of a ground state with the magnetic ordering characteristic of a generalized CE antiferromagnetic phase, having been the original CE phase proposed by J. Goodenough [Phys. Rev. 100, 564 (1955)] for the lanthanum halfdoped manganites La_0.5D_0.5MnO_3. At the same time, the former authors stated that their INS results allowed to exclude as ground state the Zener polaron dimer phase, proposed by Daoud-Aladine et al. in Pr_0.6Ca_0.4MnO_3 to explain their neutron diffraction data [Phys. Rev. Lett. 89, 097205 (2002)]. Beyond these two scenarios, Efremov, Van den Brin Khomskii proposed a new phase, which was called the intermediate phase [Nat. Mater. 3, 853 (2004)], which they indicated might be relevant to reconcile different aspects of the CE and Zener polaron phases, and interpolate between them. Our research work is the first to study the spin dynamics of the intermediate phase, and to indirectly use it to explore the nature of the fundamental ground state in half-doped manganites. The intermediate phase consists of spin dimers located along the Mn zig-zag chains present in the planes of the half-doped manganites. The dimers may be composed by Mn spins of different magnitudes, representing in this way an eventual Mn charge disproportionation. The intermediate phase also allows to consider the possibility that the spins of consecutive dimers along a zig-zag chain are not strictly parallel, an eventual constant rotation angle between the spin directions of consecutive dimers being present. To study the spin dynamics of layered and bilayer manganites we proposed a model consisting of localized and interacting spins, for which we found that the intermediate phase is stable in the classical limit. In the quantum case, we calculated the magnetic excitations of the model obtained for the intermediate phase. We made a thorough study of the effect on the magnon bands of various magnetic coupling parameters included in the model, in particular the new parameters we introduced to improve previous fits of the INS experiments in La_0.5Sr_1.5MnO_4, Pr(Ca_0.9Sr_0.1)_2Mn_2O_7 and Nd_0.5Sr_0.5MnO_3 [Ulbrich et al., Phys. Rev. B. 84, 094453 (2011)]. In addition, from our analysis of the stability of the quantum intermediate phase, we conclude that the orthogonal intermediate phase, with π/2 angle between consecutive dimers proposed by Efremov et al. is unstable. For single-layer manganite La_0.5Sr_1.5MnO_4, for which only the magnon excitations up to 40 meV were measured [Senff et al., Phys. Rev. Lett. 96, 257-201 (2006)], we were able to fit the measured magnons with generalized CE phases as well as with a dimer phase. We predict important differences between the excitations of these different phases for the higher-energy spin excitation bands, above the gap between the lower and upper magnons branches (above 40 meV), not yet measured. Our results predict that the measurement of the magnon branches above the magnon gap, would provide the key to identify unambiguously the elusive ground state present in the layered half-doped manganites [I. Buitrago y C. Ventura, J. Supercond. Nov. Magn. 26 (6), 2303 (2012)]. In our study for bilayer manganite Pr(Ca_0.9Sr_0.1)_2Mn_2O_7 [I. Buitrago et al., sent to J. Magn. Magn. Mater , Jan. 2018] we also found differences between the phases under study, mainly for the higher-energy spin excitation bands, above the magnon gap. With the experimental data now available, none of the proposals for the ground state of layered manganites can be excluded yet. On the other hand, for bilayer Pr(Ca_0.9Sr_0.1)_2Mn_2O_7 in our work we found that it is possible to obtain an improved fit of the magnetic excitations reported [Johnstone et al., Phys. Rev. Lett. 109, 237202 (2012)] , and in particular that it is possible to describe the curvature of the magnon dispersion measured along different paths of the BZ not reproduced in previous fits, by including a new next-nearest-neighbor magnetic coupling, not taken into account before. Finally, our studies of the intermediate phase, allow us to conclude that the optimal description of the spin dynamics measured in half-doped manganites is obtained assuming that the spins of consecutive dimers along the zig-zag chains are parallel. For the RNiO_3 rare earth nickelates compounds with perovskite structure, which are investigated as possible multiferroic compounds, also several magnetic phases have been proposed for the ground state at low temperatures. In this case, the characteristic magnetic wave vector gives rise to an unusual magnetic structure, in which each Ni site is ferromagnetically coupled with three of its nearest-neighbors and antiferromagnetically with the remaining three [Garcia et al., Europhys. Lett., 20 (3), 241 (1992), Muñoz et al., J. Solid State Chem. 182, 1982 (2009)]. In addition, it has been confirmed that there is a certain degree of charge disproportionation in the Ni ions in the series RNiO_3 (R =Ho, Y, Er, Tm, Yb, Lu) [Alonso et al., Phys. Rev. Lett. 82, 3871 (1999)] where instead of the nominal valence Ni"3+ these could have the mixed valence state Ni(3−δ)+ and Ni(3+δ)+, without having been reached an agreement on the precise value of the Ni-disproportionation δ among different studies. When considering the possible NaCl-type ordering of the Ni(3−)+ and Ni(3+)+ ions, and the characteristic wave vector of the magnetic structure, there are at least three phases compatible with these experimental findings. They are: the colinear phase S, proposed on the basis of X-ray and neutron diffraction experiments in PrNiO_3 and NdNiO_3 by García et al., as well as for HoNiO_3 [Medarde et al., Phys. Rev B 64,144417 (2001)]; the colinear phase T proposed from DFT calculations [Giovannetti et al., Phys. Rev. Lett. 103, 156401 (2009)]; and the non-collinear phase N proposed on the basis of resonant soft X-ray scattering experiments at the L_2,3-Ni and -Nd edges for NdNiO3 [Scagnoli et al., Phys. Rev. B 73 , 100409 (2006)]. Since these phases are difficult to distinguish in diffraction experiments, it is useful to have predictions of the magnetic excitations expected for each of them. In this context, we have calculated the magnetic excitations of a one-dimensional (1D) spin chain, such as the ones included in the collinear and non-collinear phases proposed for these materials. Using a simplified model of interacting localized moments, we found that, even without considering the three-dimensional structure, there are significant differences in the magnon bands resulting for the different phases. In particular, the magnon spectra for the collinear and non collinear phases show differences in the number of magnon bands, and in addition, the charge disproportionation in the Ni sites also affects the excitations of each phase in different ways. These differences predicted in our work [I. Buitrago y C. Ventura, J. Magn. Magn. Mater., 394, 148 (2015)], would make it possible to distinguish between them in inelastic neutron scattering experiments, not available yet, and identify the nature of the ground state among the various proposals under analysis. In the second part of the Thesis, we studied the A-site-ordered double perovskites ACu_3Fe_4O_12 (A =Ca, Sr, Y, Ce and lanthanides), for which different types of temperatureinduced phase transitions have been experimentally reported, associated to different mechanisms of relaxation of the unusually high oxidation state of Fe in these compounds. For the CaCu_3Fe_4O_12 double perovskite, the Fe"4+ state in the high-temperature paramagneticmetallic phase relaxes disproportionating the charge in the Fe sites, so that below ≈ 210 K it was found that Fe"3+ and Fe"5+ are present in the same proportion in a ferrimagneticinsulating phase [Yamada et al., Angew. Chem. Int. Ed. 47, 7032 (2008) ]. In contrast, in LaCu_3Fe_4O_12 the state Fe"3.75+, present in the paramagnetic-metallic phase at high temperatures, is transformed into Fe"3+ in the antiferromagnetic-insulator phase below 393 K. Thus, instead of charge disproportionation in Fe, here a charge transfer from Cu to Fe takes place [Long et al., Nature 458, 60 (2009)]. In the case of the solid solutions with formula La_xCa_1−xCu_3Fe_4O_12 with x = 0.5, 0.75, 1, the problem becomes more complex, since both the Cu-Fe intersite charge transfer and the charge disproportionation in Fe occur [Chen et al., Sci. Rep. 2, 449 (2012)] and there are also indications of phase separation taking place in the samples. Decreasing temperature, first Cu-Fe charge transfer takes place in part of the sample, while at a lower temperature Fe presents charge disproportionation in the rest of the sample.[Chen et al., Sci. Rep. 2, 449 (2012)] For LaCu_3Fe_4O_12 a microscopic model existed, able to describe the main characteristics of the phase transitions induced by temperature or pressure [Allub and Alascio, J. Phys .: Condens. Matter. 24, 495601 (2012)]. In addition, various works based on first principles calculations using the density functional theory (DFT) [like e.g. Alippi et al., Eur. Phys. J. B 85, 82 (2012)] had discussed the electronic and magnetic properties of this compound. In comparison, for CaCu_3Fe_4O_12, the simplest compound in this family where Fe charge disproportionation has been reported, there are only a few DFT calculations [e.g. Hao et al., Phys. Rev. B 79, 113101 (2009)] where the structural, electronic and magnetic properties, and in particular the valence and the spin state of the Fe ions, have been investigated. In this Thesis we studied CaCu_3Fe_4O_12, proposing the first microscopic effective model to describe this compound. The model takes into account effective 3d-orbitals for the Fe sites. On one hand, we assumed localized effective t_2g orbitals, represented by a spin S = 3/2 and magnetic coupling between them. We also included two effective itinerant degenerate eg orbitals, with local and next-neighbor electronic correlations, as well as an effective hopping between eg orbitals of the same symmetry, for simplicity. By an analytical calculation based on Green’s functions to describe the bands associated to the itinerant orbitals, and a series of approximations appropriate to treat the different strong and intermediate electron correlations included in the model, we determined the free energy of the system. By its numerical minimization, the phase diagram as a function of temperature and the effective hopping between nearestneighbor Fe ions was obtained, for different sets of parameters of the model. The phase diagram includes and Fe charge disproportionated-paramagnetic phase (D-PM), a charge homogeneous-ferrimagnetic phase (H-FiM), and also the two phases which were reported experimentally by Yamada et al. in 2008: the Fe charge disproportionated-ferrimagnetic (D-FiM) phase, and the charge homogeneous-paramagnetic (H-PM) phase. In our study we identified the optimal set of parameters of the model, to explain the phase transition experimentally observed [I. Buitrago et al., sent to J. Appl. Phys., Apr. 2018]. Furthermore, in the region of the phase diagram where the experimental phase transition is found, we were also able to describe the dependence with temperature of the magnetization and the Fe-charge disproportionation, which agrees with the experimental isomeric shift measurements. Finally, in another range of parameters of the model, our results predict new phases exhibiting orbital selectivity, i.e. with asymmetric occupation of the two itinerant orbitals in each Fe site.
| Tipo de objeto: | Tesis (Tesis Doctoral en Física) |
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| Palabras Clave: | Magnons; Magnones; Phase diagrams; Diagramas de fase; [Nickelates;Niquelatos; Iron-copper oxides; Óxidos de hierro-cobre; Change disproportionation; Desproporcionación de carga; Half-doped manganites; Manganitas semidopadas] |
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| Materias: | Física > Materia condensada Física |
| Divisiones: | Gcia. de área de Investigación y aplicaciones no nucleares > Gcia. de Física > Materia condensada > Teoría de sólidos |
| Código ID: | 713 |
| Depositado Por: | Tamara Cárcamo |
| Depositado En: | 07 Sep 2018 14:50 |
| Última Modificación: | 07 Sep 2018 15:06 |
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