Propiedades electrónicas y magnéticas en óxidos multiferroicos y en materiales nanoestructurados / Electrical and magnetic properties in multiferroic oides and nanoestructure materials

Lohr, Javier H. (2017) Propiedades electrónicas y magnéticas en óxidos multiferroicos y en materiales nanoestructurados / Electrical and magnetic properties in multiferroic oides and nanoestructure materials. PhD in Physics, Universidad Nacional de Cuyo, Instituto Balseiro.

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Abstract in Spanish

El interés en los materiales multiferroicos que presentan coexistencia de orden magnético y orden ferroeléctrico, se ha incrementado en los últimos años debido a su potencial tecnológico. Dentro de la familia de los materiales multiferroicos, el mayor interés lo presentan los compuestos magnetoeléctricos, es decir aquellos en los cuales el orden ferroeléctrico esta acoplado con un orden magnético. La posibilidad de controlar la magnetización con un campo eléctrico es muy interesante para su aplicación en memorias. Por otro lado se esta avanzando en la creación de estructuras heterogéneas, conocidas como multiferroicos articiales, que combinen materiales magnéticos con materiales ferroeléctricos, las cuales permiten un amplio abanico de posibilidades y una mejora en las propiedades tanto magnéticas como ferroeléctricas. En este trabajo se presenta el estudio de nuevos materiales con posibilidades de tener propiedades multiferroicas. Para ello se utilizaron distintas estrategias. La primera fue la síntesis de una familia de compositos nanoestructurados que combinan el material ferromagnético La_0.5 Sr_0.5 CoO_3 (LSCO) con el multiferroico BiFeO_3 (BFO), x La-0.5 Sr_0.5 CoO_3-(1- x) BiFeO_3. Otra estrategia fue la sustitución cationica de compuestos con propiedades multiferroicas con la idea de mejorar sus propiedades. Se sintetizaron y se estudiaron las propiedades eléctricas y magnéticas de dos familias de óxidos en estado masivo. Una de ellas es la familia RCrMnO_5 (R=Sm, Eu, Gd, Tb, Ho y Er). Se tomo como referencia la familia de compuestos multiferroicos RMn_2O_5 en los cuales el orden magnético y el ferroeléctrico están relacionados. La segunda familia de compuestos del tipo RFe_0.5 Co_0.5 O_3 se baso en las ortoferritas multiferroicas RFeO3 como referencia. Ademas, pensado en las posibles nanoestructuras que pueden conformar una estructura heterogénea se realizaron estudios eléctricos en nanoestructuras individuales como nanotubos ferromagnéticos, nanohilos metálicos y compuestos planares que podrían utilizarse como electrodos en diferentes dispositivos. Los compositos xLSCO-(1 - x) BFO (x = 0; 0.1; 0.2; 0.5; 0.8; 0.9; 1) fueron sintetizados a partir de polvos nanoestructurados de las fases que los conforman, los cuales fueron preparados previa e individualmente por la técnica de spray pirolisis. Los polvos fueron mezclados mecanicamente en las proporciones requeridas. Primeramente se estudio el LSCO nanoestructurado, el mismo presenta propiedades que dieren de las del material masivo. Su conductividad eléctrica aumenta con la temperatura a diferencia del material masivo el cual es metálico. La conductividad eléctrica del material sintetizado se puede describir con el modelo de Glazman y Matveev en el cual la conducción es a través de dos y tres estados localizados entre barreras aislantes. En principio, estas barreras están asociadas a la gran cantidad de bordes de granos en el material. Por su parte la magnetización de saturación se ve reducida producto de una capa magneticamente muerta en los bordes de los granos. En cuanto a los compositos, todos (salvo los extremos) presentaron una señal magnética adicional a la presente en las fases individuales. Se efectuaron mediciones de permitividad eléctrica, ciclos de polarizacion vs. campo eléctrico y mediciones de acople magnetoeléctrico. Estas ultimas mostraron que en el composito x=0.1 el acople es débil comparado con otros compositos reportados en la bibliografía. En el estudio de compuestos masivos de la familia RCrMnO_5, se estudiaron en profundidad las propiedades magnéticas. A pesar de que las mediciones con difracción de neutrones no mostraron orden magnético de largo alcance (R= Er, Ho, Tb), en las mediciones de magnetización a bajas temperaturas se observa una anomalía que podría estar asociada a un orden de corto alcance debido a la frustración magnética. En el caso de R=Sm esta anomalía es mas evidente indicando un orden antiferromagnético de largo alcance. Por su parte, las mediciones de magnetocapacitancia muestran un máximo cercano a dicha transición. Anomalías similares también fueron observadas en las mediciones de la componente de perdidas eléctricas. Este comportamiento puede estar mas bien asociado a un efecto magnetorresistivo que capacitivo. La resistividad electrica muestra un comportamiento tipo variable range hopping (p / T -1/4). Los resultados mas prometedores se encontraron en los compuestos de la familia RFe_0.5 Co_0.5 O_3. En medidas de magnetización se observa una transición ferromagnética débil (antiferro alabeado) y otra a temperaturas menores que muestra una transición asociada a un reordenamiento de espín. La conductividad electrica presenta un comportamiento tipo variable range hopping y se destacan resultados de corriente piroeléctrica que muestran una polarizacion remanente en TmFe_0.5 Co_0.5 O_3 y en YbFe_0.5Co_ 0.5O_3 por debajo de la temperatura de orden magnético. Por ultimo, pensado en la formación de futuras heteroestructuras, se presenta el estudio de las propiedades eléctricas en nanoestructuras de modo macroscópico y microscópico. Se presentan mediciones in-situ de una película de oxido de grafeno altamente reducido (HRGO). Se pudo medir la resistividad de un parche de HRGO presentando una baja resistividad que también fue medida macroscopicamente. Por otro lado se realizaron mediciones en nanohilos de plata individuales. Estos presentaron un aumento de la resistividad al disminuir el diámetro del mismo el cual se pudo describir con los resultados obtenidos del modelo de Mayadas.

Abstract in English

In last years, the interest in multiferroic materials, with magnetic and ferroelectric order, has been incremented due to potential technological applications. The ability to control magnetization by applying an electric feld is very attractive for memory devices. The most interesting materials in the multiferroic family are materials where the magnetic and the ferroelectric order are strongly coupled. These are called magnetoelectric materials. On the other hand, the new advances in the fabrication of heterostructures that combine ferroelectric and magnetic materials allow enlarging the possibilities and improving the magnetic and ferroelectric properties. In this work, we present a study of new materials with the possibilities of presenting multiferroic properties. Different strategies were used. The first one was the synthesis of nanocomposites combining the ferromagnetic oxide La_0.5 Sr_0.5 CoO_3 (LSCO) with the multiferroic BiFeO_3 (BFO), x La_0.5 Sr_ 0.5 CoO_3-(1- x) BiFeO_3 The second strategy was the cationic substitution of multiferroic compounds with the aim of improving their properties. Two families of bulk oxides were synthesised and their electric and magnetic properties were studied. One of these families is RCrMnO_5 (R=Sm, Eu, Gd, Tb, Ho y Er), which is isostructural with the multiferroic reference compounds RMn2O_5. The other family under study is RFe0:5Co0:5O3, which is based on the multiferroic RFeO_3 orthoferrites. We also present electrical studies on individual nanostructures like ferromagnetic nanotubes, metallic nanowires, and planar compounds which could be used as electrodes in different devices or as integrating part of new heterostructures. Composites xLSCO-(1 - x) BFO(x = 0; 0.1; 0.2; 0.5; 0.8; 0.9; 1) were synthesized starting from nanopowders of the composite phases. The nanopowders were synthesized by the spray-pyrolysis technique and then mechanically mixed in an agate mortar in the desired stoichiometry. The nanostructured LSCO was studied in the first place, showing differences with respect to the bulk properties. The electric conductivity increases with temperature, this is opposite to the behaviour of the bulk material, which is metallic. The conductivity of nanostructured LSCO can be described with the Glazman-Matveev model. The conduction mechanism is by hopping between localized states within insulating barriers. We associate these barriers to the grain boundaries in the material. The saturation magnetization is reduced as compared to the bulk, due to the magnetic dead layer at the grain boundary. We perform electric permittivit electric polarization loops and magneto-electric effect measurements. The composite with x = 0:1 composition shows a magnetoelectric coupling which is weak compared with other reported composites. The magnetic properties of RCrMnO_5 bulk materials are extensively studied. Despite the fact that neutron diffraction measurements do not show any re ections associated to a long-range magnetic order in R=Er, Ho, Tb, magnetization measurements present an anomaly at low temperatures. This anomaly could be associated to a shortrange magnetic order due to magnetic frustration. For the R=Sm case, the magnetic anomaly is clearly visible and similar to the cases with long antiferromagnetic order. On the other hand, the magneto-capacitance measurements show a maximum near this anomaly. The anomalies were observed on the loss component of the magnetocapacitance. This behaviour can be associated to a magneto-resistive rather than capacitive effect. The electric resistivity shows a variable range hopping behaviour (p / T -1/4). The most promising results were found in the RFe_0.5Co_0.5O_3 family. The magnetic measurements show a weak ferromagnetic transition (canted antiferromagnetic) and below this temperature a second transition is observed, associated to spin reorientation. Electrical conductivity also shows a variable range hopping behaviour. It is remarkable that the remanent electric polarization was observed below the magnetic order temperature by pyroelectric measurements in TmFe_0.5 Co_0.5 O_3 y en YbFe_0.5Co_ 0.5O_3 Finally, we present macroscopic and microscopic studies of the electrical properties of nanostructures, thinking of possible heterestructures to be fabricated in the future. I-V curves were measured on a highly reduced graphene oxide film (HRGO). The electrical resistivity of a HRGO patch shows low values close to the obtained macroscopically. Also individual silver nanowires were measured. These present increasing resistivity with decreasing nanowire diameter.

Item Type:Thesis (PhD in Physics)
Keywords:Nanoestructure; Nanoestructura; Oxides; Óxido; Magnetism; Magnetismo.
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Subjects:Physics > Materia condensada
Divisions:Investigación y aplicaciones no nucleares > Física > Resonancias magnéticas
ID Code:642
Deposited By:Tamara Cárcamo
Deposited On:26 Oct 2017 11:36
Last Modified:27 Oct 2017 11:01

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