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International audience

International audience

International audience; The magnetoresistance (MR) effect is widely used in technologies that pervade the world, from magnetic reading heads to sensors. Diverse contributions to MR, such as anisotropic, giant, tunnel, colossal, and spin-Hall, are revealed in materials depending on the specific system and measuring configuration. Half-metallic manganites hold promise for spintronic applications but the complexity of competing interactions has not permitted the understanding and control of their magnetotransport properties to enable the realization of their technological potential. This study reports on the ability to induce a dominant switchable magnetoresistance in La 0.7 Sr 0.3 MnO 3 epitaxial films at room temperature (RT). By engineering an extrinsic magnetic anisotropy, a large enhancement of anisotropic magne-toresistance (AMR) is achieved which at RT leads to signal changes much larger than the other contributions such as the colossal magnetoresist-ance. The dominant extrinsic AMR exhibits large variation in the resistance in low field region, showing high sensitivity to applied low magnetic fields. These findings have a strong impact on the real applications of manganite-based devices for the high-resolution low field magnetic sensors or spintronics.

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 26-09-2014, In the current work, two different aspects of the properties of graphene epitaxially grown on metallic substrates have been studied. On the one hand, the effects of the interaction with a strong interacting Ru(0001) substrate on the lattice dynamics of the graphene, as well as the charge transfer mechanisms arisen in the adsorption of acceptor molecules like TCNQ on graphene grown on Ir(111) and Ru(0001). The study has been carried out using two different surface probe techniques: Helium Atom Scattering (HAS) and Photoemission spectroscopy (PES). The high-resolution HAS measurements of angular distributions and time-of-flight (TOF) spectra have been measured along the I M direction of the first Brillouin zone (IBZ) of graphene. HAS measurements show the coexistence of areas with different properties at the surface. The HAS diffraction profiles of the Gr/Ru surface present evidence of the large periodicity due to the Gr moiré structure, and at the same time reveal an unexpected contribution of the electron charge density of the Ru(0001) surface, even though the substrate is fully covered by the graphene monolayer. On the other hand, the TOF spectra from Gr/Ru reveal a complex scenario in which four energy dispersion branches, in addition to a low energy dispersionless mode, can be clearly distinguished. In particular, the Rayleigh phonon mode of Ru(0001) is clearly seen in the phonon spectra of Gr/Ru. This suggests that He atoms are able to interact with the Ru atoms of the topmost substrate layers, through the graphene, in a sort of long-range interaction. In addition, the dispersion of the three acoustic surface phonon branches of the graphite are preserved unchanged in this heterostructure. Of particular interest are the flexural mode (ZA), which carries information about the bending rigidity and strain of the surface, and the branch early ascribed to the shear horizontal mode (TA), which clearly appears in the spectra in spite of the fact that its excitation is in principle forbidden due to the symmetry of our scattering geometry. Therefore, the latter issue points to a re-evaluation of the origin of this energy dispersion mode. We have proposed as possible explanation an electronic origin of this mode, partly motivated by the recent observation of similar plasmonic modes in topological insulators. An analogous study has been performed for samples where Cu was intercalated underneath the graphene monolayer. In this case, it is found that graphene extends continuously over the Ru(0001) surface and the intercalated areas. However, HAS diffraction patterns indicate that besides an enhancement of the atomic corrugation, a change of the electron charge density distribution takes place with the intercalation. On the other hand, the TOF measurements show that the dispersionless mode prevails in the vibrational spectra. This trend is more clearly visible by enhancing the intercalated Cu coverage between the Gr monolayer and the Ru substrate. These low energy vibrational excitations are attributed to the modes of the ordered array of the nanostructures, formed by the Gr ripples.This characteristic may play a key role in the design of a new generation of graphene–based nanoelectromechanical devices. The physical mechanisms arising on the Gr/Ru surface due the absorption of 7,7’,8,8’ Tetracyanoquinodimethane (TCNQ) have been studied by means of X–ray and UV photoemission spectroscopy. The experimental results demonstrate that there is a site-dependent charge transfer to the adsorbed molecule. Moreover, it was possible to determine that the molecules adsorbed on the lower part of the moiré ripples are charged with electrons donated from the doped graphene overlayer, developing a magnetic moment, while those at the upper part of the ripples remain neutral. This study includes a comparison with the results obtained for TCNQ adsorbed on graphene on Ir(111). In contrast to the former case, the PES analysis shows a negligible charge transfer. These observations explain the spatially dependent long range magnetic order observed for TCNQ on Gr/Ru(0001, En esta tesis doctoral se han estudiado dos aspectos relevantes de las propiedades del grafeno crecido epitaxialmente sobre sustratos metálicos. Por un lado, se han investigado los efectos en la dinámica de la red cristalina del grafeno, introducidos por la fuerte interacción con la superficie de Ru(0001), y por otra parte los mecanismos de transferencia de carga que surgen en la adsorción de moléculas aceptoras como TCNQ en grafeno crecido sobre Ir(111) y Ru(0001). Para este propósito, se han utilizado dos técnicas de estudio de física de superficies, como son la dispersión de átomos de helio (HAS) y la espectroscopá de fotoemisión (PES). En particular, las medidas de alta resolución de HAS, tanto de distribuciones angulares como de espectros de tiempo de vuelo (TOF) para determinar las curvas de dispersión de fonones, se realizaron a lo largo de la dirección I M de la primera zona de Brillouin (IBZ) del grafeno. Las medidas HAS muestran la coexistencia de propiedades con distintas características en la superficie. Los espectros de difracción de He revelan la periodicidad debida a la estructura ordenada de moiré de grafeno crecido epitaxialmente en Ru(0001) (Gr/Ru), y al mismo tiempo una contribución inesperada de la densidad de carga del Ru(0001), aunque el sustrato metálico está completamente cubierto por la monocapa de grafeno. Por otra parte, los espectros TOF de Gr/Ru revelan un escenario aún mas complejo, en el que hay cuatro ramas de dispersión de fonones ademas de un modo vibracional sin dispersión a baja energía a lo largo de la primera zona de Brilluoin (IBZ) del grafeno. En particular, el modo de fonones de Rayleigh de Ru(0001) es visible también en los espectros de fonones de Gr/Ru. Esto sugiere que los átomos de He pueden interactuar con las capas atómicas superficiales de Ru(0001), a través del grafeno, en una interacción de largo alcance. Al mismo tiempo, se puede ver que las tres ramas de fonones acústicos de superficie observadas en grafito, conservan en esta heteroestructura la misma dispersión sin cambios aparentes. Entre estas ramas, son de particular interés el modo de flexión (ZA), que lleva información sobre la rigidez de flexión y los efectos de las deformaciones, además de influir en la conductividad térmica, y la rama que hasta la fecha había sido atribuido al modo horizontal (TA), que aparece claramente en nuestro espectros a pesar de que su excitación resulta teóricamente prohibida para nuestra configuración experimental. Por tanto, estos resultados requieren una nueva interpretación para este modo. A falta de cálculos detallados que nos permitan entender mejor su origen, dicho modo se ha interpretado tentativamente como resultado de un mecanismo electrónico, motivados en parte por la observación reciente de modos plasmónicos con dispersiones similares en aislantes topológicos. Se ha realizado un estudio similar para muestras en las que Cu se ha intercalado debajo de la monocapa. En este caso, se encuentra que el grafeno se extiende sin interrupciones sobre la superficie de Ru(0001) y las islas de Cu. Sin embargo, la superficie muestra patrones de difracción que indican que, además de un aumento de la corrugación atómica, hay un cambio de la distribución de densidad electrónica. Por otro lado, las medidas de tiempo de vuelo muestran que el modo sin dispersión prevalece en los espectros de vibración. Esta tendencia se ve con mas claridad al aumentar la cantitad de Cu intercalado entre la monocapa Gr y el Ru(0001). La disposición ordenada de las nanoestructuras de moiré, formado por la reconstrución que el grafeno tiene en este substrato metálico, puede ser responsable de esta excitación de baja energía. Esta característica podría tener gran importancia en el diseño de una nueva generación de dispositivos electromecánicos de dimensiónes nanométricas. Los mecanismos físicos que surgen en la superficie de Gr/Ru debido a la absorción de 7,7 ’, 8,8’ tetracianoquinodimetano (TCNQ) fue estudiada por medio de la espectroscopía de fotoemisión de rayos X y UV (XPS y UPS). Los resultados experimentales recogidos demuestran que en esta heteroestructura existe una transferencia de carga que resulta ser dependiente del sitio de adsorciíon de la molécula. Además, se pudo determinar que las moléculas adsorbidas en la parte inferior del moiré están cargadas con los electrones donados por el grafeno dopado por el Ru(0001), y desarrollan un momento magnético, mientras que las que están en la parte superior del moiré permanecen electrónicamente neutras. Este estudio incluye una comparación con los resultados obtenidos en TCNQ adsorbido en grafeno crecido en Ir(111). Contrariamente al primer caso, el análisis de los espectros de fotoemisión revela una transferencia de carga insignificante. Estas observaciones explican el orden magnético observado recientemente para TCNQ en Gr/Ru(0001), y su ausencia en el caso de TCNQ en Gr/Ir(111).

We report on the transport characterization in dark and under light irradiation of three different interfaces: LaAlO3/SrTiO3, LaGaO3/SrTiO3, and the novel NdGaO3/SrTiO3 heterostructure. All of them share a perovskite structure, an insulating nature of the single building blocks, a polar/non- polar character and a critical thickness of four unit cells for the onset of conductivity. The interface structure and charge confinement in NdGaO3/SrTiO3 are probed by atomic-scale- resolved electron energy loss spectroscopy showing that, similarly to LaAlO3/SrTiO3, extra electronic charge confined in a sheet of about 1.5 nm in thickness is present at the NdGaO3/SrTiO3 interface. Electric transport measurements performed in dark and under radiation show remarkable similarities and provide evidence that the persistent perturbation induced by light is an intrinsic peculiar property of the three investigated oxide-based polar/non-polar interfaces. Our work sets a framework for understanding the previous contrasting results found in literature about photoconductivity in LaAlO3/SrTiO3 and highlights the connection between the origin of persistent photoconductivity and the origin of conductivity itself. An improved understanding of the photo- induced metastable electron-hole pairs might allow to shed a direct light on the complex physics of this system and on the recently proposed perspectives of oxide interfaces for solar energy conversion., Comment: 11 pages, 7 figures

The interface between the band gap insulators LaAlO3 and SrTiO3 is known to host a highly mobile two-dimensional electron gas. Here we report on the fabrication and characterization of the NdGaO3/SrTiO3 interface, that shares with LaAlO3/SrTiO3 an all-perovskite structure, the insulating nature of the single building block and the polar-non polar character. Our work demonstrates that in NdGaO3/SrTiO3 a metallic layer of mobile electrons is formed, with properties comparable to LaAlO3/SrTiO3. The localization of the injected electrons at the Ti sites, within a few unit cells from the interface, was proved by Atomic-scale-resolved EELS analyses. The electric transport and photoconduction of samples were also investigated. We found that irradiation by photons below the SrTiO3 gap does not increase the carrier density, but slightly enhances low temperature mobility. A giant persistent photoconductivity effect was instead observed, even under irradiation by low energy photons, in highly resistive samples fabricated at non-optimal conditions. We discuss the results in the light of different mechanisms proposed for the two-dimensional electron gas formation. Both the ordinary and the persistent photoconductivity in these systems are addressed and analyzed., 24 pages, 7 figures, submitted to ACS Nano