Photoemission electron microscopy of three-dimensional magnetization configurations in core-shell nanostructures

5 páginas, 4 figuras.-- PACS number(s): 75.75.−c, 68.37.Yz, 81.07.−b.-- et al.We present a photoemission electron microscopy method that combines magnetic imaging of the surface and of the inner magnetization in three-dimensional core-shell nanostructures. The structure investigated consists of a cylindrical nickel core that is completely surrounded by a shell of iron oxide and silicon oxide layers. The method enables one to image the magnetization configuration of the nickel core even though the shell is thicker than the mean-free path of the photoelectrons. Characteristic L3 and L2 edges can be observed not only in the yield of the photoelectrons emitted from the surface of the nanostructure but also in its shadow. X-ray magnetic circular dichroism in the electron yield of the x rays absorbed and transmitted by the multilayered nanowire allows for the individual imaging of the magnetization configurations of the iron oxide tube and the nickel core. The method suggests novel approaches for the characterization of the magnetic and material properties of complex three-dimensional nanostructures.Financial support by the Deutsche Forschungsgemeinschaft via the Sonderforschungsbereich 668 and the Graduiertenkolleg 1286 as well as by the Forschungs- und Wissenschaftsstiftung Hamburg via the Cluster of Excellence Nanospintronics and by the Helmholtz-Zentrum Berlin is gratefully acknowledged.Peer reviewe

Universal behavior for magnetic entropy change in magnetocaloric materials: An analysis on the nature of phase transitions

A universal curve for the change in the magnetic entropy has been recently proposed for materials with second-order phase transitions. In this work we have studied the universal behavior of the magnetocaloric effect in the family of cobalt Laves phases, RCo2, and mixed manganites, La2/3(CaxSr(1−x))1/3MnO3, which exhibit first- and second-order phase transitions. The rescaled magnetic entropy change curves for different applied fields collapse onto a single curve for materials with second-order phase transition as opposed to the first-order phase transition compounds, for which this collapse does not hold. This result suggests that the universal curve may be used as a further criterion to distinguish the order of the phase transition

Evolución y comparación de los diferentes métodos y procedimientos de combate en zonas urbanizadas durante la última década

El trabajo, “Evolución y comparación de los diferentes métodos y procedimientos de combate en zonas urbanizadas durante la última década”, tiene como objetivo final mejorar la instrucción de las pequeñas unidades de infantería ligera en el combate en zonas urbanizadas. El resultado de este trabajo es fruto del estudio y análisis de la instrucción en combate urbanizado del Batallón de Infantería “Legazpi” I/67 (San Sebastián), el cual ha servido como muestra para comprender la situación actual de la instrucción en el Ejército de Tierra e identificar los principales problemas a los que se enfrenta. El principal problema que atañe al combate en población es la falta de una instrucción estandarizada, consecuencia de la carencia de un manual aceptado y respetado por todas las unidades de España (actualmente el único manual de instrucción oficial del combate en zonas urbanizadas es el MI4-001 2ª Edición, publicado en 2006). Ante esta situación, algunas unidades han sentido la necesidad de crear sus propios manuales o normas operativas, con el fin de establecer unos procedimientos actualizados y mejorados desde un punto de vista subjetivo. Estas aportaciones suponen en determinados aspectos ciertas desigualdades con el manual oficial. Por esta razón, este trabajo se centra en estudiar la posibilidad de renovar el mismo. Para ello ha sido necesaria la comparación de sus contenidos con los desarrollados por las distintas unidades del Ejército. Dicha comparación pretende determinar por una parte, cuantos cambios han sufrido los contenidos del manual de referencia y, por otro lado, cuantificar la intensidad de los mismos. En base a los resultados obtenidos de la frecuencia y de la magnitud de estos cambios se puede desmentir la hipótesis de que el manual oficial del Ejército de Tierra es ideal para una completa instrucción.<br /

Crecimiento y caracterización de óxidos de iridio y cobalto para espintrónica.

Los óxidos de iridio han demostrado ser de gran utilidad en la espintrónica, una nueva tecnología que combina electrónica y magnetismo y permite leer o manipular el estado magnético de un material mediante corrientes eléctricas. En este trabajo se estudiarán las propiedades de los óxidos de iridio y cobalto para su aplicación en la espintrónica, concretamente en las memorias SOT-MRAM. Para la realización de este estudio en primer lugar, se han creado láminas delgadas de IrO2 dopadas con distintas cantidades de CO mediante deposición catódica. Posteriormente estas han sido sometidas a análisis estructurales mediante reflectometría de rayos X y difracción de rayos X. Además, se ha obtenido información adicional sobre la microestructura y composición a partir de un análisis de microscopía electrónica de barrido. Por último, se ha realizado un análisis magnético con el que estudiar la presencia de imanación en el material. Una vez estudiada la estructura de las láminas de IrO2 dopadas con Co se concluye que se ha obtenido una muestra policristalina con la estructura rutilo típica del IrO2. Además, se ha observado que tanto el volumen de celda como el tamaño de grano disminuye con la cantidad de Co. Por último, en el análisis magnético se ha concluido que el comportamiento de las láminas de IrO2 dopadas con Co se corresponde con un paramagnético y puesto que la imanación de las láminas no varía con la cantidad de Co el óxido de Co depositado tiene un estado de oxidación Co3+ con configuración de bajo espín LS. Por lo tanto, para las láminas con un 50% de Co el Ir presente será Ir5+ un ion beneficioso para las memorias SOT-MRAM.<br /

Paleomagnetic evidence for dynamo activity driven by inward crystallisation of a metallic asteroid

The direction in which a planetary core solidifies has fundamental implications for the feasibility and nature of dynamo generation. Although Earth's core is outwardly solidifying, the cores of certain smaller planetary bodies have been proposed to inwardly solidify due to their lower central pressures. However, there have been no unambiguous observations of inwardly solidified cores or the relationship between this solidification regime and planetary magnetic activity. To address this gap, we present the results of complimentary paleomagnetic techniques applied to the matrix metal and silicate inclusions within the IVA iron meteorites. This family of meteorites has been suggested to originate from a planetary core that had its overlaying silicate mantle removed by collisions during the early solar system. This process is thought to have produced a molten ball of metal that cooled rapidly and has been proposed to have inwardly solidified. Recent thermal evolution models of such a body predict that it should have generated an intense, multipolar and time-varying dynamo field. This field could have been recorded as a remanent magnetisation in the outer, cool layers of a solid crust on the IVA parent core. We find that the different components in the IVA iron meteorites display a range of paleomagnetic fidelities, depending crucially on the cooling rate of the meteorite. In particular, silicate inclusions in the quickly cooled São João Nepomuceno meteorite are poor paleomagnetic recorders. On the other hand, the matrix metal and some silicate subsamples from the relatively slowly cooled Steinbach meteorite are far better paleomagnetic recorders and provide evidence of an intense (≳100 μT) and directionally varying (exhibiting significant changes on a timescale ≲200 kyr) magnetic field. This is the first demonstration that some iron meteorites record ancient planetary magnetic fields. Furthermore, the observed field intensity, temporal variability and dynamo lifetime are consistent with thermal evolution models of the IVA parent core. Because the acquisition of remanent magnetisation by some IVA iron meteorites require that they cooled below their Curie temperature during the period of dynamo activity, the magnetisation carried by Steinbach also provides strong evidence favouring the inward solidification of its parent core

Magnetism of Cr10 wheels on Au(111) and Cu(111) surfaces

Trabajo presentado al X Meeting de GEFES (División de Física de la Materia Condensada de la Real Sociedad Española de Física), celebrado en Valencia del 24 al 26 de enero de 2018We report on the magnetic properties of {Cr10} wheels (Cr10(OMe)20(O2CCMe)10) UHVsublimated on Cu(111) and Au(111) single-crystals investigated by means of by XAS & XMCD, SQUID magnetometry and STM. We discuss the magnetic anisotropy and magnetic moment field-dependence of the grafted {Cr10} molecules, as a function of the metallic substrate and layer thickness, and compare it with bulk material. Our results demonstrate that evaporated molecules show negligible anisotropy and weak dependence with the substrate. A combination of XMCD and SQUID magnetometry show that Mono- and Multilayer {Cr10} samples exhibit a magnetic behavior stemming from a S = 9 ground state, ruling out some previous results. Montecarlo simulations show that the magnetic evolution can be explained by {Cr10} molecules magneticaly consisting of two semi-crowns containing 4 Cr ions interacting ferromagnetically, separated by 2 Cr ions with antiferromagnetic interactions, giving rise to the ground S = 9 state, in agreement with previous EPR data in literature. Evaporation of {Cr10} onto a substrate producing subtle structural changes induce strong changes in the Cr-Cr interactions and distinct magnetic behavior from the bulk.Peer Reviewe

Pallasite paleomagnetism: Quiescence of a core dynamo

Recent paleomagnetic studies of two Main Group pallasites, the Imilac and Esquel, have found evidence for a strong, late-stage magnetic field on the parent body. It has been hypothesized that this magnetic field was generated by a core dynamo, driven by compositional convection during core solidification. Cooling models suggest that the onset of core solidification occurred ∼200 Ma after planetary accretion. Prior to core solidification, a core dynamo may have been generated by thermal convection; however a thermal dynamo is predicted to be short-lived, with a duration of ∼10 Ma to ∼40 Ma after planetary accretion. These models predict, therefore, a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo, when no core dynamo should be active. To test this hypothesis, we have measured the magnetic remanence recorded by the Marjalahti and Brenham pallasites, which based on cooling-rate data locked in any magnetic field signals present ∼95 Ma to ∼135 Ma after planetary accretion, before core solidification began. The cloudy zone, a region of nanoscale tetrataenite islands within a Fe-rich matrix was imaged using X-ray photoemission electron microscopy. The recovered distribution of magnetisation within the cloudy zone suggests that the Marjalahti and Brenham experienced a very weak magnetic field, which may have been induced by a crustal remanence, consistent with the predicted lack of an active core dynamo at this time. We show that the transition from a quiescent period to an active, compositionally driven dynamo has a distinctive paleomagnetic signature, which may be a crucial tool for constraining the time of core solidification on differentiated bodies, including Earth