Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e. (001), (110) and (1-18). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy with the easy axis lying along the step edges and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.Comment: 10 pages, 10 figures, 1 tabl

Tuning domain wall velocity with Dzyaloshinskii-Moriya interaction

We have studied a series of Pt/Co/M epitaxial trilayers, in which Co is sandwiched between Pt and a non magnetic layer M (Pt, Ir, Cu, Al). Using polar magneto-optical Kerr microscopy, we show that the field- induced domain wall speeds are strongly dependent on the nature of the top layer, they increase going from M=Pt to lighter top metallic overlayers, and can reach several 100 m/s for Pt/Co/Al. The DW dynamics is consistent with the presence of chiral N\'eel walls stabilized by interfacial Dzyaloshinskii-Moriya interaction (DMI) whose strength increases going from Pt to Al top layers. This is explained by the presence of DMI with opposite sign at the Pt/Co and Co/M interfaces, the latter increasing in strength going towards heavier atoms, possibly due to the increasing spin-orbit interaction. This work shows that in non-centrosymmetric trilayers the domain wall dynamics can be finely tuned by engineering the DMI strength, in view of efficient devices for logic and spitronics applications.Comment: 5 pages, 4 Figure

Surfactant induced smooth and symmetric interfaces in Cu/Co multilayers

In this work we studied Ag surfactant induced growth of Cu/Co multilayers. The Cu/Co multilayers were deposited using Ag surfactant by ion beam sputtering technique. It was found that Ag surfactant balances the asymmetry between the surface free energy of Cu and Co. As a result, the Co-on-Cu and Cu-on-Co interfaces become sharp and symmetric and thereby improve the thermal stability of the multilayer. On the basis of obtained results, a mechanism leading to symmetric and stable interfaces in Cu/Co multilayers is discussed.Comment: 7 Pages, 7 Figure

Patron espacial de un ecotono bosque subalpino-pastos alpinos (Las Cutas, Ordesa, Pirineos Centrales)

We describe the spatial pattern of a subalpine forest-alpine pasture ecotone in the Central Pyrenees, that includes altitudinal timberline and treeline, and it is dominated by Pinus uncinata Ram. A rectangular (30 x 140 m) plot was located crossing the ecotone with its longest side parallel to the slope. We measured for each P. uncinata individual inside the plot: location (coordinates x, y), and structure (e. g. height) and growth form variables (number and type —living or dead, vertical or shrubby— of stems per individual). P. uncinata individuals were classified according to their size (adults, poles, saplings and seedlings) and growth form (krummholz —shrubby and multistemmed individuals— and krummholz with vertical stems). We described quantitatively the type of substrate (bare soil, organic matter, gravel and rock) and cover of herbs, shrubs and P. uncinata using transects parallel to the slope. The ecotone structure was described through: (1) point pattern (Ripley’s K) and (2) surface pattern analyses (spatial correlograms of height); (3) the detection and description of boundaries using density, size or growth form variables; (4) the synthesis of variations of presence and diversity of substrates and herbs and shrubs; and (5) the ordination of quadrats (the plot was previously subdivided into 115 6 x 6 m quadrats), according to their spatial position in the ecotone, the type of substrate, the cover of herbs and shrubs and the number, size and growth form of P. uncinata individuals. Most P. uncinata living individuals were krummholz, located above the timberline. Krummholz individuals showed significant and positive spatial interaction with seedlings. Bigger, vertical and unistemmed individuals predominated in the lower area of the ecotone, in the forest. The change of height with increasing elevation was abrupt and masked an underlying pattern of patches of trees with similar height in the forest. The structure variables were more sensitive because they produced a greater number of boundaries. These boundaries were arranged forming a “diagonal” (in the lower and upper areas of the ecotone for big and small individuals, respectively) because of the sequential location of progressively bigger unistemmed individuals descending across the ecotone. The shrubby individuals were associated with rocky substrates, that decreased in the forest, where organic matter predominated. The snow-wind interaction can explain the location of the studied timberline that could be considered a local phenomenon. Krummholz can buffer seedlings against the harsh climatic conditions of this ecotone (strong wind, reduced snowpack, low temperature). The spatial location of the different classes of individuals, the spatial interaction between seedlings and krummholz individuals, and changes of growth form (from shrubby to vertical growth form or vice versa) can cause some inertia in the response of ecotone P. uncinata populations to environmental changes.Describimos el patrón espacial de un ecotono bosque subalpino-pastos alpinos, que incluye los límites altitudinales del bosque y del árbol, dominado por Pinus uncinata Ram. y localizado en los Pirineos Centrales. Situamos una parcela rectangular (30 x 140 m) cruzando el ecotono con su lado mayor paralelo a la pendiente. Para cada individuo de P. uncinata dentro de la parcela medimos: su localización (coordenadas x, y), y variables estructurales (p. ej. altura) y de forma de crecimiento (número y tipo —vivo o muerto, vertical o arbustivo— de pies por individuo). Los individuos vivos de P. uncinata se clasificaron según su tamaño (adultos, jóvenes, vástagos y plántulas) y forma de crecimiento (krummholz —individuos arbustivos policórmicos—, krummholz con pies verticales). Describimos cuantitativamente el tipo de sustrato (suelo, materia orgánica, grava, roca) y la cobertura de herbáceas, arbustos y P. uncinata usando transectos paralelos a la pendiente. La estructura del ecotono se describió mediante: (a) los análisis del patrón de puntos (K de Ripley) y (b) del patrón de superficies (correlogramas espaciales de la altura); (c) la detección y descripción de fronteras usando la densidad o variables de tamaño y forma de crecimiento de los indidividos; (d) la síntesis de las variaciones en la presencia y diversidad de sustratos y de herbáceas y arbustos; y (e) la ordenación de los cuadrados en que subdividimos la parcela (115 cuadrados de 6 x 6 m), de acuerdo a su posición espacial en el ecotono, al tipo de sustrato, a la cobertura de herbáceas y arbustos y al número, tamaño y forma de crecimiento de los individuos de P. uncinata. La mayoría de los individuos vivos de P. uncinata eran krummholz, situándose por encima del límite del bosque. Los individuos krummholz mostraron interacción espacial positiva con las plántulas. En el área inferior del ecotono, los individuos grandes, verticales y unicórmicos predominaban en el bosque. El cambio de altura al ascender era brusco y enmascaraba un patrón subyacente de manchas de árboles de altura similar dentro del bosque. Las variables estructurales fueron las más sensibles porque produjeron el mayor número de fronteras. Estas fronteras se dispusieron en “diagonal” (en las partes inferior y superior del ecotono para los individuos grandes y pequeños, respectivamente) debido a la situación secuencial de individuos progresivamente mayores y unicórmicos al descender por el ecotono. Los individuos arbustivos estaban asociados a sustratos rocosos, que disminuían al entrar en el bosque, donde la materia orgánica predominaba. La interacción nieve-viento permite explicar la localización de este límite del bosque que puede considerarse como un fenómeno local. Los individuos krummholz pueden proteger a las plántulas frente a las duras condiciones climáticas del ecotono (vientos fuertes, escaso espesor de nieve, bajas temperaturas). La situación espacial de las distintas clases de individuos, la interacción espacial entre plántulas y krummholz, y los cambios de forma de crecimiento (de forma arbustiva a vertical o viceversa) pueden provocar cierta inercia en la respuesta de las poblaciones de P. uncinata del ecotono frente a cambios ambientales

Drought and Forest Decline in the Iberian Peninsula: A Simple Explanation for a Complex Phenomenom?

Different episodes of forest decline have been reported in several areas of the Northern Hemisphere during the 20th century. The explanation of this process included anthropogenic (air pollution), biotic (pathogens) and climatic factors. Among the climatic factors, the effects of acute or chronic droughts have been the most common explanation for the massive dieback observed. In the Iberian Peninsula, besides the pathogenic explanation which gives a paramount relevance to the fungus Phytophtora cinnamomi in many situations, the role of an increment in aridity (chronic) or the consequences of severe droughts (acute) have been empirically supported. The evident synchronism between forest decline and abnormally adverse climatic conditions, in addition to the reversibility of the process when precipitation intensity increased, are two major arguments for supporting the climatic involvement in the phenomenon. Nevertheless, the two most affected species in the episodes of oak decline in the Iberian Peninsula were Quercus ilex and Q. suber, which are representatives of the so called Mediterranean woody flora and which have been considered as more drought resistant than the temperate oak species that co-occur in this territory. The climatic complexity ofthe Iberian Peninsula, the many mechanisms for coping with water stress that have been described within the sclerophyllous Mediterranean flora and the effects of human management partially explain this paradox. Other forest species have also been affected by massive decline. Pinus sylvestris and Abies alba, which have their southern distribution limit in the mountain ranges of the Iberian Peninsula, are two examples of this situation. Both cases, besides the inmediate effects of some climatic perturbations, need the incorporation of different predisposing factors – mainly historical aspects to obtain a complete image of the process

Wood Anatomical Traits Respond to Climate but More Individualistically as Compared to Radial Growth: Analyze Trees, Not Means

Wood encodes environmental information that can be recovered through the study of tree-ring width and wood anatomical variables such as lumen area or cell-wall thickness. Anatomical variables often provide a stronger hydroclimate signal than tree-ring width, but they show a low treeto-tree coherence. We investigate the sources of variation in tree-ring width, lumen area, and cell-wall thickness in three pine species inhabiting sites with contrasting climate conditions: Pinus lumholtzii in wet-summer northern Mexico, and Pinus halepensis and Pinus sylvestris in dry-summer north-eastern Spain. We quantified the amount of variance of these three variables explained by spring and summer water balance and how it varied among trees. Wood anatomical variables accounted for a larger inter-individual variability than tree-ring width data. Anatomical traits responded to hydroclimate more individualistically than tree-ring width. This individualistic response represents an important issue in long-term studies on wood anatomical characteristics. We emphasized the degree of variation among individuals of the same population, which has far-reaching implications for understanding tree species’ responses to climate change. Dendroclimatic and wood anatomical studies should focus on trees rather than on the mean population series

Delineating limits: Confronting predicted climatic suitability to field performance in mistletoe populations

1. Biotic stressors such as hemiparasites have a profound impact on forest functioning. However, predicting the future incidence of these stressors on forests remains challenging because climate-based distribution does not consider tree-hemiparasite interactions or the impacts of extreme climate events on stressors'' performance. 2. We use species distribution models (SDMs) and ecophysiological and demographic field data to assess whether climatic suitability is a proxy for the performance of the hemiparasite mistletoe (Viscum album) in two forests with contrasting climatic conditions. Two host tree species representing wet-cold (Scots pine) and dry-warm (Aleppo pine) conditions were selected. We fitted SDMs based on climate variables, and measured different ecophysiological variables capturing cold- (photoinhibition) and drought tolerance (intrinsic water-use efficiency, iWUE). We also assessed demographic variables related to seed germination and establishment rates of mistletoe through a translocation experiment. 3. Species distribution models showed a high climatic suitability of mistletoe in both forests. Mistletoes living in the Scots pines site presented a higher cold tolerance, while those inhabiting the Aleppo pine site showed a higher iWUE. Seedlings coming from local seeds showed a lower mortality than seedlings coming from translocated seeds. Germination and seedling establishment showed temporal mismatches when comparing local and translocated seeds. 4. Synthesis. Habitat suitability predicted by SDMs based on climate data and field performance were related in this mistletoe species. However, ecophysiological and demographic variables indicated a lower fitness of mistletoe in the dry-warm site associated with drought stress. In conclusion, predicted climate suitability based on SDMs forecasts should be refined using field data on actual performance and considering plant-to-plant interactions and extreme climate events

Summer drought and spring frost, but not their interaction, constrain European beech and Silver fir growth in their southern distribution limits

Climate warming has lengthened the growing season by advancing leaf unfolding in many temperate tree species. However, an earlier leaf unfolding increases also the risk of frost damage in spring which may reduce tree radial growth. In equatorward populations of temperate tree species, both late frosts and summer droughts impose two constraints to tree growth, but their effects on growth are understudied. We used a tree-ring network of 71 forests to evaluate the potential influence of late frosts and summer droughts on growth in two tree species that reach their southern distribution limits in north-eastern Spain: the deciduous European beech (Fagus sylvatica L.) and the evergreen Silver fir (Abies alba Mill). The occurrence of late frost events and summer drought was quantified by using a high-resolution daily temperature and precipitation dataset considering the period 1950 2012. Late frosts were defined as days with average temperature below 0 °C in the site-specific frost-free period, whereas drought was quantified using the 18 month-long August Standardized Precipitation Evapotranspiration Index (SPEI). The growth of European beech and Silver fir was reduced by the occurrence of both late frost events and summer drought. However, we did not find a significant interaction on growth of these two climate extremes. Beech was more negatively impacted by late frosts, whereas Silver fir was more impacted by summer drought. Further studies could use remote-sensing information or in situ phenological records to refine our frost index and better elucidate how late frosts affect growth, whether they interact with drought to constrain growth, and how resilience mechanisms related to post-frost refoliation operate in beech. © 2019 Elsevier B.V

Estudio dendroecológico del ataque de Epinotia subsequana Hw. (Lepidoptera: Tortricidae) sobre un abetar (Abies alba Mill.) del Pirineo aragonés (Diazas, Torla)

We found many narrow or missing annual tree-rings during the period from 1996 to 1998 in most of fir trees sampled in the Diazas forest (Torla; 42º 37’ 50’’ N, 0º 05’ 40’’ W, 1528 m a.s.l.). These tree-rings were synchronic with attacks in that area of the leaf miner Epinotia subsequana Hw. (Lepidoptera). This temporal coincidence motivated this dendroecological study. The radial growth in Diazas was compared with that observed in other 13 fir forests sampled across the Aragón Pyrenees and not affected by E. subsequana. During the 1996-98 period, the radial growth in Diazas was 12-22 % with respect to the mean growth in the rest of stands. The decrease of radial growth during these three years was the cause of the high frequency of missing rings. These were rarely found in others fir forests. This fact suggested that some local factor have caused the growth reduction. The lower radial growth was produced by the intense defoliations done by E. subsequana. The most pronounced reduction of growth was observed in 1997, one year after the maximum level of defoliation was reached (1996). This temporal lag may be due to the fact that the defoliation observed in 1996 reduced the reserves and growth potential to form the tree-ring in the next year. In fact, the radial growth did not increase notably until 3 years (1999) after the first intense growth reduction (1996) was observedLa presencia de anillos anuales de crecimiento estrechos o ausentes entre 1996 y 1998 en la mayoría de los árboles muestreados en el abetar de Diazas (Torla; 42  37’ 50’’ N, 0 05’ 40’’ W, 1528 m s.n.m.) y su coincidencia en el tiempo con ataques del lepidóptero minador Epinotia subsequana Hw. en la zona motivó el siguiente estudio dendroecológico. Se comparó el crecimiento radial del abetar de Diazas con el observado en otros 13 abetares muestreados a lo largo del Pirineo aragonés y no afectados por E. subsequana. Durante el período 1996-98, el crecimiento radial en Diazas fue de un 12-22 % respecto al crecimiento medio del resto de abetares. La reducción del crecimiento radial durante estos tres años también se manifestó en la elevada frecuencia de anillos ausentes, que raramente aparecían en el resto de abetares estudiados. Este hecho sugería que algún factor había actuado de forma local causando la reducción de crecimiento. El menor crecimiento radial fue producto de las intensas defoliaciones producidas por E. subsequana. La reducción más pronunciada del crecimiento radial se produjo en 1997, un año después de que la defoliación alcanzara niveles máximos (1996). Este desfase temporal puede deberse a que la defoliación observada en 1996 disminuyera las reservas y redujera así la capacidad de crecimiento para la producción del anillo del año posterior. De hecho, el crecimiento radial no aumentó de forma notable hasta tres años (1999) después de la primera reducción intensa de crecimiento (1996)