Enhanced thermo-spin effects in iron-oxide/metal multilayers

Since the discovery of the spin Seebeck effect (SSE), much attention has been devoted to the study of the interaction between heat, spin, and charge in magnetic systems. The SSE refers to the generation of a spin current upon the application of a thermal gradient and detected by means of the inverse spin Hall effect. Conversely, the spin Peltier effect (SPE) refers to the generation of a heat current as a result of a spin current induced by the spin Hall effect. Here we report a strong enhancement of both the SSE and SPE in Fe3O4/Pt multilayered thin films at room temperature as a result of an increased thermo-spin conversion efficiency in the multilayers. These results open the possibility to design thin film heterostructures that may boost the application of thermal spin currents in spintronics

Observation of huge thermal spin currents in magnetic multilayers

Thermal spin pumping constitutes a novel mechanism for generation of spin currents; however their weak intensity constitutes a major roadblock for its usefulness. We report a phenomenon that produces a huge spin current in the central region of a multilayer system, resulting in a giant spin Seebeck effect in a structure formed by repetition of ferromagnet/metal bilayers. The result is a consequence of the interconversion of magnon and electron spin currents at the multiple interfaces. This work opens the possibility to design thin film heterostructures that may boost the application of thermal spin currents in spintronics

Palaeolimnological evidence for an east-west climate see-saw in the Mediterranean since AD 900

During the period of instrumental records, the North Atlantic Oscillation (NAO) has strongly influenced inter-annual precipitation variations in the western Mediterranean, while some eastern parts of the basin have shown an anti-phase relationship in precipitation and atmospheric pressure. Here we explore how the NAO and other atmospheric circulation modes operated over the longer timescales of the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). High-resolution palaeolimnological evidence from opposite ends of the Mediterranean basin, supplemented by other palaeoclimate data, is used to track shifts in regional hydro-climatic conditions. Multiple geochemical, sedimentological, isotopic and palaeoecological proxies from Estanya and Montcortés lakes in northeast Spain and Nar lake in central Turkey have been cross-correlated at decadal time intervals since AD 900. These dryland lakes capture sensitively changes in precipitation/evaporation (P/E) balance by adjustments in water level and salinity, and are especially valuable for reconstructing variability over decadal-centennial timescales. Iberian lakes show lower water levels and higher salinities during the 11th to 13th centuries synchronous with the MCA and generally more humid conditions during the 'LIA' (15th-19th centuries). This pattern is also clearly evident in tree-ring records from Morocco and from marine cores in the western Mediterranean Sea. In the eastern Mediterranean, palaeoclimatic records from Turkey, Greece and the Levant show generally drier hydro-climatic conditions during the LIA and a wetter phase during the MCA. This implies that a bipolar climate see-saw has operated in the Mediterranean for the last 1100. years. However, while western Mediterranean aridity appears consistent with persistent positive NAO state during the MCA, the pattern is less clear in the eastern Mediterranean. Here the strongest evidence for higher winter season precipitation during the MCA comes from central Turkey in the northeastern sector of the Mediterranean basin. This in turn implies that the LIA/MCA hydro-climatic pattern in the Mediterranean was determined by a combination of different climate modes along with major physical geographical controls, and not by NAO forcing alone, or that the character of the NAO and its teleconnections have been non-stationary. © 2011 Elsevier B.V

Role of the surface states in the magnetotransport properties of ultrathin bismuth films

We have investigated the magnetotransport properties of ultrathin films of Bi grown on thermally oxidized Si(001) substrates with thickness ranging from 10 to 100 nm at temperatures down to 2 K and in magnetic fields up to 90 kOe. Remarkable differences both in temperature and field dependence of the Hall resistivity are found for the films with thickness above and below 20 nm. These observations can be explained due to the presence of surface states, which play an important role in determining the electronic transport properties of the thinnest films. The estimated surface carrier density 4 x 10^(13) cm^(-2) at room temperature correlates well with that recently reported from angle-resolved photoemission spectroscopy on ultrathin Bi(001) films

Temperature dependence of the spin Seebeck effect in [Fe3O4/Pt]n multilayers

We report temperature dependent measurements of the spin Seebeck effect (SSE) in multilayers formed by repeated growth of a Fe3O4/Pt bilayer junction. The magnitude of the observed enhancement of the SSE, relative to the SSE in the single bilayer, shows a monotonic increase with decreasing the temperature. This result can be understood by an increase of the characteristic length for spin current transport in the system, in qualitative agreement with the recently observed increase in the magnon diffusion length in Fe3O4 at lower temperatures. Our result suggests that the thermoelectric performance of the SSE in multilayer structures can be further improved by careful choice of materials with suitable spin transport properties

Tuning coherent-phonon heat transport in LaCoO3/SrTiO3 superlattices

Accessing the regime of coherent phonon propagation in nanostructures opens enormous possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. In this paper we show that coherent phonons contribute substantially to the thermal conductivity of LaCoO3/SrTiO3 oxide superlattices, up to room temperature. We show that their contribution can be tuned through small variations of the superlattice periodicity, without changing the total superlattice thickness. Using this strategy, we tuned the thermal conductivity by 20% at room temperature. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices. © 2021 The Authors. Published by American Chemical Society

Nature of antiferromagnetic order in epitaxially strained multiferroic SrMnO3 thin films

Epitaxial films of SrMnO3 and bilayers of SrMnO3/La0.67Sr0.33MnO3 have been deposited by pulsed laser deposition on different substrates, namely, LaAlO3 (001), (LaAlO3)0.3(Sr2AlTaO6)0.7 (001), and SrTiO3 (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures (TN) of the SrMnO3 films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, TN is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around 1.6±0.1%. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the SrMnO3 layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the SrMnO3 and La0.67Sr0.33MnO3 films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the SrMnO3 layer

Vegetation changes and hydrological fluctuations in the Central Ebro Basin (NE Spain) since the Late Glacial period: Saline lake records

The final version is available at: http://www.sciencedirect.com/science/journal/00310182Although the Central Ebro Basin (Northeastern Iberian Peninsula) is both the northernmost semi-arid area in Europe and one of the regions with the largest biodiversity, it has been insufficiently studied in terms of past climate variability due to the scarcity of suitable sites for palaeoenvironmental analyses. Previous studies from ephemeral saline lakes in the area, mainly based on palynological data, show abrupt and rapid arid/humid transitions throughout the last glacial cycle highlighting a complex palaeohydrological evolution. New cores from two saline lakes (La Playa and La Salineta) in the Los Monegros area provide multi-proxy records including sedimentology, geochemistry, and pollen indicators. This study, together with a detailed and comprehensive review of the main saline records from the Central Ebro Basin, enables us tos reconstruct a comprehensive picture of the palaeoclimate evolution during the last glacial cycle. One of the main results of this study is the alternation of humid and dry phases as a characteristic of the climate evolution during the Lateglacial. Additionally, the study suggests an important role of the increased flow from the Pyrenean rivers during deglaciation in the hydrological balance of the Central Ebro Basin. It is found that the Early Holocene is the wettest period over the sequence studied contrasting with the arid Middle Holocene interval, which is frequently absent as a result of intense aeolian erosive processes. Although anthropogenic activity partially masks the climate signal from the palynological data in the uppermost part of the sequences studied, there are some sedimentological evidences for a climate change during the last 2000 years resulting in a recovery of average saline lake levels in the Central Ebro Basin.This work was part of the project “Arid periods in the Mediterranean area since the Last Glacial Maximum” (REN 2000-1136/CLI), and LIMNOCLIBER (REN 2003-09130-C02-02), funded by the CICYT, and the project “Environmental evolution and effects of the anthropogenic impact in the lacustrine complex of Bujaraloz–Sástago” (PO23/2001), funded by the Aragon regional government (DGA). We acknowledge the Pyrenean Institute of Ecology (CSIC) for financial support.Peer reviewe