Bence Tóth

Giant magnetoresistance in electrodeposited Co-Cu/Cu multilayers: Origin of the absence of oscillatory behavior

A detailed study of the evolution of the magnetoresistance was performed on electrodeposited Co/Cu multilayers with Cu-layer thicknesses ranging from 0.5 to 4.5 nm. For thin Cu layers up to 1.5 nm, anisotropic magnetoresistance AMR was observed, whereas multilayers with thicker Cu layers exhibited clear giant magnetoresistance GMR behavior. The GMR magnitude increased up to about 3.5–4 nm Cu-layer thickness and slightly decreased afterward. According to magnetic measurements, all samples exhibited ferromagnetic FM behavior. The relative remanence turned out to be about 0.75 for both AMR- and GMR-type multilayers. This clearly indicates the absence of an antiferromagnetic AF coupling between adjacent magnetic layers for Cu layers even above 1.5 nm where the GMR effect occurs. The AMR behavior at low spacer thicknesses indicates the presence of strong FM coupling due to, e.g., pinholes in the spacer and/or areas of the Cu layer where the layer thickness is very small. With increasing spacer thickness, the pinhole density reduces and/or the layer thickness uniformity improves, which both lead to a weakening of the FM coupling. This improvement in multilayer structure quality results in a better separation of magnetic layers and the weaker coupling or complete absence of interlayer coupling enables a more random magnetization orientation of adjacent layers, all this leading to an increase in the GMR. Coercive field and zero-field resistivity measurements as well as the results of a structural study reported earlier on the same multilayers provide independent evidence for the microstructural features established here. A critical analysis of former results on electrodeposited Co/Cu multilayers suggests the absence of an oscillating GMR in these systems. It is pointed out that the large GMR reported previously on such Co/Cu multilayers at Cu-layer thicknesses of around 1 nm can be attributed to the presence of a fairly large superparamagnetic SPM fraction rather than being due to a strong AF coupling. In the absence of SPM regions as in the present study, AMR only occurs at low spacer thicknesses due to the dominating FM coupling.

Massive mortality of invasive bivalves as a potential resource subsidy for the adjacent terrestrial food web

Large-scale mortality of invasive bivalves was observed in the River Danube basin in the autumn of 2011 due to a particularly low water discharge. The aim of this study was to quantify and compare the biomass of invasive and native bivalve die-offs amongst eight different sites and to assess the potential role of invasive bivalve die-offs as a resource subsidy for the adjacent terrestrial food web. Invasive bivalve die-offs dominated half of the study sites and their highest density and biomass were recorded at the warm water effluent. The density and biomass values recorded in this study are amongst the highest values recorded for aquatic ecosystems and show that a habitat affected by heated water can sustain an extremely high biomass of invasive bivalves. These mortalities highlight invasive bivalves as a major resource subsidy, possibly contributing remarkable amounts of nutrients and energy to the adjacent terrestrial ecosystem. Given the widespread occurrence of these invasive bivalves and the predicted increase in the frequency and intensity of extreme climatic events, the ecological impacts generated by their massive mortalities should be taken into account in other geographical areas as well.

Magnetothermopower and magnetoresistance of single Co-Ni/Cu multilayered nanowires

The magnetothermopower and the magnetoresistance of single Co-Ni/Cu multilayered nanowires with various thicknesses of the Cu spacer are investigated. Both kinds of measurement are performed as a function of temperature (50–325 K) and under applied magnetic fields perpendicular to the nanowire axis, with magnitudes up to −15% at room temperature. A linear relation between thermopower S and electrical conductivity σ of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the S vs σ relation and the Mott formula, the energy derivative of the resistivity is determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the nondiffusive thermopower contributions of the nanowire can be neglected, it is found that the magnetic-field-induced changes of thermopower and resistivity are equivalent. The emphasis in the present paper is on the comparison of the magnetoresistance and magnetothermopower results and it is found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various individual nanowires.

Impact of Dreissena fouling on the physiological condition of native and invasive bivalves: interspecific and temporal variations

Abstract The impact of Dreissena fouling on unionids has hardly been studied in Europe, despite the fact that in some ecosystems (e.g. Lake Balaton, Hungary) infestations of several hundreds to a thousand individuals per unionid have been observed. At present, the zebra mussel Dreissena polymorpha is a dominant species in Lake Balaton and in the last decade three other invasive bivalves were introduced, potentially increasing the pressure on native unionid survival. We examined whether the fouling of dreissenids (zebra and quagga (D. rostriformis bugensis) mussels) has a negative impact on native (Anodonta anatina, Unio pictorum and U. tumidus) and invasive (Corbicula fluminea and Sinanodonta woodiana) bivalves and whether there are any interspecific and temporal variations in fouling intensity and physiological condition measured by standard condition index and glycogen content. A significant negative impact was detected on native unionids only in July and September (no impact was detected in May), when the fouling rate was high. For invasive species, a significant negative impact was detected on S. woodiana with a high level of dressenid infestation; whereas no significant impact was detected on C. fluminea. Overall, this study confirms that Dreissena may threaten unionid species including the invasive S. woodiana, although high interspecific and temporal variations were observed. This situation should be taken into account in future ecological and conservational assessments because species respond differently to Dreissena fouling and effects seem to be more pronounced in late summer/early autumn. In addition, this study provides the first evidence that the invasive C. fluminea appear to be less vulnerable to dressenid fouling.

Spatial and temporal variation in biomass and size structure of Corbicula fluminea in Danube River catchment, Hungary

The aim of this study was to reveal the spatial and temporal variation in biomass, density, and size structure of the invasive clam, Corbicula fluminea in the Danube River catchment and to explore the environmental factors determining these patterns. Samples were taken seasonally during the years 2007 and 2008 at 15 sites located along a river continuum from a second order stream to the Danube River. C. fluminea was the most abundant species in the main arm of the Danube; however, it was found in great density in the side arms, too. In the Ipoly River it only occured close to the conjuction, and it was not present at all in the second and third order streams. Regression models were developed to predict dry mass (DW, g) from linear body dimension [shell length (L), mm]. For the description of length-dry mass relationship, the power function (DW = a * Lb) was applied. The highest density, biomass, and mean shell length were 178 ind. m−2, 24.9697 g DW m−2, and 21.99 mm, respectively. ANOVA detected significant spatial variation in biomass; however, significant temporal variation was not recorded between the years, only among the seasons. Multiple linear regressions were used to find the relationship between the biomass and the environmental parameters. The coarse and very fine sediment fractions and the high water temperature provided suitable habitat for C. fluminea. Although statistically significant correlation was not detected between the chlorophyll-a content of water and the biomass, the highest biomass values were recorded at sites with high chlorophyll-a value.

Wartime scars or reservoirs of biodiversity? The value of bomb crater ponds in aquatic conservation

Considering the ongoing loss of aquatic habitats, anthropogenic ponds are gaining importance as substitute habitats. It is therefore important to assess their functioning in comparison to their natural precursors. Here we assess the biodiversity value of sodic bomb crater ponds by comparing their gamma diversity to that of natural reference habitats, astatic soda pans, and assess their importance on the landscape level by studying alpha and beta diversity. We studied aquatic organisms ranging from algae to vertebrates in a dense cluster of 54 sodic bomb crater ponds in Central Europe. Despite the overall small area of the pond cluster, gamma diversity was comparable to that found in surveys of natural habitats that encompassed much wider spatial and temporal scales. We also found a considerable number of species shared with reference habitats, indicating that these anthropogenic habitats function as important refuge sites for several species that are associated with the endangered soda pans. Moreover, we found a number of regionally or worldwide rare species. Among the components of beta diversity, species replacement dominated community assembly. Individual ponds contributed similarly to beta diversity in terms of replacement, being equally important for maintaining high gamma diversity and emphasising the role of the pond network rather than individual ponds. This pattern was seen in all studied groups. Bomb crater ponds therefore acted as important contributors to aquatic biodiversity. Considering the tremendous losses of ponds throughout Europe, anthropogenic ponds should be taken into consideration in nature conservation, especially when occurring in pond networks.

NAVICULA NOVAESIBERICA LANGE-BERTALOT; A NEW RECORD OF THE SPECIES FROM THE DANUBE RIVER IN HUNGARY

Navicula novaesiberica Lange-Bertalot was recorded for the first time on the Hungarian stretch of the Danube River. The species was found in low relative abundance values in the epilithon at Göd (riv.km. 1669). N. novaesiberica was described more than ten years ago, it is a relatively large and conspicuous species. However, there are only a few records of it in the literature. Some additional details on the morphology and autoecology of this species are provided here.

Thermoelectric power factor enhancement by spin-polarized currents – a nanowire case study

In this work, thermoelectric (TE) measurements have been performed on the workhorses of today’s data storage devices, namely nanostructured materials exhibiting either the giant or the anisotropic magnetoresistance effect (GMR and AMR). In particular, the temperature-dependent (50 K - 300 K) and magnetic field-dependent (up to 1 T) TE power factor (PF) has been determined for several Co-Ni alloy nanowires with varying Co:Ni ratios as well as for Co-Ni/Cu multilayered nanowires with various Cu layer thicknesses, which were all synthesized via a template-assisted electrodeposition process. A systematic investigation of the resistivity, (rho), as well as the Seebeck coefficient, S, was performed for Co-Ni alloy nanowires exhibiting AMR and Co-Ni/Cu multilayered nanowires exhibiting GMR. At room temperature, measured values of TE PFs up to 3.6 mWK-2m-1 for AMR samples and 2.0 mWK-2m-1 for GMR nanowires were obtained. Furthermore, the TE PF was found to increase by up to 13.1 % for AMR Co-Ni alloy nanowires and by up to 52 % for GMR Co-Ni/Cu samples in an external applied magnetic field. According to these measurements, the magnetic nanowires exhibit TE PFs that are of the same order of magnitude as TE PFs of Bi-Sb-Se-Te based thermoelectric materials and, additionally, give the opportunity to adjust the TE power output to changing loads and hot spots through external magnetic fields.

Morphological and genetic variability of assemblages of Cyclotella ocellata Pantocsek/C. comensis Grunow complex (Bacillariophyta, Thalassiosirales)

Centric diatom taxa in the Cyclotella ocellata and C. comensis complexes show high morphological variability and often apparently continuous morphological transitions. In this study, we investigated natural assemblages of the C. ocellata/C. comensis complex from Hungarian and Croatian lakes and from Turkish streams using morphological and molecular methods. The studied assemblages contained cells with morphologies resembling C. ocellata as well as other, closely related, species: C. comensis, C. pseudocomensis, C. costei, and C. trichnoidea. The goal of our paper was to assess whether the observed morphological differences were due to intraspecific variability or suggest the existence of several, putatively distinct species. Ten morphometric characters were measured, which, either individually, or in pairs, did not differentiate the nominal taxa in our assemblages. However, multivariate discriminant analysis has revealed a group including C. ocellata and C. trichonidea morphologies could be separated from another containing C. comensis, C. pseudocomensis and C. costei. A nuclear (18S rDNA) and a chloroplast (rbcL) gene were amplified and partially sequenced from environmental DNA or from isolated cells. The sequences showed little variability among the assemblages and nominal species. Although general congruence of molecular and morphometric separation supports the species level separation of C. ocellata/trichonidea from the probably conspecific C. comensis/pseudocomensis/costei, sequence divergences between the groups are in the same range as within them, so that a conspecificity of all four taxa cannot be unequivocally excluded.

Composition and dynamics of benthic organic matter in the middle Danube section

The organic matter of the sediment is prerequisite for the detritivorous community in the river ecosystem. To quantify this organic matter content and its changes in time and space, investigations were performed at two typical sites of the Hungarian Danube, which differ in their local hydrological characteristics due to the deposition and erosion processes. The distribution of the benthic particulate matter and the organic matter content of that were analysed in the core samples of the river side and the near shore line in three layers (0-5, 5-10 and 10-15 cm) and in four fractions: C = coarse (2360-710 μm), F = fine (710-250 μm), V = very fine (250-63 μm), and U = ultra fine (<63 μm). The variability of the organic matter content was highest in the coarse fraction (CV: 86 %), while it was the lowest in the ultra fine fraction (CV: 20 %). Sites differed mainly in the organic matter content of the C and F fractions, and the vertical changes of the benthic organic matter. The first TBOM records from the studied section of the Danube show relatively high values in comparison with the literature. Results show that mainly the quantity of the fractions (i.e. hydrological characteristics) determined the BOM content of the bed sediments at the investigated sites.