Ildikó Vincze

Microscopic study of the magnetic coupling in a nanocrystalline soft magnet

The magnetic behaviour of nanosize ferromagnetic bcc granules embedded in an amorphous tissue (i.e., partially crystallized amorphous alloy) was studied by Mossbauer spectroscopy. The results are compared with the bulk counterparts: bcc-Fe and amorphous . Size dependent enhancement of the Curie point of the nanosize amorphous phase was not observed. At temperatures well above the Curie point of the amorphous phase superparamagnetic relaxation of the bcc crystallites is observed opening new possibilities to study the anisotropy energy of nanosize ferromagnetic grains.

Anomalous magnetic properties of the nano-size residual amorphous phase in nanocrystals

The composition dependence of the Curie temperature and the magnetic moment of the nano-size residual amorphous phase in partially crystallized Fe92-xZr7BxCu1 (2 x 23) amorphous alloys were determined by 57Fe M?ssbauer spectroscopy. Both quantities show broad minima and for increasing relative Zr content (i.e. for decreasing B concentration) surpass the usual monotonic decrease observed for the bulk counterparts. The deviation does not scale with the characteristic size of the residual amorphous regions, which was found to be constant, ruling out explanations based on interphase exchange interaction. A magnetovolume origin of the observed anomalous composition dependence and the improved soft-magnetic characteristics of these nanocrystals is proposed.

Holocene treeline and timberline changes in the South Carpathians (Romania): Climatic and anthropogenic drivers on the southern slopes of the Retezat Mountains

Two high-altitude lake-sediment sequences (Lake Lia, 1910 m a.s.l. and Lake Bucura, 2040 m a.s.l.) from the Retezat Mountains (South Carpathians, Romania) were analysed using multi-proxy methods to study responses of treeline, timberline and alpine/subalpine vegetation to climate change and human impact during the past 16,000 years. Woody species (Pinus mugo, Pinus cembra, Picea abies and Juniperus communis) reached Lake Lia between 12,000 and 11,800 cal. yr BP, whereas P. mugo colonised the shores of Lake Bucura at 9600 cal. yr BP. Lake Lia was in the timberline ecotone between 8000 and 3200 cal. yr BP, in semi-open P. cembra and Picea abies woodland, probably mixed with P. mugo on the steeper slopes. Lake Bucura was surrounded by the upper part of the krummholz zone during the mid-Holocene. The increase in P. cembra after c. 6000 cal. yr BP around Lake Lia suggests that the composition of the timberline forest changed. The disappearance of P. cembra and Picea abies around Lake Lia at ~3000 cal. yr BP reflects descent of the timberline. A large mean July temperature decline between 3300 and 2800 cal. yr BP may have driven or at least contributed to the descent of the Picea abies–P. cembra forests. An increase in human indicator pollen types in Lake Bucura around 4200 cal. yr BP may reflect human impact in the naturally open alpine zone in the Late Bronze Age. In contrast, human impact likely appeared considerably later, around 2650 cal. yr BP (Early Iron Age) around Lake Lia in the upper subalpine zone. Human impact likely intensified after 2200 cal. yr BP at both sites that resulted in the lowering of the krummholz zone. We conclude that climate change and human impact both played an important role in the lowering of the treeline and timberline in the late-Holocene.

Structure and magnetic properties of nanocrystalline soft ferromagnets

The structure and magnetic properties of soft magnetic nanocrystalline composites crystallised from amorphous ribbons are reviewed. The Fe-Early Transition Metal-Boron (Nanoperm-type) nanostructures are discussed in details and compared to the Si containing (Finemet) alloys. The nanocrystallization process was studied by calorimetry; the spatial dimension, the composition and the relative fraction of the bcc and the residual amorphous phases were investigated by X-ray diffraction and Mössbauer spectroscopy. A small fraction of Fe atoms (about 4%) was found which cannot be assigned either to the residual amorphous or to the bcc phase. It is suggested that the magnetic anisotropy of the bcc phase is decreased due to the dissolved Zr and B impurities. The Curie point and Fe atomic moments in the residual amorphous tissue are determined and compared to that of a macrosized amorphous phase of similar composition. The observed deviations do not scale with the average characteristic size of the amorphous phase and thus cannot be explained in the framework of the existing models. Magnetic dipolar coupling and tensile stresses between the grains of the different phases are suggested for explaining the soft magnetic behaviour of the nanostructures.

Small-scale moisture availability increase during the 8.2-ka climatic event inferred from biotic proxy records in the South Carpathians (SE Romania)

In this paper, we present high-resolution early Holocene pollen, plant macrofossil, charcoal, diatom, biogenic silica, and loss-on-ignition records from a mountain lake in the South Carpathians in order to reveal ecosystem response to the 8.2-ka climatic oscillation. We found significant changes both in terrestrial vegetation and lake diatom assemblages in the northern slope of the Retezat Mts between c. 8300 and 8000 cal. yr BP. Rapid changes in relative frequencies and pollen accumulation rates of the major deciduous pollen types associated with peaks in microcharcoal accumulation rates suggested that vegetation disturbance mainly took place in the mixed-deciduous forest zone, where woodland fires partially destroyed the populations of Fraxinus excelsior, Quercus, and Corylus avellana and facilitated the establishment of Carpinus betulus in the forest openings. The diatom record furthermore showed the spread of a planktonic diatom species, Aulacoseira valida, at 8150 cal. yr BP, coincidently with a short-lived expansion of C. betulus. Since diatom blooms mainly occur in spring in the Retezat Mts, increased spring water depth and increased water turbulence were inferred from these data. The expansion of C. betulus against F. excelsior and C. avellana at the same time suggested a modest increase in available moisture during the growing season. Taken together, these data imply that during the 8.2-ka event, winter and spring season available moisture increased, while summers were characterized by alternating moist/cool and dry/warm conditions.

Inter-grain coupling in nanocrystalline soft magnets

The magnetic coupling between nanosize ferromagnetic bcc crystalline grains embedded in a residual amorphous matrix was studied by Mossbauer spectroscopy in nanocrystalline Fe - Zr - B - Cu alloys studied above the Curie temperature of the amorphous phase. It is shown that the ferromagnetic coupling is mediated via an about two atomic layer thick interface region between the nanocrystallites.

Drivers of Holocene treeline and timberline changes in the Retezat Mountains (South Carpathians, Romania)

Four high-altitude lake sediment sequences (Lake Brazi, 1740 m .as.l., Lake Gales 1990 m a.s.l., Lake Bucura, 2040 m a.s.l. and Lake Lia, 1910 m a.s.l.) were analyzed using multi-proxy methods (pollen, stomata, plant macrofossil and micro- and macrocharcoal) in order to study responses of treeline and alpine/subalpine vegetation to climate change and human impact during the last 15000 years. Observing and reconstructing the changes of the position and structure of the treeline can provide valuable information on biotic and other factors such as human activities. Sediment cores were taken from two lakes on the northern slope (Lake Brazi and Lake Gales) and two lakes from the southern slope (Lake Lia and Lake Bucura) in the Retezat Mountains, South Carpathians (Romania).

Hyperfine field at grain boundary atoms in iron nanostructures

Iron nanocrystallites of ball-milled iron powder, partially crystallized melt quenched amorphous alloys and polycrystalline multilayers were studied. The change in the hyperfine field at iron atoms due to grain boundaries does not exceed the experimental linewidth.

Temperature and external magnetic field dependence of the spin freezing in amorphous Fe93Zr7

Abstract The rotation of the Fe magnetic moments toward the collinear state was studied by 57Fe Mossbauer spectroscopy as a function of temperature in applied magnetic fields up to 7 T in amorphous Fe93Zr7 exhibiting a single paramagnetic to spin glass transition. The spin freezing temperature decreased linearly as a function of the external magnetic field.

Magnetic properties of melt-quenched amorphous Fe2(B1-yZry)

Abstract:Two-dimensional (2D) photonic band gaps (PBG) structure fabricated from anisotropic dielectric is studied by solving Maxwells equations with use of plane-wave expansion method. Numerical simulations show that absolute photonic band gaps can be substantially improved in two dimensional square and triangular lattices of cylinders by introducing anisotropy in material dielectricity. Owing to different refractive indices for electromagnetic waves with E- and H-polarization, the quasi-independent adjustment of band gaps for the E- and H-polarization modes can be implemented by uniaxial crystals with their extraordinary axis parallel to the cylinders. Large absolute band gaps can be created for uniaxial cylinders in air with a positive anisotropy. In the case of air holes in background uniaxial dielectric with even a weak negative anisotropy, the absolute band gap can be increased 2-3 times. Large absolute band gap can also be obtained in other complex configurations of uniaxial and biaxial materials and this enables a full exploitation of potential utilization for anisotropic materials available in nature. Such a mechanism of band gap adjustment should open up a new scope for designing band gaps in 2D PBG structures.Amorphous Fe2(B1-yZry) alloys, which are important as the residual amorphous matrices in nanocrystalline Fe - Zr - B - Cu systems, were prepared by rapid quenching from the melt for . The iron magnetic moment determined by 57Fe Mossbauer spectroscopy and magnetic measurements shows an unexpected initial increase when Zr is substituted for B. This behaviour, together with the uncommon crystallization sequence, is attributed to the highly attractive B - Zr interaction.Amorphous Fe2(B1-yZry) alloys, which are important as the residual amorphous matrices in nanocrystalline Fe - Zr - B - Cu systems, were prepared by rapid quenching from the melt for . The iron magnetic moment determined by 57Fe Mossbauer spectroscopy and magnetic measurements shows an unexpected initial increase when Zr is substituted for B. This behaviour, together with the uncommon crystallization sequence, is attributed to the highly attractive B - Zr interaction.