76 research outputs found
Microwave properties of DyBa_2Cu_3O_(7-x) monodomains and related compounds in magnetic fields
We present a microwave characterization of a DyBaCuO
single domain, grown by the top-seeded melt-textured technique. We report the
(a,b) plane field-induced surface resistance, , at 48.3 GHz,
measured by means of a cylindrical metal cavity in the end-wall-replacement
configuration. Changes in the cavity quality factor Q against the applied
magnetic field yield at fixed temperatures. The temperature
range [70 K ; T_c] was explored. The magnetic field 0.8 T was
applied along the c axis. The field dependence of does not
exhibit the steep, step-like increase at low fields typical of weak-links. This
result indicates the single-domain character of the sample under investigation.
exhibits a nearly square-root dependence on H, as expected for
fluxon motion. From the analysis of the data in terms of motion of Abrikosov
vortices we estimate the temperature dependences of the London penetration
depth and the vortex viscosity , and their zero-temperature
values 165 nm and 3 10 Nsm, which are
found in excellent agreement with reported data in YBaCuO
single crystals. Comparison of microwave properties with those of related
samples indicate the need for reporting data as a function of T/T_c in order to
obtain universal laws.Comment: 6 pages, 4 figures, LaTeX, submitted to Journal of Applied Physic
Surface Effects in Superparamagnetic Magnetite Particles *
Abstract. We have studied the properties of unshielded and shielded (hybrid) nanosized spherical magnetite particles with diameter 10 ± 2 nm, which are superparamagnetic at room temperature, through magnetometry, X-ray powder diffraction, magnetic force microscopy imaging and Mössbauer spectroscopy. The unshielded material was prepared by co-precipitation either in air or in inert atmosphere and part of it was shielded subsequently by β-cyclodextrin (β-C42H70O35). The studies indicated that in the unshielded particles there is a surface layer with a depth of 3 nm wherein an exponential rise in the number of vacancies is observed in the octahedral sublattice ([B]-sites), so that the particle surface is highly defective and could be represented by the general formula (Fe , where x = 0-0.3 and denotes vacancies. In contrast, the hybrid particles, being protected from oxidation, are structurally close to bulk magnetite so that the surface magnetic effects could be understood as due to the chaotic orientation of the magnetic moments at the magnetic-nonmagnetic material interface. An empirical model for a spherical single domain particle was developed in attempting to describe the evolution of structural defects in the surface layer
“Magnetic Force Microscopy and Energy Loss Imaging of Superparamagnetic Iron Oxide Nanoparticles”
We present quantitative, high spatially resolved magnetic force microscopy imaging of samples based on 11 nm diameter superparamagnetic iron oxide nanoparticles in air at room temperature. By a proper combination of the cantilever resonance frequency shift, oscillation amplitude and phase lag we obtain the tip-sample interaction maps in terms of force gradient and energy dissipation. These physical quantities are evaluated in the frame of a tip-particle magnetic interaction model also including the tip oscillation amplitude. Magnetic nanoparticles are characterized both in bare form, after deposition on a flat substrate, and as magnetically assembled fillers in a polymer matrix, in the form of nanowires. The latter approach makes it possible to reveal the magnetic texture in a composite sample independently of the surface topography
Evidence of a cubic iron sub-lattice in t-CuFe2O4 demonstrated by X-ray Absorption Fine Structure
Copper ferrite, belonging to the wide and technologically relevant class of spinel ferrites, was grown in the form of t-CuFe2O4 nanocrystals within a porous matrix of silica in the form of either an aerogel or a xerogel, and com-pared to a bulk sample. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed the presence of two different sub-lattices within the crystal structure of t-CuFe2O4, one tetragonal and one cubic, defined by the Cu2+ and Fe3+ ions respectively. Our investigation provides evidence that the Jahn-Teller distortion, which occurs on the Cu2+ ions located in octahedral sites, does not affect the coordination geometry of the Fe3+ ions, regardless of their location in octahedral or tetrahedral sites
European agricultural landscapes, common agricultural policy and ecosystem services: a review
Since the 1950s, intensification and scale enlargement of agriculture have changed agricultural landscapes across Europe. The intensification and scale enlargement of farming was initially driven by the large-scale application of synthetic fertilizers, mechanization and subsidies of the European Common Agricultural Policy (CAP). Then, after the 1990s, a further intensification and scale enlargement, and land abandonment in less favored areas was caused by globalization of commodity markets and CAP reforms. The landscape changes during the past six decades have changed the flows and values of ecosystem services. Here, we have reviewed the literature on agricultural policies and management, landscape structure and composition, and the contribution of ecosystem services to regional competitiveness. The objective was to define an analytical framework to determine and assess ecosystem services at the landscape scale. In contrast to natural ecosystems, ecosystem service flows and values in agricultural landscapes are often a result of interactions between agricultural management and ecological structures. We describe how land management by farmers and other land managers relates to landscape structure and composition. We also examine the influence of commodity markets and policies on the behavior of land managers. Additionally, we studied the influence of consumer demand on flows and values of the ecosystem services that originate from the agricultural landscape
On the Magnetization Mechanisms in Polycrystalline Hexaferrites
An attempt was made to adapt the domain-wall size model to the explanation of magnetization mechanisms in real polycrystalline hard ferrites (SrFe12O19). Some new data concerning the total anisotropy specifically for polycrystals were discussed and its value deduced from natural spin resonance measurements. It was observed that samples with different Heff grain size and shape, can present different hysteresis loops. To explain these differences, we applied the theoretical expressions for the magnetization curve and the hysteresis loop developed earlier
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