250 research outputs found
Evidence of Skyrmion-Tube Mediated Magnetization Reversal in Modulated Nanowires
Magnetic nanowires, conceived as individual building blocks for spintronic devices, constitute a well-suited model to design and study magnetization reversal processes, or to tackle fundamental questions, such as the presence of topologically protected magnetization textures under particular conditions. Recently, a skyrmion-tube mediated magnetization reversal process was theoretically reported in diameter modulated cylindrical nanowires. In these nanowires, a vortex nucleates at the end of the segments with larger diameter and propagates, resulting in a first switching of the nanowire core magnetization at small fields. In this work, we show experimental evidence of the so-called Bloch skyrmion-tubes, using advanced Magnetic Force Microscopy modes to image the magnetization reversal process of FeCoCu diameter modulated nanowires. By monitoring the magnetic state of the nanowire during applied field sweeping, a detected drop of magnetic signal at a given critical field unveils the presence of a skyrmion-tube, due to mutually compensating stray field components. That evidences the presence of a skyrmion-tube as an intermediate stage during the magnetization reversal, whose presence is related to the geometrical dimensions of the cylindrical segments
Efeito da bracatinga (Mimosa scabrella) na produção de matéria seca de pastagem sob sistema silvipastorial.
Edición de las Memorias de la 22a. Reunión ALPA, 2011, Montevideo. E68 Poster
Plasmonic coupling in closed-packed ordered gallium nanoparticles
Plasmonic gallium (Ga) nanoparticles (NPs) are well known to exhibit good performance in numerous applications such as surface enhanced fluorescence and Raman spectroscopy or biosensing. However, to reach the optimal optical performance, the strength of the localized surface plasmon resonances (LSPRs) must be enhanced particularly by suitable narrowing the NP size distribution among other factors. With this purpose, our last work demonstrated the production of hexagonal ordered arrays of Ga NPs by using templates of aluminium (Al) shallow pit arrays, whose LSPRs were observed in the VIS region. The quantitative analysis of the optical properties by spectroscopic ellipsometry confirmed an outstanding improvement of the LSPR intensity and full width at half maximum (FWHM) due to the imposed ordering. Here, by engineering the template dimensions, and therefore by tuning Ga NPs size, we expand the LSPRs of the Ga NPs to cover a wider range of the electromagnetic spectrum from the UV to the IR regions. More interestingly, the factors that cause this optical performance improvement are studied with the universal plasmon ruler equation, supported with discrete dipole approximation simulations. The results allow us to conclude that the plasmonic coupling between NPs originated in the ordered systems is the main cause for the optimized optical responseThe research is supported by the MINECO (CTQ2014-53334-C2-2-R, CTQ2017-84309-C2-2-R and MAT201676824-C3-1-R) and Comunidad de Madrid (P2018/NMT4349 and S2018/NMT-4321 NANOMAGCOST) projects. ARC acknowledges Ramón y Cajal program (under contract number RYC-2015-18047
Domain Wall Propagation and Pinning Induced by Current Pulses in Cylindrical Modulated Nanowires
The future developments of three-dimensional magnetic nanotechnology require
the control of domain wall dynamics by means of current pulses. While this has
been extensively studied in planar magnetic strips (planar nanowires), few
reports exist in cylindrical geometry, where Bloch point domain walls are
expected to have intriguing properties. Here we report this investigation in
cylindrical magnetic Ni nanowires with geometrical notches. Experimental work
based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with
photoemission electron microscopy (PEEM) indicates that large current densities
induce domain wall nucleation while smaller currents move domain walls
preferably against the current direction. In the region where no pinning
centers are present we found domain wall velocity of about 1 km/s. The domain
wall motion along current was also detected in the vicinity of the notch
region. Pinning of domain walls has been observed not only at geometrical
constrictions but also outside of them. Thermal modelling indicates that large
current densities temporarily raise the temperature in the nanowire above the
Curie temperature leading to nucleation of domain walls during the system
cooling. Micromagnetic modelling with spin-torque effect shows that for
intermediate current densities Bloch point domain walls with chirality parallel
to the Oersted field propagate antiparallel to the current direction. In other
cases, domain walls can be bounced from the notches and/or get pinned outside
their positions. We thus find that current is not only responsible for the
domain wall propagation but is also a source of pinning due to the Oersted
field action
Optical Writing of Magnetic Properties by Remanent Photostriction.
We present an optically induced remanent photostriction in BiFeO_{3}, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO_{3}/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO_{3}. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications
Direct observation of transverse and vortex metastable magnetic domains in cylindrical nanowires
We present experimental evidence of transverse magnetic domains, previously observed only in nanostrips, in CoNi cylindrical nanowires with designed crystal symmetry and tailored magnetic anisotropy. The transverse domains are found together with more conventional vortex domains along the same cylindrical nanowire, denoting a bistable system with similar energies. The surface and the inner magnetization distribution in both types of domains are analyzed by photoemission electron microscopy with x-ray magnetic circular dichroism contrast, and hysteresis loop in individual nanowires are measured by magneto-optical Kerr effect. These experimental data are understood and compared with complementary micromagnetic simulations
Radial Star Formation Histories in 32 Nearby Galaxies
The spatially resolved star formation histories are studied for 32 normal
star-forming galaxies drawn from the the Spitzer Extended Disk Galaxy
Exploration Science survey. At surface brightness sensitivities fainter than 28
mag arcsec, the new optical photometry is deep enough to complement
archival ultraviolet and infrared imaging and to explore the properties of the
emission well beyond the traditional optical extents of these nearby galaxies.
Fits to the spectral energy distributions using a delayed star formation
history model indicate a subtle but interesting average radial trend for the
spiral galaxies: the inner stellar systems decrease in age with increasing
radius, consistent with inside-out disk formation, but the trend reverses in
the outermost regions with the stellar age nearly as old as the innermost
stars. These results suggest an old stellar outer disk population formed
through radial migration and/or the cumulative history of minor mergers and
accretions of satellite dwarf galaxies. The subset of S0 galaxies studied here
show the opposite trend compared to what is inferred for spirals:
characteristic stellar ages that are increasingly older with radius for the
inner portions of the galaxies, and increasingly younger stellar ages for the
outer portions. This result suggests that either S0 galaxies are not well
modeled by a delayed- model, and/or that S0 galaxies have a more
complicated formation history than spiral galaxies.Comment: Accepted for publication in the Astronomical Journal. arXiv admin
note: text overlap with arXiv:1511.0328
Toward Rare-Earth-Free Permanent Magnets: A Combinatorial Approach Exploiting the Possibilities of Modeling, Shape Anisotropy in Elongated Nanoparticles, and Combinatorial Thin-Film Approach
The objective of the rare-earth free permanent magnets (REFREEPM) project is to develop a new generation of high-performance permanent magnets (PMs) without rare earths. Our approach is based on modeling using a combinatorial approach together with micromagnetic modeling and the realization of the modeled systems (I) by using a novel production of high-aspect-ratio (>5) nanostructrures (nanowires, nanorods, and nanoflakes) by exploiting the magnetic shape anisotropy of the constituents that can be produced via chemical nanosynthesis polyol process or electrodeposition, which can be consolidated with novel processes for a new generation of rare-earth free PMs with energy product in the range of 60 kJ/m3 < (BH)max < 160 kJ/m3 at room temperature, and (II) by using a high-throughput thin-film synthesis and high-throughput characterization approach to identify promising candidate materials that can be stabilized in a tetragonal or hexagonal structure by epitaxial growth on selected substrates, under various conditions of pressure, stoichiometry, and temperature. In this article, we report the progress so far in selected phases.This work is supported by European Commission (REFREEPERMAG project) grant number GA-NMP3-SL-2012-280670
Detection of Intracellular Bacterial Communities in Human Urinary Tract Infection
Analyzing urine specimens from women with bladder infections, Scott Hultgren and colleagues find evidence for intracellular bacterial communities, which have been associated with recurrent urinary tract infections in mice
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