•Electron-micrographs, bulk and MFM magnetization of pyrolytic oxidized-graphenic nanoplatelets are presented.•Ferromagnetism order at room-temperature induced by topographic defects was ...experimentally observed.•Increased defect density, results in enhanced magnetization.
This work reports on room-temperature ferromagnetism of pyrolytic oxidized-graphene nanoplatelets obtained from bamboo pyroligneous acid by varying the density of extended defects. Topographic defects, created during the fabrication process, arise from a natural formation of clusters; such clusters drastically distort the graphitic basal plane, giving rise to abrupt surface curvatures. Topographic defects were found to be sources of the magnetic signal, as evidenced by bulk magnetization and magnetic force microscopy measurements. Increased defect density, tuned by carbonization temperature, results in enhanced magnetization.
Atomic force microscopy (AFM) was used to study the field emission (FE) properties of a dense array of long and vertically quasi-aligned multi-walled carbon nanotubes grown by catalytic chemical ...vapor deposition on a silicon substrate. The use of nanometric probes enables local field emission measurements to be made allowing the investigation of effects that are not detectable with a conventional parallel plate setup, where the emission current is averaged over a large sample area. The micrometric inter-electrode distance allows one to achieve high electric fields with a modest voltage. These features made us able to characterize field emission for macroscopic electric fields up to 250
V/μm and attain current densities larger than 10
5
A/cm
2. FE behaviour is analyzed in the framework of the Fowler–Nordheim theory. A field enhancement factor
γ
≈
40–50 and a turn-on field
E
turn-on ∼15
V/μm at an inter-electrode distance of 1
μm are estimated. Current saturation observed at high voltages in the
I-
V characteristics is explained in terms of a series resistance of the order of MΩ. Additional effects, such as electrical conditioning, CNT degradation, response to laser irradiation and time stability are investigated and discussed.
Impressive changes in the transport and ferromagnetic properties of Co-doped ZnO thin films have been obtained by postgrowth hydrogen irradiation at temperatures of 400 °C. Hydrogen incorporation ...increases the saturation magnetization by one order of magnitude (up to ∼1.50 μB/Co) and increases the carrier density and mobility by about a factor of two. In addition to the magnetic characterization, the transport and structural properties of hydrogenated ZnO:Co have been investigated by Hall effect, local probe conductivity measurements, micro-Raman, and X-ray absorption spectroscopy. Particular care has been given to the detection of Co oxides and metal Co nanophases, whose influence on the increase in the transport and ferromagnetic properties can be excluded on the ground of the achieved results. The enhancement in ferromagnetism is directly related to the dose of H introduced in the samples. On the contrary, despite the shallow donor character of H atoms, the increase in carrier density n is not related to the H dose. These apparently contradictory effects of H are fully accounted for by a mechanism based on a theoretical model involving Co–VO (Co–O vacancy) pairs.
While Abrikosov vortices repel each other and form a uniform vortex lattice in bulk type-II superconductors, strong confinement potential profoundly affects their spatial distribution eventually ...leading to vortex cluster formation. The confinement could be induced by the geometric boundaries in mesoscopic-size superconductors or by the spatial modulation of the magnetic field in superconductor/ferromagnet (S/F) hybrids. Here we study the vortex confinement in S/F thin film heterostructures and we observe that vortex clusters appear near magnetization inhomogeneities in the ferromagnet, called bifurcations. We use magnetic force microscopy to image magnetic bifurcations and superconducting vortices, while high resolution scanning tunneling microscopy is used to obtain detailed information of the local electronic density of states outside and inside the vortex cluster. We find an intervortex spacing at the bifurcation shorter than the one predicted for the same superconductor in a uniform magnetic field equal to the thermodynamical upper critical field H
. This result is due to a local enhanced stray field and a competition between vortex-vortex repulsion and Lorentz force. Our findings suggest that special magnetic topologies could result in S/F hybrids that support superconductivity even when locally the vortex density exceeds the thermodynamic critical threshold value beyond which the superconductivity is destroyed.
Multimaterial optical coatings are a promising viable option to meet the challenging requirements (in terms of transmittance, absorbance, and thermal noise) of next-generation gravitational wave ...detector mirrors. In this paper we focus on ternary coatings consisting of quarter-wavelength-thick layers, where a third material (H^{′}) is added to the two presently in use, namely, silica (L) and titania-doped tantala (H), featuring higher dielectric contrast (against silica) and lower thermal noise (compared with titania-doped tantala), but higher optical losses. We seek the optimal material sequences, featuring minimal thermal (Brownian) noise under prescribed transmittance and absorbance constraints, by exhaustive simulation over all possible configurations, for different values of the optical density and extinction coefficient of the third material, including the case of amorphous silicon and silicon nitride operating at ambient and cryogenic temperatures. In all cases studied, the optimal designs consist of a stack of (H^{′}|L) doublets topped by a stack of (H|L) doublets, confirming previous heuristic assumptions, and the achievable coating noise power spectral density reduction factor ranges from ∼0.5 at 290 K down to ∼0.1 at 20 K. The robustness of the found optimal designs against layer thickness deposition errors and uncertainties and/or fluctuations in the optical losses of the third material is also investigated. Possible margins for further thermal noise reduction by layer thickness optimization, and strategies to implement it, are discussed.
Metallic cantilever in high vacuum atomic force microscope has been used as anode for field emission experiments from densely packed vertically aligned multi-walled carbon nanotubes. The high spatial ...resolution provided by the scanning probe technique allowed precise setting of the tip-sample distance in the submicron region. The dimension of the probe (curvature radius below 50
nm) allowed to measure current contribution from sample areas smaller than 1
μm
2. The study of long-term stability evidenced that on these small areas the field emission current remains stable (within 10% fluctuations) several hours (at least up to 72
h) at current intensities between 10
−5 and 10
−8
A. Improvement of the current stability has been observed after performing long-time conditioning process to remove possible adsorbates on the nanotubes.
We use low temperature magnetic force microscopy and global magnetometry measurements to study the influence of magnetic domains on the Abrikosov vortex pinning in planar superconducting/ferromagnet ...bilayers. The superconducting/ferromagnet bilayers consist of a 200 nm superconducting Nb film covering a Permalloy film, with an insulating layer in between to avoid proximity effect. The periodic stripe domain in the Permalloy film produces a potential for directing vortex motion in the adjacent superconducting film. We observed an enhancement of vortex pinning by a factor of 3 that occurs in bilayers with a magnetic stripe domains w ≈ 500 nm, close to the superconducting critical temperature (T/T c = 0.9) . At lower temperatures, when T/T c = 0.6 the channeled vortex motion and the intrinsic pinning favor vortex avalanches.
This work summarises the results of a series of experiences made on Panasonic UD-802AS, a multi-element dosemeter that is currently used in Caorso Nuclear Power Plant for personnel external ...monitoring. Two main topics have been considered: energy response and the influence of working place dose rate on relative readings of the elements at the end of the monitoring period. The presence of four elements of different materials and filtrations gives the possibility of evaluating radiation energy by a simple algorithm, so that an accurate estimate of the personal dose equivalent can be provided; the good results of the participation of Caorso Individual Monitoring Service (IMS) to 'EURADOS 2008 intercomparison on whole-body dosemeters' account for the reliability of the method. A study on the response in low-dose rate fields is also reported, giving in practice interesting information about operational conditions.
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