1,720,984 research outputs found
Local atomic order of a metallic glass made visible by scanning tunneling microscopy
No full textAbstract Exploring the atomic level structure in amorphous materials by STM becomes extremely difficult due to the localized electronic states. Here we carried out STM studies on a quasi-low-dimensional film of metallic glass Zr65Cu27.5Al7.5 which is ‘ultrathin’ compared with the localization length and/or the length scale of short range order. The local electronic structure must appear more inherent, having states at Ef available for tip-sample tunneling current. To enhance imaging contrasts between long-range and short-range orders, the highly oriented pyrolytic graphite was chosen as substrate, so that the structural heterogeneity arising from competition between the glass former ability and the epitaxy can be ascertained. A chemical order predicted for this system was observed in atomic ordered regimes (1–2 monolayers), accompanied with a superstructure with the period Zr–Cu(Al)–Zr along three hexagonal axes. The result implies a chemical short range order in disordered regimes, where polyhedral clusters are dominant with the solute atom Cu(Al) in the center. An attempt for the structural modelling was made based on high resolution STM images, giving icosahedral order on the surface and different Voronoi clusters in 3D space
Oscillation of low-bias tunnel conductance with applied magnetic field in manganite/alumina tunnel structures
No full textWe present magnetotransport measurements on tunnel structures Ir/LCMO/Al2O3/LCMO/Al, where the Ir and Al layers were used to guarantee the homogeneity of tunnel current. Epitaxial growth of Ir and LCMO layers on MgO(001) was confirmed by X-ray diffractions. At T 0.2 V, the magnetotransport behavior becomes simply colossal MR (CMR)-like. The novel behaviors observed near T-c could be attributed to the field-induced enhancement in density of e(g) electron state near the Fermi level
Superconductive spin-valve effect in CoFeHf/Pb/CoFeHf layered structures
No full text(CoFe)75Hf25/Pb/(CoFe)75Hf25 spin-valve–like structures were investigated to show an influence from the relative orientation of ferromagnetic layers on the superconductivity of the Pb space layer, where (CoFe)75Hf25 is amorphous and thus soft ferromagnetic. The Pb layer deposited at 77 K is super smooth as indicated by X-ray diffraction. A proximity effect arises from the ferromagnetic layers and destroys the superconductivity of Pb with a layer thickness ds<35 nm. In order to minimize the proximity effect, thin oxide layers of CoFeHfO (1 nm) were inserted at relevant interfaces. The superconductivity of Pb is alive then even for layered samples with ds=4 nm, showing a sharp superconductive transition at the critical temperature Tc=6 K. A shift of ΔTc∼30 mK was observed by switching the ferromagnetic layers from parallel to antiparallel
Local elasticity and mobility of twin boundaries in martensitic films studied by atomic force acoustic microscopy
No full textNanoscale elastic properties of twinned martensite NiMnGa films were characterized by means of atomic force acoustic microscopy using cantilever contact-resonance spectra to measure the local contact stiffness k and the local damping Q(-1), which contains information on the crystallographic anisotropy of martensitic twin variants and the dissipative motion of twin boundaries (TBs). Images of k and indentation modulus maps were obtained. Similar to topography images measured by conventional atomic force microscopy in contact mode, they show the nature of the twin structure and thus a regular variation in local elastic modulus. A correlation between k and Q(-1) was observed and mirrors the motion of the TB accompanied by a viscoelastic procedure. The k -image and the topography image measured are opposite in contrast, which likely arises from mobile and immobile TBs depending on the geometry of twinning. Multi-resonance spectra were measured, which can be related to martensitic multivariants and are explainable as different types of nanotwins. A critical stress, defined as the starting point of softening due to TB movement was determined to be about 0.5 GPa for a thick film (1 mu m) and 0.75 GPa for a thin film (0.15 mu m), respectively. The values are much larger than that measured for bulk materials, but reasonable due to a large internal stress in the films.BMBF through the project MSM-Sens [13N10061]Open-Access-Publikationsfonds 201
Thermophysical properties of a Si50Ge50 melt measured on board the International Space Station
Full text linkThermophysical properties of highly doped Si50Ge50 melt were measured contactlessly in the electromagnetic levitation facility ISSEML on board the International Space Station. The sample could be melted, overheated by about 375 K, and cooled down in 350 mbar Argon atmosphere. A large undercooling of about 240 K was observed and a quasi-homogeneous nucleation on the droplet surface occurred. During the cooling phase, high-resolution videos were taken from the side and the top. The density and
thermal expansion were evaluated with digital image processing; the viscosity and the surface tension were measured by means of the oscillating drop technique. Inductive measurements of the electrical resistivity were conducted by a dedicated electronics. All data were taken as a function of temperature T from the overheated melt down to the undercooled range. We found a nonlinear
thermal expansion, suggesting a many body effect in the liquid beyond the regular pair interaction, an enhanced damping of surface oscillations likely related to an internal turbulent flow, and an increment of the electrical resistivity with decreased T in the undercooled range regarding a demixing of the components
A novel spin transfer torque effect in Ag2Co granular films
No full textWe studied a spin transfer torque ( STT) effect in Ag2Co granular films, induced by a high current density 10(8-9) A cm(-2) injected via a point contact. The system consists of single domain Co nanoparticles randomly embedded in Ag matrix, with a mean distance corresponding to the typical quantity of layer thicknesses used for the nonmagnetic space in multilayer nanopillars. A large giant magnetoresistance ( GMR) effect of 55% measured at 4.2K is an indication for high spin scattering anisotropy which is required for STT observations. Supposedly, a certain amount of large-sized Co particles saturated in external magnetic field H acts here as the spin polarizer for the injected current I and small-sized particles unsaturated at 4.2K and H-max ( 90 kOe) even act as the detector for switching. A novel STT effect was observed thereby as I rises across a threshold value I-c, showing a sharp decrease in R ( Delta R/R = 130% with two steps), which arises accordingly from further alignment of the small-sized Co granules. The behavior is polar and hysteretic, similar to properties measured for multilayer nanopillars. The two-step behavior could be an effect related to a lognormal particle size distribution. Depending on the spin polarization, Ic is found to be field disproportional, indicating a larger STT efficiency at a higher H
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