593 research outputs found
Domain wall motion in thin ferromagnetic nanotubes: Analytic results
Dynamics of magnetization domain walls (DWs) in thin ferromagnetic nanotubes subject to weak longitudinal external fields is addressed analytically in the regimes of strong and weak penalization. Exact solutions for the DW profiles and formulas for the DW propagation velocity are derived in both regimes. In particular, the DW speed is shown to depend nonlinearly on the nanotube radius
Domain wall structure in magnetic bilayers with perpendicular anisotropy
We study the magnetic domain wall structure in magnetic bilayers (two
ultrathin ferromagnetic layers separated by a non magnetic spacer) with
perpendicular magnetization. Combining magnetic force and ballistic electron
emission microscopies, we are able to reveal the details of the magnetic
structure of the wall with a high spatial accuracy. In these layers, we show
that the classical Bloch wall observed in single layers transforms into
superposed N\'eel walls due to the magnetic coupling between the ferromagnetic
layers. Quantitative agreement with micromagnetic calculations is achieved.Comment: Author adresses AB, SR, JM and AT: Laboratoire de Physique des
Solides, CNRS, Universit\'e Paris Sud, UMR 8502, 91405 Orsay Cedex, France ML
: Laboratoire PMTM, Institut Galil\'ee, CNRS, Universit\'e Paris-13, UPR
9001, 93430 Villetaneuse, Franc
Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy
The time and field dependence of the magnetic domain structure at
magnetization reversal were investigated by Kerr microscopy in interacting
ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local
inhomogeneous magnetostatic fields favor mirroring domain structures and domain
decoration by rings of opposite magnetization. The long range nature of these
magnetostatic interactions gives rise to ultra-slow dynamics even in zero
applied field, i.e. it affects the long time domain stability. Due to this
additionnal interaction field, the magnetization reversal under short magnetic
field pulses differs markedly from the well-known slow dynamic behavior.
Namely, in high field, the magnetization of the coupled harder layer has been
observed to reverse more rapidly by domain wall motion than the softer layer
alone.Comment: 42 pages including 17 figures. submitted to JA
Distribution of naturally occurring radionuclides (U, Th) in Timahdit black shale (Morocco)
Attention has been focused recently on the use of Moroccan black oil shale as the raw material for production of a new type of adsorbent and its application to U and Th removal from contaminated wastewaters. The purpose of the present work is to provide a better understanding of the composition and structure of this shale and to determine its natural content in uranium and thorium. A black shale collected from Timahdit (Morocco) was analyzed by powder X-ray diffraction and SEM techniques. It was found that calcite, dolomite, quartz and clays constitute the main composition of the inorganic matrix. Pyrite crystals are also present. A selective leaching procedure, followed by radiochemical purification and α-counting, was performed to assess the distribution of naturally occurring radionuclides. Leaching results indicate that 238U, 235U, 234U, 232Th, 230Th and 228Th have multiple modes of occurrence in the shale. U is interpreted to have been concentrated under anaerobic conditions. An integrated isotopic approach showed the preferential mobilization of uranium carried by humic acids to carbonate and apatite phases. Th is partitioned between silicate minerals and pyrite
Photovoltaic response around a unique180° ferroelectric domain wall in single crystalline BiFeO3
Using an experimental setup designed to scan a submicron sized light spot and collect the photogenerated current through larger electrodes, we map the photovoltaic response in ferroelectric BiFeO3 single crystals. We study the effect produced by a unique 180° ferroelectric domain wall (DW) and show that the photocurrent maps are significantly affected by its presence and shape. The effect is large in its vicinity and in the Schottky barriers at the interface with the Au electrodes, but no extra photocurrent is observed when the illuminating spot touches the DW, indicating that this particular entity is not the heart of specific photo-electric properties. Using 3D modelling, we argue that the measured effect is due to the spatial distribution of internal fields which are significantly affected by the charge of the DW due to its distortion
Highly asymmetric magnetic domain wall propagation due to coupling to a periodic pinning potential
Magneto-optical microscopy and magnetometry have been used to study
19 magnetization reversal in an ultrathin magnetically soft [Pt/Co]2 ferromagnetic film
20 coupled to an array of magnetically harder [Co/Pt]4 nanodots via a predominantly
21 dipolar interaction across a 3 nm Pt spacer. This interaction generates a spatially
22 periodic pinning potential for domain walls propagating through the continuous
23 magnetic film. When reversing the applied field with respect to the static nanodot
24 array magnetization orientation, strong asymmetries in the wall velocity and switching
25 fields are observed. Asymmetric switching fields mean that the hysteresis of the film is
26 characterized by a large bias field of dipolar origin which is linked to the wall velocity
27 asymmetry. This latter asymmetry, though large at low fields, vanishes at high fields
28 where the domains become round and compact. A field-polarity-controlled transition
29 from dendritic to compact faceted domain structures is also seen at low field and a
30 model is proposed to interpret the transition
Construction of a Tethered Poly(ethylene glycol) Surface Gradient For Studies of Cell Adhesion Kinetics
Surface gradients can be used to perform a wide range of functions and represent a novel experimental platform for combinatorial discovery and analysis. In this work, a gradient in the coverage of a surface-immobilized poly(ethylene glycol) (PEG) layer is constructed to interrogate cell adhesion on a solid surface. Variation of surface coverage is achieved by controlled transport of a reactive PEG precursor from a point source through a hydrated gel. Immobilization of PEG is achieved by covalent attachment of the PEG molecule via direct coupling chemistry to a cystamine self-assembled monolayer on gold. This represents a simple method for creating spatial gradients in surface chemistry that does not require special instrumentation or microfabrication procedures. The structure and spatial distribution of the PEG gradient are evaluated via ellipsometry and atomic force microscopy. A cell adhesion assay using bovine arteriole endothelium cells is used to study the influence of PEG thickness and chain density on biocompatibility. The kinetics of cell adhesion are quantified as a function of the thickness of the PEG layer. Results depict a surface in which the variation in layer thickness along the PEG gradient strongly modifies the biological response
Probing the interface magnetism in the FeMn/NiFe exchange bias system using magnetic second harmonic generation
Second harmonic generation magneto-optic Kerr effect (SHMOKE) experiments,
sensitive to buried interfaces, were performed on a polycrystalline NiFe/FeMn
bilayer in which areas with different exchange bias fields were prepared using
5 KeV He ion irradiation. Both reversible and irreversible uncompensated spins
are found in the antiferromagnetic layer close to the interface with the
ferromagnetic layer. The SHMOKE hysteresis loop shows the same exchange bias
field as obtained from standard magnetometry. We demonstrate that the exchange
bias effect is controlled by pinned uncompensated spins in the
antiferromagnetic layer.Comment: submitted to Phys. Rev. Let
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