Exchange bias like effect induced by domain walls in FeGd/FeSn bilayers

A study of exchange bias phenomenon in ferrimagnetic /ferromagnetic FeGd/ FeSn bilayers is presented. The amorphous FeSn and FeGd alloys have been grown by co-evaporation. Specific growth conditions allow to induce an uniaxial anisotropy in both alloys in a parallel direction. After saturation of the bilayers under a positive field, the hysteresis loop of one of the layer is shifted towards a positive field H _E . The sign of the exchange bias field H _E is shown to be due to the antiferromagnetic coupling between the net magnetizations of both alloys. The field H _E is studied as a function of the thickness of each layer and of the temperature. Using ac-susceptibility measurements and polarized neutron reflectometry, it is shown that the reversal of magnetization of the bilayers is dominated by the presence of a domain wall at the interface. This exchange bias system is shown to act as a potential well for the magnetic domain wall. Within this assumption and thanks to a precise magnetic characterization of each alloy, the evolution of H _E with the thickness of the layers is well reproduced using simple one-dimensional analytical models for the domain wall or a more elaborate numerical approach.Received: 20 February 2003, Published online: 9 September 2003PACS: 75.60.Ch Domain walls and domain structure - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces - 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.)     

Structural Snapshots of Cluster Growth from {U6} to {U38} During the Hydrolysis of UCl4

Herein we report the assembly of large uranium(IV) clusters with novel nuclearities and/or shapes from the controlled hydrolysis of UCl₄ in organic solution and in the presence of the benzoate ligands. {U₆}, {U₁₃}, {U₁₆}, {U₂₄}, {U₃₈} oxo and oxo/hydroxo clusters were isolated and crystallographically characterized. These structural snapshots indicate that larger clusters are slowly built from the condensation of octahedral {U₆} building blocks. The uranium/benzoate ligand ratio, the reaction temperature and the presence of base play an important role in determining the structure of the final assembly. Moreover, the isolation of different size cluster {U₆} (few hours), {U₁₆} (3 days), {U₂₄} (21 days) from the same solution in a chosen set of conditions shows that the assembly of uranium oxo clusters in hydrolytic conditions is time dependent.     

Formation and stability of a suspended biomimetic lipid bilayer on silicon submicrometer-sized pores

We report the fabrication of a thin silicon membrane with an array of micrometer and submicrometer pores that acts as a scaffold for suspending a lipid bilayer. We successfully deposited a lipid bilayer by the Langmuir-Blodgett method on a synthetic silicon membrane bearing arrays of pores with sizes of 1000, 650, and 300 nm. Topographic images obtained by AFM showed a suspended lipid film spanning the pores, whatever the pore size. Higher stability of bilayers supported on smaller pores was shown by AFM characterization. These results represent an important first step to creating a biomimetic environment to study cell membrane dynamics and/or in developing a biosensor.     

Second-order phase transition induced by deterministic fluctuations in aperiodic eight-state Potts models

We investigate the influence of aperiodic modulations of the exchange interactions between nearest-neighbour rows on the phase transition of the two-dimensional eight-state Potts model. The systems are studied numerically through intensive Monte-Carlo simulations using the Swendsen-Wang cluster algorithm for different aperiodic sequences. The transition point is located through duality relations, and the critical behaviour is investigated using FSS techniques at criticality. While the pure system exhibits a first-order transition, we show that the deterministic fluctuations resulting from the aperiodic coupling distribution are liable to modify drastically the physical properties in the neighbourhood of the transition point. For strong enough fluctuations of the sequence under consideration, a second-order phase transition is induced. The exponents , and are obtained at the new fixed point and crossover effects are discussed. Surface properties are also studied. Received: 18 March 1998 / Revised: 29 June 1998 / Accepted: 10 September 1998     

Hybridization potential of oligonucleotides comprising 3′-O-methylated altritol nucleosides

A series of 3??-O-methylated-d-altrohexitol nucleoside analogs (MANA) was synthesized comprising all four base moieties, adenine, cytosine, uracil, and guanine. These monomers were incorporated into oligonucleotides (ONs) by automated solid phase synthesis and the thermal and thermodynamic stability of all new modified constructs were evaluated. Data were compared with results obtained for both anhydrohexitol (HNAs) and 3??-O-altrohexitol-modified ONs (ANAs). We hereby demonstrate that ONs modified with MANA monomers have an improved thermal and thermodynamic stability compared to RNA, ANA, or HNA containing ONs of which the extent depends on the number of incorporated moieties and their position in the sequence. Thermodynamic analysis afforded comparable or even improved results in comparison with the incorporation of locked nucleic acids. While the specificity of these new synthons is slightly lower compared to mismatches within RNA double strands, it is similar to the discrimination potential of other hexitol modifications (HNA and ANA) which already proved their biologic interest, highlighting the potential of MANA constructs in antisense and in siRNA applications.     

Adaptive mesh refinement for stochastic reaction–diffusion processes

We present an algorithm for adaptive mesh refinement applied to mesoscopic stochastic simulations of spatially evolving reaction–diffusion processes. The transition rates for the diffusion process are derived on adaptive, locally refined structured meshes. Convergence of the diffusion process is presented and the fluctuations of the stochastic process are verified. Furthermore, a refinement criterion is proposed for the evolution of the adaptive mesh. The method is validated in simulations of reaction–diffusion processes as described by the Fisher–Kolmogorov and Gray–Scott equations.     

Contour instabilities in early tumor growth models

Recent tumor growth models are often based on the multiphase mixture framework. Using bifurcation theory techniques, we show that such models can give contour instabilities. Restricting to a simplified but realistic version of such models, with an elastic cell-to-cell interaction and a growth rate dependent on diffusing nutrients, we prove that the tumor cell concentration at the border acts as a control parameter inducing a bifurcation with loss of the circular symmetry. We show that the finite wavelength at threshold has the size of the proliferating peritumoral zone. We apply our predictions to melanoma growth since contour instabilities are crucial for early diagnosis. Given the generality of the equations, other relevant applications can be envisaged for solving problems of tissue growth and remodeling.