Soft Multilayer Research Articles

Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers

Abstract Magnetization reversal and magnetic exchange coupling of (Co/Ni)×N/Ru/(Co/Pt)×12 were investigated as a function of the temperature. The number of repeats N of the soft multilayer (Co/Ni) was varied from 4 to 8 bilayers while the number of repeats of the hard bilayers (Co/Pt) was fixed to 12. Two steps hysteresis loops were observed for coupled structure with only 4 repeats of (Co/Ni) in a wide range of temperature (25–300 K). From the shift of the minor hysteresis loop, the antiferromagnetic exchange coupling Hex was measured and then the interlayer exchange coupling Jex was calculated. A non-monotonous dependence of Jex with temperature was observed for N = 4 with a maximum Jex of 0.13 erg/cm2 at 150 K. The annealing process performed on the same structure confirms the unusual behavior of interlayer exchange coupling Jex. As the repetition number N increases to 8 bilayers the two steps hysteresis loops disappeared in the investigated temperature range, however a small kink appeared in the range of 125 and 225 K for the case of 6 bilayers. From the analysis of the coupled and uncoupled structures, it seems that the dipolar energy overcomes the antiferromagnetic coupling for thicker (Co/Ni) multilayer.

Viscoelastic adlayers of collagen and lysozyme studied using quartz crystal microbalance with dissipation monitoring

In this study the effects of molecular structure of proteins on their adsorption behaviour and viscoelasticity are investigated using a QCM-D method. The adsorption measurements show that spherical lysozyme is rapidly adsorbed on the gold surface as a stiff monolayer, as indicated by a sharp drop in the oscillation frequency (Deltaf) of the sensor, and by a very small increase in the energy dissipation of the adlayer (DeltaD). Fibrous calfskin collagen (CSC) is, however, adsorbed on the same surface rather slowly in two steps to form a thick and soft multilayer (large Deltaf and DeltaD) at pH 3 in salt-free conditions. The two-step adsorption is not observed for stiffly aggregated CSC. This study clearly demonstrates that the polymer structure strongly affects not only how adsorption develops but also the viscoelastic properties of the adlayer.

Impact of Chemical and Structural Anisotropy on the Electrophoretic Mobility of Spherical Soft Multilayer Particles: The Case of Bacteriophage MS2

We report a theoretical investigation of the electrohydrodynamic properties of spherical soft particles composed of permeable concentric layers that differ in thickness, soft material density, chemical composition, and flow penetration degree. Starting from a recent numerical scheme developed for the computation of the direct-current electrophoretic mobility (μ) of diffuse soft bioparticles, the dependence of μ on the electrolyte concentration and solution pH is evaluated taking the known three-layered structure of bacteriophage MS2 as a supporting model system (bulk RNA, RNA-protein bound layer, and coat protein). The electrokinetic results are discussed for various layer thicknesses, hydrodynamic flow penetration degrees, and chemical compositions, and are discussed on the basis of the equilibrium electrostatic potential and hydrodynamic flow field profiles that develop within and around the structured particle. This study allows for identifying the cases where the electrophoretic mobility is a function of the inner structural and chemical specificity of the particle and not only of its outer surface properties. Along these lines, we demonstrate the general inapplicability of the notions of zeta potential (ζ) and surface charge for quantitatively interpreting electrokinetic data collected for such systems. We further shed some light on the physical meaning of the isoelectric point. In particular, numerical and analytical simulations performed on structured soft layers in indifferent electrolytic solution demonstrate that the isoelectric point is a complex ionic strength-dependent signature of the flow permeation properties and of the chemical and structural details of the particle. Finally, the electrophoretic mobilities of the MS2 virus measured at various ionic strength levels and pH values are interpreted on the basis of the theoretical formalism aforementioned. It is shown that the electrokinetic features of MS2 are to a large extent determined not only by the external proteic capsid but also by the chemical composition and hydrodynamic flow permeation of/within the inner RNA-protein bound layer and bulk RNA part of the bacteriophage. The impact of virus aggregation, as revealed by decreasing diffusion coefficients for decreasing pH values, is also discussed.