Sima Valizadeh

Aging phenomena in ferrofluids suitable for magnetic biosensor applications

Aging phenomena were investigated in three different ferrofluids containing submicron sized magnetic beads consisting of a cluster of iron oxide nanoparticles embedded in a dextrane matrix with primary amine groups on the surface (one fresh sample and one sample aged during three months) and with a plain surface (only dextrane on the surface, aged for three months), suitable for magnetic biosensor applications. The main characterization technique used was measurement of the frequency dependent complex magnetization using a superconducting quantum interference device. The experimental factors considered were the effect of ultrasonication, temperature, dc bias magnetic field, and addition of sodium dodecyl sulphate surfactant. The stability against aging was found to depend strongly on the bead surface, and the reproducibility of a refreshing procedure involving addition of surfactant in combination with ultrasonication was shown to be low. Aggregation was shown to be strongly affected by the presence of ev...

Focused ion beam fabrication procedures of x-ray micro Fresnel zone plates

In this paper the use of focused ion beam (FIB) mask-less lithography is presented as a novel and simple way to fabricate x-ray Fresnel zone plates (FZPs), and a prototype of a FIB-made 100 nm resolution FZP with 38 zones is described. Considerations for future developments—the maximum aspect ratio achievable by FIB lithography in nickel, a way to produce zones with a parabolic transverse profile, as theoretically required for the highest efficiency in focusing—are also reported.

Structure of Fe-Co/Pt(001) superlattices : a realization of tetragonal Fe-Co alloys

The structural properties of a tetragonally distorted Fe1−xCox alloy, in the form of Fe1−xCox/Pt(001) superlattices with x = 0.64, have been investigated experimentally. The study follows recent theoretical predictions on the enhanced uniaxial magnetocrystalline anisotropy of such alloys with specific combinations of chemical composition and tetragonal distortion. The ratio between out-of-plane and in-plane lattice parameters in the Fe0.36Co0.64 layers, c/a, was found to vary between 1.18 and 1.31, depending on the thickness ratio between the alloy and the spacer. This covered the range of interest c/a = 1.20–1.25 in the previous calculations and should be compared to c/a = 1 in the original bcc alloy lattice. Simulations of x-ray diffraction patterns as well as density functional calculations support the derivation of the Fe0.36Co0.64 lattice parameters.