M. Reissner

Volume magnetostriction and structure of copper mold-cast polycrystalline Fe-Ga alloys

The effect of the volume expansion on the total linear magnetostriction of Fe85Ga15, Fe80Ga20, and Fe71Ga29 mold-cast bulk alloys was investigated by measuring the change in length with applied field longitudinal and perpendicular to the temperature gradient during solidification. In the magnetically saturated state, due to the contribution of the volume expansion, the saturation magnetostriction for all three alloys was about 66 % of the total linear magnetostriction. The magnetostriction is strongly dependent on the direction of the temperature gradient, being larger in this direction. The substitution of Fe by Ga atoms increases the lattice constant and causes a change to the A2 crystal structure, which was confirmed by Mossbauer spectroscopy. The thermal-expansion coefficient increases with Ga content at temperatures between 4.2 and 150K.

Electronic structure of a σ -FeCr compound

The electronic structure of a σ-FeCr compound in a paramagnetic state was calculated for the first time in terms of isomer shifts and quadrupole splittings. The former were calculated using the charge self-consistent Korringa-Kohn-Rostoker (KKR) Greens function technique, while the latter were estimated from an extended point charge model. The calculated quantities combined with recently measured site occupancies were successfully used to analyze a Mössbauer spectrum recorded at room temperature using only five fitting parameters namely background, total intensity, linewidth, IS0 (necessary to adjust the refined spectrum to the used Mössbauer source) and the QS proportionality factor. Theoretically determined changes of the isomer shift for the σ-FeCr sample were found to be in line with the corresponding ones measured on a α-FeCr sample.

Variable blocking temperature of a porous silicon/Fe3O4 composite due to different interactions of the magnetic nanoparticles

In the frame of this work, the aim was to create a superparamagnetic nanocomposite system with a maximized magnetic moment when magnetized by an external field and a blocking temperature far below room temperature. For this purpose, iron oxide nanoparticles of 3.8-, 5- and 8-nm size have been infiltrated into the pores of porous silicon. To fabricate tailored magnetic properties of the system, the particle size and the magnetic interactions among the particles play a crucial role. Different concentrations of the particles dispersed in hexane have been used for the infiltration to vary the blocking temperature TB, which indicates the transition between the superparamagnetic behavior and blocked state. TB is not only dependent on the particle size but also on the magnetic interactions between them, which can be varied by the particle-particle distance. Thus, a modification of the pore loading on the one hand and of the porous silicon morphology on the other hand results in a composite material with a desired blocking temperature. Because both materials, the mesoporous silicon matrices as well as the Fe3O4 nanoparticles, offer low toxicity, the system is a promising candidate for biomedical applications.

Fe3O4-nanoparticles within porous silicon: Magnetic and cytotoxicity characterization

The magnetic properties of porous silicon/Fe3O4 composites are investigated with respect to the adjustability of the blocking temperature along with an evaluation of any size-dependent changes in cytocompatibility. Fe3O4-nanoparticles have been infiltrated within mesoporous silicon, resulting in a system with tunable magnetic properties due to the matrix-morphology, the loading of the nanoparticles, and their size. In order to provide basic information regarding its suitability as a therapeutic platform, the cytotoxicity of these composites have been investigated by a trypan blue exclusion assay with respect to human embryonic kidney 293 cells, and the results compared with cell-only and known cytotoxic controls.

Current densities of MgB2 wires by combined ex situ/in situ process

SiC doped MgB2/Ti/Cu wires have been made by powder-in-tube technique using precursor prepared by mixing of MgB2 (ex situ) with Mg, B (in situ) in variable ex situ/in situ ratio. Wires containing 0%, 23%, 50%, 70%, and 100% of MgB2 powder were two axially rolled to 1.2 mm and annealed in Ar at temperatures 700–850 °C/0.5 h. Increased critical current density [Jc(μ0H) performance] has been measured for 23% and 50% of added MgB2, which is attributed to improved grain connectivity combined with more effective grain boundaries acting as pinning centers. This novel process utilizes the advantages of both techniques: lower MgB2 porosity typical ex situ and better grain connectivity and effective C-substitution through SiC addition obtained by in situ.

Discovery and characterization of magnetism in sigma-phase intermetallic Fe-Re compounds

Systematic experimental (vibrating sample magnetometry) and theoretical (electronic structure calculations using charge and spin self-consistent Korringa-Kohn-Rostoker Green function method) studies were performed on a series of intermetallic sigma-phase Fe(100-x)Re(x) (x = 43-53) compounds. Clear evidence was found that all investigated samples exhibit magnetism with an ordering temperature ranging between 65 K for x = 43 and 23 K for x = 53. The magnetism was revealed to be itinerant and identified as a spin-glass (SG) possibly having a re-entrant character. The SG was found to be heterogeneous viz. two regimes could be distinguished as far as irreversibility in temperature dependence of magnetization is concerned: (1) of a weak irreversibility and (2) of a strong one. According to the theoretical calculations the main contribution to the magnetism comes from Fe atoms occupying all five sub lattices. Re atoms have rather small moments. However, the calculated average magnetic moments are highly (ferromagnetic ordering model) or moderately (antiparallel ordering model) overestimated relative to the experimental data.

Effective activation energy in high Tc superconductors

Measurements of the time dependence of the magnetization are reported for ceramic samples from the different classes of high Tc superconductors. The obtained variation in the activation energies for the different classes points to pinning mechanisms characteristic for each structure type.

Enhanced magnetic anisotropy of Ni nanowire arrays fabricated on nano-structured silicon templates

The magnetic function of a Ni-nanowire/silicon-template system has been explored in corporation with an advanced process. Arrays of nanopores with a mean diameter of 35 nm have been fabricated by anodization of silicon wafers under an external magnetic field (8 T) perpendicular to the substrate. Due to a guided supply of holes from the substrate during the anodization, well controlled straight nanopores have been formed with a high aspect ratio, and then isolated Ni nanowires have been grown along these nanopores by electrodeposition. The fabricated samples show a significantly enhanced magnetic anisotropy with little crosstalk between adjacent pores.

Site occupancy and lattice parameters in sigma‐phase Co–Cr alloys

Neutron powder diffraction was used to study the distribution of Co and Cr atoms over different lattice sites as well as the lattice parameters of sigma-phase compounds Co(100 - x)Cr(x) with x = 57.0, 62.7 and 65.8. From the diffractograms recorded in the temperature range of 4.2-300 K it was found for the five crystallographically independent sites that A (2a) and D (8i) are predominantly occupied by Co atoms, while sites B (4f), C (8i) and E (8j) mainly accommodate Cr atoms. The lattice parameters a and c exhibit linear temperature dependencies, with different expansion coefficients in the temperature ranges of 4.2-100 and 100-300 K.

Influence of the Tl-content on the properties of Bi-doped Tl-1223 superconductors

Abstract Bi-doped Tl-1223 superconductors of the composition Tl x Bi 0.4 Sr 1.6 Ca 2 Cu 3 O y ( x − 0.59–0.92) were prepared from oxidic Bi-Ba-Sr-Ca-Cu precursor material. Upon calcination of the spray-dried metal nitrates, the respective amounts of Tl 2 O 3 were added to the oxidic powders by milling. The sintered and annealed samples consisted predominantly of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. T c(0) -values ranged between 108 and 116 K with transition widths as small as 1 K. T c -onsets from magnetic measurements varied between 110 and 116 K. The transition widths in the magnetization curves were 2–4 K. Transport critical current densities up to 2.1 kA cm −2 at 77 K were measured. Texturing could be improved by grinding, repressing and resintering of the superconducting material increasing the transport critical current density in bulk material to 4.2 kA cm −2 at 77 K.