Criterion of multi-switching stability for magnetic nanoparticles

We present a procedure to study the switching and the stability of an array of magnetic nanoparticles in the dynamical regime. The procedure leads to the criterion of multi-switching stability to be satisfied in order to have stable switching. The criterion is used to compare various magnetic-field-induced switching schemes, either present in the literature or suggested in the present work. In particular, we perform micromagnetic simulations to study the magnetization trajectories and the stability of the magnetization after switching for nanoparticles of elliptical shape. We evaluate the stability of the switching as a function of the thickness of the particles and the rise and fall times of the magnetic pulses, both at zero and room temperature. Furthermore, we investigate the role of the dipolar interaction and its influence on the various switching schemes. We find that the criterion of multi-switching stability can be satisfied at room temperature and in the presence of dipolar interactions for pulses shaped according to CMOS specifications, for switching rates in the GHz regime

Simulation of structural and electronic properties of amorphous tungsten oxycarbides

Electron beam induced deposition with tungsten hexacarbonyl W(CO)6 as precursors leads to granular deposits with varying compositions of tungsten, carbon and oxygen. Depending on the deposition conditions, the deposits are insulating or metallic. We employ an evolutionary algorithm to predict the crystal structures starting from a series of chemical compositions that were determined experimentally. We show that this method leads to better structures than structural relaxation based on guessed initial structures. We approximate the expected amorphous structures by reasonably large unit cells that can accommodate local structural environments that resemble the true amorphous structure. Our predicted structures show an insulator to metal transition close to the experimental composition at which this transition is actually observed. Our predicted structures also allow comparison to experimental electron diffraction patterns.Comment: 17 Pages, 11 figure

Fabrication of FeSi and Fe3Si compounds by electron beam induced mixing of [Fe/Si]2 and [Fe3/Si]2 multilayers grown by focused electron beam induced deposition

Fe-Si binary compounds have been fabricated by focused electron beam induced deposition by the alternating use of iron pentacarbonyl, Fe(CO)5, and neopentasilane, Si5H12 as precursor gases. The fabrication procedure consisted in preparing multilayer structures which were treated by low-energy electron irradiation and annealing to induce atomic species intermixing. In this way we are able to fabricate FeSi and Fe3Si binary compounds from [Fe=Si]2 and [Fe3=Si]2 multilayers, as shown by transmission electron microscopy investigations. This fabrication procedure is useful to obtain nanostructured binary alloys from precursors which compete for adsorption sites during growth and, therefore, cannot be used simultaneously

High-κ\kappa field-effect transistor with copper-phthalocyanine

The use of SrTiO$_3$ dielectrics as high-permittivity insulator in organic thin film field effect transistors (FET) is evaluated. Field-effect transistors with sputtered SrTiO$_3$ and copper-phthalocyanine (CuPc) as semiconducting layer were fabricated. The device preparation was performed in-situ in an ultra high vacuum chamber system. The dielectric in the transistors had a permittivity of up to 200 which led to low driving voltages of 3 V. The field effect transistors were p-type and reached mobilities of about $\mu = 1.5\times 10^{-3}$ cm$^2$/Vs and an on/off ratio of $10^3$. These properties are compared to devices based on other dielectric materials.Comment: 9 pages, 6 figure

Magnetotransport properties of iron microwires fabricated by focused electron beam induced autocatalytic growth

We have prepared iron microwires in a combination of focused electron beam induced deposition (FEBID) and autocatalytic growth from the iron pentacarbonyl, Fe(CO)5, precursor gas under UHV conditions. The electrical transport properties of the microwires were investigated and it was found that the temperature dependence of the longitudinal resistivity (rhoxx) shows a typical metallic behaviour with a room temperature value of about 88 micro{\Omega} cm. In order to investigate the magnetotransport properties we have measured the isothermal Hall-resistivities in the range between 4.2 K and 260 K. From these measurements positive values for the ordinary and the anomalous Hall coefficients were derived. The relation between anomalous Hall resistivity (rhoAN) and longitudinal resistivity is quadratic, rhoAN rho^2 xx, revealing an intrinsic origin of the anomalous Hall effect. Finally, at low temperature in the transversal geometry a negative magnetoresistance of about 0.2 % was measured