Perpendicular magnetic anisotropy in chemically disordered FePd-FeV(100) alloy thin films

We find that the use of V(100) buffer layers on MgO(001) substrates for the epitaxy of FePd binary alloys yields to the formation at intermediate and high deposition temperatures of a FePd¿FeV mixed phase due to strong V diffusion accompanied by a loss of layer continuity and strong increase of its mosaic spread. Contrary to what is usually found in this kind of systems, these mixed phase structures exhibit perpendicular magnetic anisotropy (PMA) which is not correlated with the presence of chemical order, almost totally absent in all the fabricated structures, even at deposition temperatures where it is usually obtained with other buffer layers. Thus the observed PMA can be ascribed to the V interdiffusion and the formation of a FeV alloy, being the global sample saturation magnetization also reduced.

Measurement and simulation of anisotropy in the Infrared and Raman spectra of beta-FeSi2 single crystals

In this paper we show that the orthorhombic phase of FeSi2 (stable at room temperature) displays a sizable anisotropy in the infrared spectra, with minor effects in the Raman data too. This fact is not trivial at all, since the crystal structure corresponds to a moderate distortion of the fluorite symmetry. Our analysis is carried out on small single crystals grown by flux transport, through polarization-resolved far-infrared reflectivity and Raman measurements. Their interpretation has been obtained by means of the simulated spectra with tight-binding molecular dynamics.

Hysteresis and avalanches in the random anisotropy Ising model

ches. The critical point is characterized by a set of critical exponents, which are consistent with the universal values proposed from the study of other simpler models.

Probing elastic anisotropy from defect dynamics in Langmuir monolayers

We study the dynamics of annihilation of point defects in Langmuir monolayers. The absence of hydrodynamic effects allows us to quantitatively relate the asymmetry in defect mobility to the elastic anisotropy of the material, which in turn can be varied through the control of the surface pressure applied to the monolayer. Using the proposed theoretical analysis, we are able to obtain rather elusive equilibrium properties out of relatively simple dynamical measurements. In particular, we measure the elastic constants and their pressure dependence.

Viscous fingering in liquid crystals: Anisotropy and morphological transitions

We show that a minimal model for viscous fingering with a nematic liquid crystal in which anisotropy is considered to enter through two different viscosities in two perpendicular directions can be mapped to a twofold anisotropy in the surface tension. We numerically integrate the dynamics of the resulting problem with the phase-field approach to find and characterize a transition between tip splitting and side branching as a function of both anisotropy and dimensionless surface tension. This anisotropy dependence could explain the experimentally observed (reentrant) transition as temperature and applied pressure are varied. Our observations are also consistent with previous experimental evidence in viscous fingering within an etched cell and simulations of solidification.

Quadratic tranverse anisotropy term due to dislocations in Mn12 acetate crystals directly observed by EPR spectroscopy

High-sensitivity electron paramagnetic resonance experiments have been carried out in fresh and stressed Mn12 acetate single crystals for frequencies ranging from 40 GHz up to 110 GHz. The high number of crystal dislocations formed in the stressing process introduces a E(Sx2-Sy2) transverse anisotropy term in the spin Hamiltonian. From the behavior of the resonant absorptions on the applied transverse magnetic field we have obtained an average value for E=22 mK, corresponding to a concentration of dislocations per unit cell of c=10-3.

Vortex state and effect of anisotropy in sub-100 nm magnetic nanodots

Magnetic properties of Fe nanodots are simulated using a scaling technique and Monte Carlo method, in good agreement with experimental results. For the 20-nm-thick dots with diameters larger than 60¿nm, the magnetization reversal via vortex state is observed. The role of magnetic interaction between dots in arrays in the reversal process is studied as a function of nanometric center-to-center distance. When this distance is more than twice the dot diameter, the interaction can be neglected and the magnetic properties of the entire array are determined by the magnetic configuration of the individual dots. The effect of crystalline anisotropy on the vortex state is investigated. For arrays of noninteracting dots, the anisotropy strongly affects the vortex nucleation field and coercivity, and only slightly affects the vortex annihilation field

Probing elastic anisotropy from defect dynamics in Langmuir monolayers

We study the dynamics of annihilation of point defects in Langmuir monolayers. The absence of hydrodynamic effects allows us to quantitatively relate the asymmetry in defect mobility to the elastic anisotropy of the material, which in turn can be varied through the control of the surface pressure applied to the monolayer. Using the proposed theoretical analysis, we are able to obtain rather elusive equilibrium properties out of relatively simple dynamical measurements. In particular, we measure the elastic constants and their pressure dependence.

Chemical etching without acids: from early etching solutions to today's corrosive salts

Conferència impartida a l'Accademia di Belle Arti di Bologna en motiu de de les jornades "Atossico incisione e la sua introduzione in istruzione superiore", del 25 al 27 de gener del 2011 a Bologna

Creative and Multidisciplinary Etching in the Digital Age

In our capacity as creative artists, researchers and fine arts professors, most part of our activity focuses on the relationships between people and creativity, technology, and resources, and, most frequently, what we aim to offer are new enjoyable and subversive ways of interacting with these three fields. As art teachers, we ask 'why and how' to teach dynamic bearing in mind that digital technology will undoubtedly impact on contemporary art practice.

Optimization of KOH etching process to obtain textured substrates suitable for heterojunction solar cells fabricated by HWCVD

In this work, we have studied the texturization process of (100) c-Si wafers using a low concentration potassium hydroxide solution in order to obtain good quality textured wafers. The optimization of the etching conditions have led to random but uniform pyramidal structures with good optical properties. Then, symmetric heterojunctions were deposited by Hot-Wire CVD onto these substrates and the Quasi-Steady-State PhotoConductance technique was used to measure passivation quality. Little degradation in the effective lifetime and implicit open circuit voltage of these devices (< 20 mV) was observed in all cases. It is especially remarkable that for big uniform pyramids, the open-circuit voltage is comparable to the values obtained on flat substrates.

Interplay between surface anisotropy and dipolar interactions in an assembly of nanomagnets

We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization, taking into account temperature, applied field, core and surface anisotropy, and dipolar interparticle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly magnetization derived here: the overall sign of the product of the two parameters governing the surface and the dipolar contributions determines whether intrinsic and collective terms compete or have synergistic effects on the magnetization. This is illustrated by the magnetization curves of γ-Fe2O3 nanoparticle assemblies in the low concentration limit.

Influence of elastic anisotropy on structural nanoscale textures

We study the influence of elastic anisotropy on nanoscale precursor textures that exist in some shape-memory alloys and show that tweed occurs in the limit of high elastic anisotropy while a nanocluster phase-separated state occurs for values of anisotropy inhibiting the formation of martensite. These results are consistent with specific heat data, elastic constant measurements, and zero-field cooling or field cooling experiments in nonstoichiometric NiTi alloys.