Smooth driving of M?ssbauer electromechanical transducers

Quality of Mössbauer spectra is strongly related to the performance of source velocity modulator. Traditional electromechanical driving techniques demand hard-edged square or triangular velocity waveforms that introduce long settling times and demand careful driver tuning. For this work, the behavior of commercial velocity transducers and drive units was studied under different working conditions. Different velocity reference waveforms in constant-acceleration, constant-velocity and programmable-velocity techniques were tested. Significant improvement in spectrometer efficiency and accuracy was achieved by replacing triangular and square hard edges with continuous smooth-shaped transitions. A criterion for best waveform selection and synchronization is presented and attainable enhancements are evaluated. In order to fully exploit this driving technique, a compact microprocessor-based architecture is proposed and a suitable data acquisition system implementation is presented. System linearity and efficiency characterization are also shown.     

Magneto-crystalline properties of BaFe12−2x M x Sn x O19 (M = Sn, Ni, Zn) ferrite powders

BaFe_12?2x M _x Sn _x O₁₉ compounds, where M?=?Sn²⁺, Ni²⁺ or Zn²⁺ ions, were synthesized by mechanical milling and partially by citrate precursor methods. Analysis of magneto-crystalline structure has been carried out by Mössbauer spectroscopy. The Sn⁴⁺ ions replace Fe³⁺ ions on 2b and slightly on 2a?+?4f₁ sites, Zn²⁺ ions strongly prefer 4f₁ sites, Sn²⁺ ions prefer 4f₁ sites too and Ni²⁺ ions occupy 4f₂ and 12k or 2a sites. The magnetic properties were evaluated by the vibrating sample magnetometry and the thermomagnetic analysis. A large variation of the intrinsic coercivity H _c (330 to 78 kA/m) and of temperature coefficient of coercivity of ΔH _c /Δ? (0.39 to 0.22 kA/m°C) were achieved as a function of the (Zn–Sn) and (Sn–Sn) substitutions, respectively. The Curie temperature T _c decreased with the (Ni–Sn) substitution from 447 to 399°C.     

Mössbauer Study of Microstructure and Magnetic Properties (Co,Ni)–Zr Substituted Ba Ferrite Particles

The β-NMR of short-lived β-emitter in a rutile TiO₂ single crystal has been measured as functions of temperature and external magnetic field. Atomic motion induced spin lattice relaxation was observed for two known sites, O substitutional and interstitial sites. The data were analyzed in terms of the thermal atomic jump, which suggests that the motion of defects around the substitutional ¹²N atom for O, and of the interstitial ¹²N atom are attributed to the spin lattice relaxation. The electric field gradients have shown temperature dependence for both sites, which is probably due to the thermal expansion of rutile.     

Separation of vibration fringe data from rotating object fringes using pulsed ESPI

In industrial and other types of non-controlled environments, an unbalanced rotating object may present characteristic out-of-plane vibration amplitude at a specific frequency. For this type of cases and as a first step towards a complete evaluation, it is only desired to visualize the effect of the vibration on the rotating object, or vice versa, for instance to achieve object balancing. Real time optical non-intrusive measurement techniques such as pulsed electronic speckle pattern interferometry (ESPI), are well suited to study this rotating-vibrating object. The advantage offered by ESPI is that real-time fringe data is qualitatively analyzed while being observed on a TV monitor. The present paper proposes a qualitative method, based on pulsed ESPI, to separate rotation fringes from fringes solely related to vibration. The method relies on a high precision scheme that synchronizes and fixes an object point during rotation, without the use of an optomechanical object derotator.     

A Model for second harmonic generation in a TWO-Dimensional array of quantum dots

We present a model for second harmonic generation in a two-dimensional array of quantum dots. We show that the combined effect of the electromagnetic local field of the array and the intrinsic electronic resonances due to the spatial confinement in the quantum dot produce giant enhancements of the second order non-linear susceptibilities, hence giving a very large efficiency in the second harmonic generation.     

Second harmonic response of the Si(111)7 × 7 surface

Optical second harmonic generation spectra have been experimentally obtained from a clean Si(111) 7 × 7 in two different polarization configurations isolating the rotational anisotropic and isotropic contributions. The energy of the fundamental photon is varied from 0.8 eV to 2.5 eV. For comparison, we also use a microscopic formulation based on the semi-empirical tight binding method to evaluate the nonlinear surface susceptibility tensor χ(2ω). Good agreement between theory and experiment is obtained with respect to the number of resonances, their position in energy, and surface or bulk character.     

Structural properties and scaling of the radius of gyration of two-dimensional star-branched polymers grown by diffusion

We built up star-branched polymers, whose morphology is fully determined by diffusion, with p=1,3,6 and 12 branches with a total of 30,000 monomer units. We investigated their structural properties by calculating the monomer-monomer correlation functions. A detailed finite size scaling analysis of the radius of gyration was also performed to determine the exponent and the corrections to scaling. From these results we calculated the fractal dimension of the branched aggregates and obtained: d_f=1.222(7), for the linear chain, d_f=1.2305(8), d_f=1.247(8) and d_f=1.261(8) for the three, six and twelve branches polymer, respectively.     

The role of Fe during hydride formation in the Mg–Fe system: a Mössbauer investigation

The mechanically assisted hydriding of Mg_1???x Fe _x powder mixtures with x?=?0.02, 0.03, 0.10, 0.20 and 0.33 was studied. The hydriding kinetics up to saturation was followed from the hydrogen absorption during milling. All the steady state samples and some partially hydrided ones, with x?=?0.03 , were characterized by X-ray diffraction and Mössbauer spectroscopy. The catalytic action of Fe in the H₂ sorption by Mg based systems and the influence of complex ternary hydride formation in such processes are discussed.