Nature of phase transition in magnetic thin films

We study the critical behavior of magnetic thin films as a function of the film thickness. We use the ferromagnetic Ising model with the high-resolution multiple histogram Monte Carlo (MC) simulation. We show that though the 2D behavior remains dominant at small thicknesses, there is a systematic continuous deviation of the critical exponents from their 2D values. We observe that in the same range of varying thickness the deviation of the exponent $\nu$ is very small from its 2D value, while exponent $\beta$ suffers a larger deviation. Moreover, as long as the film thickness is fixed, i. e. no finite size scaling is done in the $z$ direction perpendicular to the film, the 3D values of the critical exponents cannot be attained even with very large (but fixed) thickness. The crossover to 3D universality class cannot therefore take place without finite size scaling applied in the $z$ direction, in the limit of numerically accessible thicknesses. From values of exponent $\alpha$ obtained by MC, we estimate the effective dimension of the system. We conclude that with regard to the critical behavior, thin films behave as systems with effective dimension between 2 and 3.Comment: 8 pages, 17 figures, submitted to Phys. Rev.

Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, in order to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. We show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure

Possible determination of critical exponents from experimental magnetization data of ferromagnetic thin films

The influence of finite-size effects on the determination of the critical exponent of the magnetization β is critically discussed. We show that the usual fitting procedure, of fitting a power law to experimental data, is not correct for finite-size disturbed data and only accidentally results in nearly correct values for the critical exponent. The critical temperature resulting from a fit procedure is always too high, because the fitting range includes finite-size disturbed data. A new method is proposed, which circumvents the problems arising in fitting a power law to the experimental magnetization data. This method uses an effective critical exponent and allows the determination of β and Tc even from finite-size disturbed data. The method will be reliable for evaluating experimental data in the currently interesting field of confined magnetic films

Nanolithographie par anodisation locale en microscopie à force atomique sur le phosphore d'indium pour des applications optoélectroniques

Dans le cadre de cette thèse, nous examinons les possibilités offertes par la microscopie à force atomique (AFM) pour la nanolithographie de matériaux semi-conducteurs. Deux méthodes sont présentées sur phosphore d'indium (InP): le mode contact/tension continue et le mode contact intermittent/une tension alternative. La première méthode est caractérisée par une cinétique logarithmique et une dépendance linéaire en tension. Nous interprétons ces résultats par l'existence d'une charge d'espace limitant l'oxydation (homogénéité et résolution). Les améliorations de la méthode sont en résolution et homogénéité. Nous avons réalisé une caractérisation électrique par différents modes AFM. La qualité électronique de l'oxyde a été estimée par des mesures en Tunneling-AFM et phase (AFM). Les déchargements électriques de l'oxyde ont été étudiés par EFM et par SCM. Nous interprétons ainsi les mécanismes d''oxydation. Pour finir, nous montrons l'intérêt de ces méthodes pour l'organisation de boîtes quantiques.We study local anodisation using Atomic Force Microscopy on Indium Phosphite. We develop two methods: a standard AFM oxidation (contact mode and constant bias) and a non usual method (intermittent contact mode and a modulated voltage). The first method results are logarithmic growth kinetic and a linear variation with applied bias voltage. These results had been related to charging effect that limits the oxidation (homogeneity and resolution). To reduce these limits we develop the second method. We obtain an improvement for: resolution, homogeneity, control on oxide shape, and lithography velocity. Electrical characterisations by using different AFM mode measurements had been realised: Tunnelling-AFM and Phase measurement to probe the oxide quality and Scanning Capacitance Microscopy and Electrostatic Force Microscopy measurement to probe charge dissipation. These measurements had been used to understand the oxidation process. We study these methods for Quantum Dots localisation.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Adsorption of molecular and atomic hydrogen on vacuum-cleaved V₂O₅(001)

The (001) surface of a vacuum-cleaved V2O5 single crystal has been studied using high-resolution electron energy loss spectroscopy (HREELS), UV-photoelectron spectroscopy angular resolved photoemission spectra (ARUPS) and X-ray photoelectron spectroscopy (XPS). Special emphasis was put onto the interaction with molecular and atomic hydrogen and the existence of hydroxyl groups on the surface. Dosage of atomic hydrogen induces reduction of the V2O5(001) surface as is obvious from the evolution of intensity near to the Fermi edge in the photoelectron spectra. Even after strong reduction no indications of OH groups could be observed in the spectra. Detailed inspection of the ARUPS and HREELS data and comparison with theory leads to the conclusion that the hydrogen induced defects on the surface are most likely missing bridging oxygen atoms whereas the terminal oxygen atoms of the vanadyl groups appear to be stable with respect to removal by atomic hydrogen

Temperature behaviour of irreversibility fields of various Hg- based superconducting ceramics

A variety of Hg-1223 high-T-c superconducting: systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples. (C) 2002 Elsevier Science B.V. All rights reserved