Papers by Philippe Djemia
Brillouin light scattering (BLS) was conducted on melt-grown ZnO bulk crystals and ZnO thin films... more Brillouin light scattering (BLS) was conducted on melt-grown ZnO bulk crystals and ZnO thin films grown by pulsed laser deposition. The bulk ZnO crystals presented both longitudinal and transverse bulk acoustic waves. Theoretical calculations agreed well with there being one piezoelectric longitudinal branch and two transverse branches. BLS measurements conducted on ZnO thin films also revealed Rayleigh surface acoustic waves (R-SAW) guided by only the surface of the layer and Sezawa modes, guided by the film thickness. Measurements were conducted for three incidence angles in order to investigate different SAW wave numbers. Higher frequency features were identified as being related to a new class of guided longitudinal (LG) SAW modes which are not usually detected for ZnO thin films. The LG-SAW modes were observed for two incidence angles (θ=45° and 55°) corresponding to frequencies of 17.88 and 20.75 GHz, respectively. BLS measurements enable us to estimate the LG-SAW velocity as 6500 m/s. This value is three times higher than that of the currently used R-SAW. Theoretical simulations were coherent with the presence of LG modes in the ZnO layers. Such LG-SAW modes are promising for the development of novel, higher-speed SAW devices operating in the GHz-band and which could be readily incorporated in Si-based integrated circuitry.
Brillouin light scattering (BLS) was conducted on melt-grown ZnO bulk crystals and ZnO thin films... more Brillouin light scattering (BLS) was conducted on melt-grown ZnO bulk crystals and ZnO thin films grown by pulsed laser deposition. The bulk ZnO crystals presented both longitudinal and transverse bulk acoustic waves. Theoretical calculations agreed well with there being one piezoelectric longitudinal branch and two transverse branches. BLS measurements conducted on ZnO thin films also revealed Rayleigh surface acoustic waves (R-SAW) guided by only the surface of the layer and Sezawa modes, guided by the film thickness. Measurements were conducted for three incidence angles in order to investigate different SAW wave numbers. Higher frequency features were identified as being related to a new class of guided longitudinal (LG) SAW modes which are not usually detected for ZnO thin films. The LG-SAW modes were observed for two incidence angles (θ=45° and 55°) corresponding to frequencies of 17.88 and 20.75 GHz, respectively. BLS measurements enable us to estimate the LG-SAW velocity as 6500 m/s. This value is three times higher than that of the currently used R-SAW. Theoretical simulations were coherent with the presence of LG modes in the ZnO layers. Such LG-SAW modes are promising for the development of novel, higher-speed SAW devices operating in the GHz-band and which could be readily incorporated in Si-based integrated circuitry.
Journal of Applied Physics, 2004
Picosecond ultrasonics and Brillouin light scattering were used to investigate the acoustical pro... more Picosecond ultrasonics and Brillouin light scattering were used to investigate the acoustical properties of periodic ͑Py-Al 2 O 3 ͒ multilayers constituted of ultrathin polycrystalline layers of permalloy (Py corresponding to a Ni 80 Fe 20 alloy) separated by a dielectric spacer of amorphous alumina ͑Al 2 O 3 ͒. Picosecond ultrasonics gives access to the elastic properties of multilayers both through the travel time of acoustic echoes propagating in the whole structure and through the frequencies of localized vibrational modes lying within the gaps induced by the periodicity of the system. These measurements show that the effective longitudinal elastic constant along the bilayer stacking axis, softens from 7% to 40%, with respect to a reference value obtained in thick samples, when the period decreases from 14.8 to 5.3 nm. This strong softening is ascribed to interfacial effects between permalloy and alumina layers. In contrast with the longitudinal elastic constant, the softening of the in-plane shear elastic constant, derived from Brillouin light scattering measurements, does not exceed 10% for the smallest period sample.
Physical Review B, 2004
Picosecond ultrasonics and Brillouin light scattering were used to investigate the acoustical pro... more Picosecond ultrasonics and Brillouin light scattering were used to investigate the acoustical properties of periodic ͑Py-Al 2 O 3 ͒ multilayers constituted of ultrathin polycrystalline layers of permalloy (Py corresponding to a Ni 80 Fe 20 alloy) separated by a dielectric spacer of amorphous alumina ͑Al 2 O 3 ͒. Picosecond ultrasonics gives access to the elastic properties of multilayers both through the travel time of acoustic echoes propagating in the whole structure and through the frequencies of localized vibrational modes lying within the gaps induced by the periodicity of the system. These measurements show that the effective longitudinal elastic constant along the bilayer stacking axis, softens from 7% to 40%, with respect to a reference value obtained in thick samples, when the period decreases from 14.8 to 5.3 nm. This strong softening is ascribed to interfacial effects between permalloy and alumina layers. In contrast with the longitudinal elastic constant, the softening of the in-plane shear elastic constant, derived from Brillouin light scattering measurements, does not exceed 10% for the smallest period sample.
EPJ Web of Conferences, 2010
Performing a complete in-situ strain measurement of polycrystalline thin films using X-ray diffra... more Performing a complete in-situ strain measurement of polycrystalline thin films using X-ray diffraction is time consuming with most standard diffraction beamlines at synchrotron facilities and not realistic with laboratory diffractometers. Two dimensional detection is shown to enable relatively fast and reliable X-ray strain measurements during continuous in-situ tensile testing of metallic films deposited on polyimide substrates. We show in this paper the advantages to perform this kind of measurements as compared to those with punctual detection.
Dans les matériaux massifs, la spectroscopie Brillouin constitue une des techniques importantes d... more Dans les matériaux massifs, la spectroscopie Brillouin constitue une des techniques importantes de mesure des propriétés élastiques aux côtés de méthodes plus connues, parce que plus anciennes et, peut-être, plus directes, comme les mesures mécaniques reliant contraintes et déformations, mettant ou non à profit des résonances, et les mesures ultrasonores analysant la vitesse de propagation des ondes acoustiques. La spectroscopie Brillouin donne des informations dans un domaine de fréquences s'étendant typiquement de quelques GHz à quelques dizaines de GHz, souvent qualifié d'hypersonique, domaine particulièrement intéressant pour de nombreuses applications en électronique. Elle présente de plus l'avantage de pouvoir être pratiquée sur des échantillons de faible volume, de dimensions parfois submillimétriques. Cependant, pendant longtemps le recours à cette technique n'a guère pu être envisagé que dans le cas de matériaux transparents dans le spectre électromagnétique visible : rappelons en effet qu'on mesure le rayonnement diffusé issu d'un faisceau lumineux incident monochromatique se propageant dans le composé étudié ; la polarisabilité du milieu subit une modulation photoélastique sous l'effet des ondes acoustiques d'origine thermique, d'où une variation de fréquence pour le faisceau diffusé observé reliée à la fréquence des ondes élastiques sondées. Plus précisément, pour un matériau massif, on montre aisément que la différence entre les vecteurs d'onde des faisceaux incident, k i , et diffusé, k d , est égale ou opposée au vecteur d'onde K de l'onde élastique concernée et qu'en conséquence, la variation de la fréquence lumineuse (f i -f d ) est, respectivement, égale (diffusion "Stokes") ou opposée (diffusion "anti-Stokes") à la fréquence F de celle-ci. Notons qu'à l'interprétation classique s'appuyant sur la comparaison à chaque instant et en tout point des phases des ondes mises en jeu, on peut substituer une interprétation "corpusculaire" : le phénomène étudié correspond, dans ce schéma, à la création (diffusion
The European Physical Journal Conferences
Performing a complete in-situ strain measurement of polycrystalline thin films using X-ray diffra... more Performing a complete in-situ strain measurement of polycrystalline thin films using X-ray diffraction is time consuming with most standard diffraction beamlines at synchrotron facilities and not realistic with laboratory diffractometers. Two dimensional detection is shown to enable relatively fast and reliable X-ray strain measurements during continuous in-situ tensile testing of metallic films deposited on polyimide substrates. We show in this paper the advantages to perform this kind of measurements as compared to those with punctual detection.
Surface and Coatings Technology, 2014
ABSTRACT Multicomponent alloying is an effective design route to increase toughening in hard nitr... more ABSTRACT Multicomponent alloying is an effective design route to increase toughening in hard nitride-based coatings. We provide density functional theory calculations and experimental assessment of the elastic properties of a series of ternary transition metal nitride coatings. M11 − xM2xN films (M1, M2 = Ti, Zr, Ta) were elaborated by reactive magnetron co-sputtering. The structural (texture and microstructure) and growth morphology evolution as a function of film thickness and chemical composition were investigated by X-ray diffraction, X-ray reflectivity, electron probe microanalysis and scanning electron microscopy. The effective elastic constants, C33 and C44, were obtained from longitudinal and transverse sound velocity measurements, using picosecond ultrasonics (PU) and Brillouin light scattering (BLS), respectively. We employ Blackman diagram, as well as Pugh and Pettifor criteria, to derive trends in elastic anisotropy, bonding character and brittleness vs. ductility. The influence of microstructure and preferred orientation on the elastic response of polycrystalline films is discussed. For Ta-rich TiTaN and TaZrN systems, the formation of a nanocomposite system consisting of a mixture of cubic and hexagonal phases has a beneficial contribution to toughening.
EPJ Web of Conferences, 2010
Performing a complete in-situ strain measurement of polycrystalline thin films using X-ray diffra... more Performing a complete in-situ strain measurement of polycrystalline thin films using X-ray diffraction is time consuming with most standard diffraction beamlines at synchrotron facilities and not realistic with laboratory diffractometers. Two dimensional detection is shown to enable relatively fast and reliable X-ray strain measurements during continuous in-situ tensile testing of metallic films deposited on polyimide substrates. We show in this paper the advantages to perform this kind of measurements as compared to those with punctual detection.
EPJ Web of Conferences, 2010
Magnetic thin layers show properties that may differ from their bulk counterparts. Understanding ... more Magnetic thin layers show properties that may differ from their bulk counterparts. Understanding the properties of these systems requires an accurate characterisation of their structure and their morphology, in order to determine the influence of mechanical stresses and reduced sizes. In order to accurately study the elastic properties of thin films, we have developed mechanical testing in-situ coupled with x-ray diffraction [1]. This method is not yet widely used to study such systems. The principle is to apply known stresses to thin films, while intra-granular strains are determined by x-ray diffraction.
Physical Review B, 2000
We present an approach to the implementation of the virtual crystal approximation (VCA) for the s... more We present an approach to the implementation of the virtual crystal approximation (VCA) for the study of properties of solid solutions in the context of density-functional methods. Our approach can easily be applied to any type of pseudopotential, and also has the advantage that it can be used to obtain estimates of the atomic forces that would arise if the real atoms were present, thus giving insight into the expected displacements in the real alloy. We have applied this VCA technique within the Vanderbilt ultrasoft-pseudopotential scheme to predict dielectric and piezoelectric properties of the Pb(Zr0.5Ti0.5)O3 solid solution in its paraelectric and ferroelectric phases, respectively. Comparison with calculations performed on ordered alloy supercells and with data on parents compounds demonstrates the adequacy of using the VCA for perovskite solid solutions. In particular, the VCA approach reproduces the anomalous Born effective charges and the large value of the piezoelectric coefficients.
In this paper we report on both magnonic and phononic properties of a Ni-based inverse ferromagne... more In this paper we report on both magnonic and phononic properties of a Ni-based inverse ferromagnetic opal film with the [111] axis of fcc structure along the normal to the film. FMR (FerroMagnetic Resonance) measurements have revealed multiple spin wave (SW) resonances for the magnetic field applied perpendicular to the film. For the field applied in the film plane a broad band of microwave absorption is observed which does not contain a fine structure. Theoretical analysis allows concluding that SW resonanaces localised on chains of quasi-tetrahedron -quasi-cube -quasi-tetrahedron basic elements are weakly coupled, which excludes pronounced collective "magnonic" behavior. On the contrary, our preliminary "pump-probe" experimental studies of acoustic properties of such structures have revealed a presence of Bloch-type phonons.
We have performed strip line ferromagnetic resonance on weak stripe domains in permalloy layers. ... more We have performed strip line ferromagnetic resonance on weak stripe domains in permalloy layers. We used Thiele equation of motion to interprete our experimental data : the lowest frequency mode corresponds to the motion of vortices inside the layer, its frequency is evaluated through the relation ω = k G where G is the gyrovector magnitude and k is a stiffness which mainly depends on demagnetizing effects.
ABSTRACT Some general features concerning inelastic light scattering by hypersonic waves are expo... more ABSTRACT Some general features concerning inelastic light scattering by hypersonic waves are exposed and it is shown that Brillouin spectroscopy can be fruitfully applied in order to study the elastic properties of thin and ultrathin films as well as of multilayers and superlattices. A brief presentation of the various kinds of surface waves and of the mechanisms which monitor the scattering by these excitations is provided. Examples concerning transparent and opaque structures are given. The important influence of the interfaces is pointed out.
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Papers by Philippe Djemia