Quesada-Peñate Isariebel
email: isariebel_quesada@yahoo.es

Julcour-Lebigue Carine

Jáuregui-Haza Ulises-Javier

Wilhelm Anne-Marie

Adsorption of Levopoda from aqueous solution on granular activated carbon

Keywords: Levodopa, pharmaceutical waste waters, adsorption isotherms, activated carbon

 Jovanovic-Freundlich and Radke-Prausnitz, 1972) were evaluated to fit the experimental data. The characteristic parameters for each isotherm were calculated and the selection of the most adequate model was performed using the Fisher's test. From the obtained experimental results, the adsorption process can be well described with Jovanovic-Freundlich model (q=369,21 mg/g, a=0,0042 and γ=0,52) with an average standard error of estimation of 3,62 %.

Introduction

The occurrence of pharmaceuticals in the aquatic environment is an emerging concern. Studies in many countries have found more than 80 compounds, including analgesics, antibiotics, antiepileptics, bblockers and lipid regulators. The presence of drugs in waterways has been established for almost 30 years, but recently concerns have arisen that toxic effects of these compounds may be seen even at very low concentrations (Webb, 2001a ;Webb, 2001b). Pharmaceutically active compounds are produced and used in very large volumes and their use and diversity is increasing every year. Although exact data are rarely available, estimates based on the number of prescriptions suggest that, for example, around 100 tonnes of drugs were prescribed in Germany in 1995 (Ternes, 1998). In the UK in 2000, usage exceeded 10 tonnes per year for all of the top 25 compounds [START_REF] Jones | [END_REF]. Most drugs are designed so that they retain their chemical structure long enough to do their therapeutic work and this property, combined with their continuous input, may enable them to remain in the environment for extended periods of time [START_REF] Ternes | Pharmaceuticals: occurrence in rivers, groundwater and drinking water[END_REF]. Research has shown that many pharmaceuticals are not completely removed during wastewater treatment and, as a result, this has led to their occurrence being reported in WTW effluents, rivers and lakes, and more rarely in groundwater (Kolpin and al., 2004;Jones-Lepp and al., 2004;Stackelberg and al., 2004;Bound and Voulvoulis, 2004). Considering the potential impacts of pharmaceutical products, it is highly important to remove them from wastewater before discharge. The current data suggest that wastewater treatment processes (e.g. activated sludge) have variable performances in removing them. Therefore, it is essential to install additional treatment processes for secondary treatment step. Granular activated carbon (GAC) filtration has been successfully used in wastewater and drinking water treatment plants, to remove different pollutants and it is the most efficient conventional treatment method for the purification of water contaminated by other pollutants like pesticides. Activated carbon is the most efficient adsorbent for organic contaminants due to its high surface area, low specificity and fast adsorption kinetics. Extensive experimental and modeling studies have been reported to date on the activated carbon adsorption of a broad spectrum of hazardous compounds from aqueous solution (Crittenden and al., 1987;[START_REF] Noll | Adsorption Technology for Air and Water Pollution Control[END_REF]. [START_REF] Yenkie | [END_REF] have reported the adsorption of some organic compounds like phenol, aniline, o-cresol, benzoic -acid, and p-methoxyphenol. Specifically, the adsorption of different pharmaceuticals has become one of the aims of the researchers in the world. It is possible to find studies for very commons pharmaceuticals like paracetamol (Terzyk, 2000;[START_REF] Terzyk | [END_REF]Terzyk, 2002;Terzyk and Rychlicki, 2000;Terzyk and al., 2003), aspirine, theophylline (Navarrete Casas and al., 2006) and 17b-estradiol (Zhang and Zhou, 2005;Fukuhara and al., 2006). Adsorption mechanisms are so complicated that no simple theory can adequately represent all experimental data. Many expressions have been published to describe the equilibrium relationships between the most commonly used sorbate and adsorbent (activated carbon). Langmuir and Freundlich isotherms are the most common ones and sometimes piecewise equations are used for a selected range of concentration where implementation of only one expression fails badly (Khan and al., 1997). Levodopa, which molecular structure is represented in Figure 1, is one of the several drugs used to treat Parkinson disease. No reports appear in the literature in concern to the treatment of this product. The goals of this study were to determine the adsorption isotherms of levodopa on GAC at 25 o C and to model obtained data by five models.

Experimental section

Levodopa for synthesis (Sigma-Aldrich) containing more than 99% of pure compound was used for the preparation of the initial solution. Levodopa solutions were prepared from a stock solution and deionised water. The other chemicals and solvents used in chemical analysis were purchased from MERCK Activated carbon (L27) was supplied by PICA (VEOLIA EAU). Their BET surface area, pore volume, and average pore sizes were measured by nitrogen sorptometry, using an ASAP 2010 version 5.02 analyzer. Specific surface area, S, was calculated from BET plots in the relative pressure range of 0,01-0,1. (Brunauer and al., 1938). Pore volume, V p , was determined from the amount of nitrogen adsorbed at relative pressure of 0.985 (Cranston and Inkley, 1957). Mean pore diameter, dp, was calculated from dp =4V p /S, the pore system being assumed to consist of uniform cylindrical nonintersecting capillaries [START_REF] Gregg | Adsorption, Surface Area and Porosity[END_REF]. Table 1 shows the physical properties of the activated carbon. Equilibrium adsorption experiments were carried out using L-27 carbon to evaluate adsorption mechanisms such as the adsorption capacity of the adsorbent. This was done by mixing 100 mL of a solution of levodopa with initial concentration between 0,031g/L and 1,281 g/L with a fixed amount of activated carbon (0,1 g) for 24 hours in a thermostated bath (Polystat 71) at 25 oC. This time was found to be sufficient for the system to reach equilibrium. After mixing, the adsorbent was separated from the adsorbate solution and the concentration of levodopa remaining in solution was measured using an HPLC with a Varian ProStar 310 UV/Vis detector (wave length 278 nm). The equilibrium concentration of levodopa on solid phase was calculated from its initial and final concentrations in aqueous solution. Each experiment was triplicated under identical conditions.

Results and discussion

The experimental adsorption data of Levodopa on activated carbon L-27 at 25 o C are plotted in Figure 2 (experimental data available by request). The average relative error of the measured concentration in the liquid phase was 4.04 %. Adsorption isotherms are important to understand how adsorbates interact with the support. In this regard, the correlation of adsorption data using either a theoretical or empirical equation is essential for practical propose. Different isotherm models have been used to correlate single component isotherm data. The majority of those models were proposed originally for the treatment of gas mixtures; however, their application to liquid mixtures is supported by theoretical and practical reasons [START_REF] Jaroniec | Physical Adsorption on Heterogeneous Solids[END_REF]. Among them several types are found: simple isotherm models for homogeneous surfaces without lateral interactions, like the Langmuir model (Langmuir, 1916) and the Jovanovic model (Jovanovic, 1969); isotherm equations for heterogeneous surfaces without lateral interactions, like the the Freundlich model (Freundlich, 1906) and the Jovanovic-Freundlich model (Quiñones and Guiochon, 1996) and empiric isotherm equations, like the Radke-Prausnitz model (Radke and Prausnitz, 1972). The five above mentioned isotherm models (Table 2) were checked in the present study for correlating the equilibrium data. Fitting of the adsorption isotherm models to the experimental data was performed using a non linear regression (Levenberg Marquardt method). The procedure calculates the values of the isotherm parameters which minimize the residual sum of squares (RSS):

(

2 1 , exp, ∑ = - = n i i t i q q RSS ) (1)
where and are the experimental and calculated values for each data point, respectively.

i q exp, i t q ,
We used a Fisher's test to compare models that have different numbers of parameters. The selected model exhibited the highest value of the Fisher parameter F calc [START_REF] Ajnazarova | Metodi Optimisatsi Eksperimenta v Khimicheskoy Teknologui[END_REF]:

( ) ( ) ( ) ( ) ∑ ∑ = = - - - - = n i i t i n i i calc q q n q q l n F 1 2 , exp, 1 2 exp exp, 1 (2)
Where is the mean value of the vector and l is the number of adjusted parameters of the model.

exp

___ q i q exp,
The standard error of estimation (SEE) was calculated by Equation 3

∑ = - ⋅ = n i i i t i q q q n SEE 1 exp, , exp, 100 (3)
Table 3 summarizes the results of the nonlinear regression analysis. The best fit to the experimental data was obtained for Jovanovic-Freundlich and Radke-Prausnitz models as SEE and F calc values shows. The value of adsorption capacity in Jovanovic-Freundlich model is comparable with those obtained for others products like salicylic acid (351 mg/L) [START_REF] Otero | [END_REF], o-chlorophenol (380,2), p-chlorophenol (422,1 mg/L) [START_REF] Aksu | [END_REF], 2,4 dichlorophenol (339 mg/L) and 2,4 dinitrophenol (232 mg/L) (Daifullah and Girgis, 1998). Regarding the surface heterogeneity, we can observe that the parameter γ in Jovanovic-Freundlich model is far from the unity, so it can be considered that adsorption of levodopa takes place on heterogeneous surface of L-27 carbon.

Conclusion

The present study investigated the adsorption of Levodopa onto activated carbon L-27 at 25 °C. The isotherms were correlated by five models among which the Jovanovic-Freundlich and Radke-Prausnitz equations were found to provided the best fit to the experimental data with an average error of estimation of 3,62 % and 4,05 % respectively. The value of adsorption capacity in Jovanovic-Freundlich model is comparable with those obtained for others products and the surface heterogeneity parameter γ suggests that the adsorption of Levodopa takes place on heterogeneous surface of L-27 carbon.

Figure 1 .

 1 Figure 1. Molecular structure of levodopa

Figure 2 .

 2 Figure 2. Comparison between experimental adsorption data of levodopa on activated carbon L-27 at 25 o C and values calculated using Jovanovic-Freundlich model.

Table 1 .

 1 Physical properties of the a ctivated carbon ( S: BET surface a rea; Vp: to tal p ore volume; dp: average pore diameter; lp: diameter of the particles).

	PROPERTIES	L 27
	S, m 2 g -1	1860
	Vp, cm³/g	1,16
	dp, Å	10
	lp, mm	0,20-0,40
	Microporous volume, cm³/g	0,7

Table 2 .

 2 Adsorption isotherm models. q e the amount of adsorbate adsorbed at equilibrium per unit amount of a dsorbent and q s the monolayer capacity. c e is th e concentration of adsorbate in aq ueous phase at equilibrium, ν is the heterogen eity parameter, k, n, and qo other parameters.

	Model	Equation
	Langmuir	qe=(qs K Ce)/(1+ K Ce)
	Jovanovic	qe=qs(1-e^-(KCe))
	Freundlich	qe=K (Ce)^γ
	Jovanovic-Freundlich	qe=qs(1-e^-(KCe)^γ)
	Radke-Prausnitz	qe=qoKCe/(1+K(Ce)^n)

Table 3 .

 3 Comparison of model correlations. q s is the monolayer capacity, γ is the heterogeneity parameter. K, n and qo are oth er parameters. SEE is the aver age standard error of estimation. F cal is the calculated Fisher parameter.

	Model and parameters	Carbon L-27
	1-Langmuir
	qs (mg/g)	317,93
	K (L/mg)	0,0149
	Fcalc	29,39
	SEE	17,55
	2-Jovanovic
	qs (mg/g)	278,50
	K	0,0117
	Fcalc	12,13
	SEE	24,82
	3-Freundlich
	K	35,11
	γ	0,3352
	Fcalc	49,31
	SEE	13,74
	4-Jovanovic-Freundlich
	qs (mg/g)	369,21
	K	0,0042
	γ	0,52
	Fcalc	340,98
	SEE	3,62
	5-Radke-Prausnitz
	qo	72,33
	K	0,18
	n	0,77
	Fcalc	387,64
	SEE	4,05

Acknowledgements.

I.Q.P expresses her gratitude to ALFA-Programme of the European Community and INP-ENSIACET for the financial support of her travel and stay at the ENSIACET.


Jean Canselier
email: jeanpaul.canselier@ensiacet.fr

Paul

Power Ultrasound in the Formulation of Dispersed Systems

Keywords: ultrasound, emulsion, suspension, nanoparticle, aerosol, foam

Fundamental aspects and applications of power ultrasound in the preparation of dispersed systems are reviewed. As in cleaning, degassing, crystallization and precipitation, its mechanical effects originate from cavitation. Thus, power ultrasound (of low frequency) provides energy under a form particularly suitable for product formulation, more often under the form of dispersions. Therefore, insonation shows very efficient in emulsion (L/L), suspension (S/L), nanoparticle and aerosol (L/G or S/G) formation as well as in foam control.

Introduction 1.1 Power ultrasound

Beyond the audible acoustic spectrum (16 Hz -16 kHz), power ultrasound (US) frequencies mainly range from 16 to 100 kHz. In fact, almost no interaction with matter occurs above about 2 MHz. Contrary to electromagnetic radiation, US waves only propagate in material media and not in vacuo. Propagation speed and attenuation depend on the media (density, homogeneity and elasticity of solids, liquids or gas) and on US frequency and intensity. Whereas US provokes heating of solids, also modifying apparent friction coefficients, and generates acoustic winds in gas, and apart from turbulence arising from acoustic streaming, the mechanical and derived effects of power US in liquid medium originate from acoustic cavitation, usually more intense than the corresponding hydrodynamic phenomenon. Associated with energy dissipation, acoustic cavitation is the driving force of sonochemistry (Lorimer and Mason, 1987;[START_REF] Suslick | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF][START_REF] Mason | Large scale sonochemical processing: aspiration and actuality[END_REF]. Cavitation bubbles (Ø = 5 -150 mm), containing dissolved gas or vapour of the insonated liquid, are produced in the bulk liquid, on dust or on the vessel wall when forces generated in depression regions (acoustic pressure, P a , at a minimum, -P A ) prevail on intermolecular attraction (mostly Van der Waals) forces. They are rather stable for low acoustic intensity values (I = P A 2 /2Z, where Z is the acoustic impedance of the medium), i.e. get bigger on the whole during repeated oscillations before imploding, whereas for higher intensities, they have a very short lifetime, however doubling in size during less than one cycle. These bubbles appear as "hot spots" where temperature and pressure can reach 4000 -5000 K and more than 100 MPa, respectively. This results in an enormous concentration of energy: the amplification factor between the acoustic field generated and that of the collapsed cavitation bubble is of more than eleven orders of magnitude [START_REF] Suslick | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF]. The cavitation threshold increases with US frequency and maximum efficiency (resonance) is obtained if this frequency matches the proper oscillation frequency of the bubble (e.g. 500 -20 kHz for Ø = 5 -100 mm). On the other hand, according to Sirotyuk (1970a,b), the stabilization of cavitation bubbles in water is attributable to the presence of surface-active substances. Cavitation is a local phenomenon, restricted to the vicinity of the US probe. Finally, all the acoustic energy is converted into heat through viscous dissipation. The main technological aspects of US production and the applications of power US in Chemistry and Chemical Engineering have been reviewed until recently [START_REF] Ashley | Ultrasonics in chemical processing[END_REF][START_REF] Mason | Ultrasonic intensification of chemical processing and related operations[END_REF][START_REF] Mason | Ultrasound in synthetic organic chemistry[END_REF]2000;[START_REF] Suslick | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF][START_REF] Ashokkumar | Ultrasound assisted chemical processes[END_REF][START_REF] Keil | Reactors for sonochemical engineering -Present status[END_REF][START_REF] Canselier | Ultrasound emulsification: an overview[END_REF][START_REF] Poux | Procédés d'émulsification : techniques et appareillage[END_REF][START_REF] Poux | Procédés d'émulsification : techniques et appareillage[END_REF][START_REF] Canselier | Ultrasons et formulation : préparation de systèmes dispersés[END_REF].

Dispersed systems

Formulation, i.e. mixing the required number of ingredients in the relevant proportions to prepare an endproduct available directly for professional or domestic use, more often leads to heterogeneous media on a macroscopic or a microscopic scale. That is why a dispersion step is required for obtaining thermodynamically metastable systems possessing long-term kinetic stability. Let us now only consider colloidal or nearly colloidal dispersed systems, as regards dimensions and stability. They include the following types (dispersed/continuous phase): L/G, S/G, G/L, L/L, S/L and G/S (solid foams) for the two-phase systems, and L/L/L (W/O/W and O/W/O multiple emulsions), S/L/L and (G+L+S)/L (whipped suspo-emulsion). The present study is only related to L/G, S/G, G/L, L/L and S/L systems (Table 1). The interactions between power US and colloids or, more generally, dispersed media have been a subject of interest to scientists for many decades. In fact, emulsification and aerosol generation were reported among the first applications of power US [START_REF] Wood | Physical and biological effects of high-frequency sound waves[END_REF] and, very soon afterwards, in 1928, Freundlich opened the "ultrasonic way" in the colloid field: US interaction with colloids, viscosity reduction or thixotropy were the subject of a number of papers [START_REF] Gillings | Adv. in Sonochem[END_REF]. Due to cavitation, US induces mechanical and chemical effects on dispersed systems, especially through their various actions on liquid media:

-intimate mixing of immiscible liquids, -dispersions of solids in liquids, -acceleration of liquid diffusion across membranes, -atomization (spraying) of liquids, -cleaning and erosion of solids covered with liquids, -chemical reaction enhancement Following a first paper on US and formulation of dispersed systems [START_REF] Canselier | Ultrasons et formulation : préparation de systèmes dispersés[END_REF], in this work we try to review the various effects of US in preparing dispersed systems, principally with the goal of formulating end-products. The reverse effects of US in modifying or destabilizing such systems will also be addressed.

Emulsions

Overview

Acoustic methods of emulsification have been known for a long time [START_REF] Wood | Physical and biological effects of high-frequency sound waves[END_REF], their efficiency has been proven (Rooney, 1988) and industrial applications have been developed or foreseen [START_REF] Ashley | Ultrasonics in chemical processing[END_REF]Shoh, 1988, Duée and[START_REF] Duée | Processes using ultrasound[END_REF]. [START_REF] Canselier | Ultrasound emulsification: an overview[END_REF] have reviewed ultrasound emulsification. So, we just deal here with some complementary information.

O/W systems

US processing is a very efficient emulsification technique, namely with respect to mechanical agitation and can be compared with high pressure homogenization and membrane systems as yielding very fine, highly stable emulsions. Probably partly due to low foam formation, the amount of surfactant required is generally lower than with other techniques [START_REF] Canselier | Ultrasound emulsification: an overview[END_REF]. To prepare air/liquid-filled proteinaceous microspheres, possessing a wide range of biomedical applications, Suslick and coworkers (1998) "emulsified" nonaqueous liquid or air in water under US, obtaining a mm-sized dispersion, but the high stability of these objects was due to protein cross-linkage thanks to free radicals generated by US. Very fine emulsions, suitable for pharmaceutical production, can be prepared by means of a contamination-free system: sound waves transmitted via pressurized water to a glass tube installed inside an excited steel jacket [START_REF] Freitas | Continuous contact-and contamination-free ultrasonic emulsification -a useful tool for pharmaceutical development and production[END_REF]. Emulsion polymerization can also be conducted with US application. The use of US during at least part of the process allows to prepare spherical silicone polymer fine particles useful as fillers for electronic materials [START_REF] Kikuchi | Preparation method of spherical silicone polymer fine particles useful as fillers for electronic materials[END_REF], polystyrene nanocapsules (Ø ~ 200 nm) encapsulating oil phase [START_REF] Paul | Manufacture of nanocapsules by ultrasonic waves[END_REF] or particles of polyamides, polyurethanes, polyesters or polyureas with lower porosity, via interfacial polycondensation [START_REF] Touraud | Procedure for preparation of particles of polyamide, polyurethane, polyester or polyurea by interfacial polycondensation in an emulsion with ultrasound application[END_REF]. Terpolymer nanoparticles of butyl acrylate/styrene/acrylamide were even obtained by emulsifier-free emulsion polymerization with only water as a solvent [START_REF] Yin | Preparation of BA/ST/AM nano particles by ultrasonic emulsifier-free emulsion polymerization[END_REF].

W/O systems

The production of sonicated W/O emulsions has been far less studied than that of O/W systems [START_REF] Behrend | Influence of continuous phase viscosity on emulsification by ultrasound[END_REF][START_REF] Canselier | Ultrasound emulsification: an overview[END_REF][START_REF] Canselier | Ultrasons et formulation : préparation de systèmes dispersés[END_REF]. In viscous oils cavitation occurs but is limited to a tiny volume around the emitting surface of the probe and, even with the H 2 O/kerosine system, complete emulsification was obtained by [START_REF] Abismaïl | Comparaison de procédés d'émulsification par agitation mécanique et par ultrasons de puissance: granulométrie et stabilité[END_REF] only while combining US with gentle mechanical stirring. Besides, for the development of liquid surfactant membrane separations, the effects of emulsification time, surfactant and carrier concentrations, ultrasonic power and volume ratio of the internal aqueous phase to organic phase were evaluated and droplet sizes much smaller than those reported previously by mechanical agitation were obtained [START_REF] Juang | Ultrasound-assisted production of W/O emulsions in liquid surfactant membrane processes[END_REF]. As a general feature of O/W and W/O systems, it can be stated that US delivers a high power input in a small volume: this advantage on the laboratory-scale becomes detrimental when scaling up, so that the process often needs a mechanical pre-emulsification and is well-suited for continuous operation.

Demulsification

Chinese researchers have been especially active in this domain for a few years. Crude oil emulsions can be broken by means of US [START_REF] Han | Study on crude oil emulsion breaking via ultrasound, Nanjing Gongye Daxue Xuebao[END_REF]. Chemical demulsifier and US field, especially their combination, can speed up demulsification and dewatering of coal tar or dirty oil emulsions [START_REF] Han | Applying ultrasound to demulsification of dirty oil for water removal, Nanjing Gongye Daxue Xuebao[END_REF][START_REF] Shi | Study on the demulsification and dewater of coal-tar emulsified[END_REF], even at lower temperature (Zhang et al., 2004a) and the feasibility and superiority of US irradiation to separate water and oil were verified: under given conditions (formulation including demulsifier, temperature, time, etc.), the dewatering rate was much higher after US treatment (Zhang et al., 2004b). An US device (f = 5-100 kHz) for treating water-containing oil slops performs rapid oil-water separation (2-30 min) in a thermal settling tank, thus with low operating cost [START_REF] Lu | Process for dehydrating slops from oil refinery or oil field by ultrasonic treatment device[END_REF]. Another method for demulsifying an oil-water emulsion, used in all kinds of oil and water separation technologies, in petroleum recovery and refining, comprises co-current and countercurrent US probes acting simultaneously on the emulsion. The demulsified mixture is dewatered through settlement separation under the effect of electric field. [START_REF] Gou | A method and apparatus for demulsifying an oil-water emulsion via ultrasonic effect[END_REF][START_REF] Li | Ultrasonic demulsifying device for oil-water separation[END_REF].

Suspensions and nanoparticles

Dispersion of solids into liquids

When cavitation occurs near extended solid-liquid interfaces, microjet impact and shockwave damage may be invoked to explain high velocity interparticle collisions (for particles of Ø ~200 mm) inducing abrasion of surface crystallites and even effective melting of metals, such as Cr and Mo. For instance, with 160 mm size Ni powder, US irradiation causes particle size reduction (down to ~ 80 mm), smoothing and removal of passivating oxide coating [START_REF] Suslick | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF]. Ultrasonic probes are currently used to prepare laboratory dispersions: the cavities, formed next to surfaces, are thus able to shatter bonds holding clumps together. The main result is desagglomeration, reducing particle size. On a large scale, a continuous process should be preferred since cavitation only occurs in a small volume around the probe (Nelson Jr., 1996). An US homogenizer was operated at 12,000 L/hr. to achieve a dispersion of pyrogenic silicagel in polyester/polystyrene resins, resulting in a 40% reduction of this expensive additive [START_REF] Mason | Sonochemistry: theory, applications and uses of ultrasound in chemistry[END_REF]. [START_REF] Shoh | Ultrasound: its Chemical, Physical and Biological Effects[END_REF] reports a lot of other examples of industrial applications, from magnetic oxides for audio, video and computer devices to dye pigments (TiO 2 ), china clay, to solid epoxy curing agents mixed with liquid epoxy resins.

Precipitation and preparation of inorganic nanoparticles

US irradiation of fluids enhances precipitation thanks to pressure waves, which bring dissolved molecules together, and to cavitational collapse allowing fragmentation of seed crystals. The production of extremely fine, uniform particles is especially suitable for medical applications: oral or sub-cutaneous administration of drugs as stable suspensions are readily assimilated into the body [START_REF] Mason | Chemistry with Ultrasound[END_REF]. Already in the 50's, through mixing and precipitation in an US reactor (~100 kHz), small (5-15 mm) and rather monodisperse particles of procaine penicillin were obtained compared with those produced by conventional mixing (100-200 mm) [START_REF] Mason | Sonochemistry: theory, applications and uses of ultrasound in chemistry[END_REF]. [START_REF] Ashokkumar | Ultrasound assisted chemical processes[END_REF] have reviewed the synthesis of nanostructured inorganic materials, e.g. metal, metal oxides and semiconductor particles, by sonication-induced precipitation. [START_REF] Suslick | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF] cited remarkable examples of enhanced catalytic activity of Fe-SiO 2 high-surface aggregates for the Fischer-Tropsch synthesis and of Mo 2 C tiny clusters (~ 2 nm) for dehydrogenation. More recently, numerous species, amorphous or microcrystalline, or technical variations, e.g. combination with electrochemical methods or plasma discharge, could be added to the list: filled or hollow silica spheres [START_REF] Enomoto | Effect of ultrasound on synthesis of spherical silica[END_REF][START_REF] Fan | Synthesis of silica hollow spheres assisted by ultrasound[END_REF], Sn [START_REF] Li | A facile way for preparing tin nanoparticles from bulk tin via ultrasound dispersion[END_REF], Pt [START_REF] Mizukoshi | Preparation of nanoparticles by sonochemical reduction of Pt(IV) ions: role of surfactants[END_REF], W [START_REF] Lei | Synthesis of tungsten nanoparticles by sonoelectrochemistry[END_REF], Ag [START_REF] Xie | Preparation method for silver nanoparticle sol using reverse-phase microemulsion[END_REF], Au [START_REF] Reed | The effects of microgravity on nanoparticle size distributions generated by the ultrasonic reduction of an aqueous gold-chloride solution[END_REF][START_REF] Park | Synthesis of multiple shapes of gold nanoparticles with controlled size in aqueous solution using ultrasound[END_REF], C nanoparticles and C encapsulated metal (Cu) or carbide (TiC) nanoparticles [START_REF] Sergiienko | Synthesis of amorphous carbon nanoparticles and carbon encapsulated metal nanoparticles in liquid benzene by an electric plasma discharge in ultrasonic cavitation field[END_REF], C nanotubes [START_REF] Katoh | Sonochemical production of a carbon nanotube[END_REF], CeO 2 [START_REF] Khalil | Preparation and characterization of thermally stable porous ceria aggregates formed via a sol-gel process of ultrasonically dispersed cerium(IV) isopropoxide[END_REF], a-MnO 2 nanoneedles [START_REF] Kumar | Organized and highly dispersed growth of MnO 2 nano-rods by sonochemical hydrolysis of Mn(3)acetate[END_REF], Mn 3 O 4 [START_REF] Kumar | A comparison between hot-hydrolysis and sonolysis of various Mn(II) salts[END_REF], nanosized Sb 2 O 3 flame retardant [START_REF] Zhu | Preparation of nanosized Sb 2 O 3 flame retardant by homogeneous precipitation in ultrasonic field[END_REF], Au-Fe 2 O 3 [START_REF] Mizukoshi | Sonochemical preparation of composite particles of Au/g-Fe 2 O 3 and magnetic separation of glutathione[END_REF], ZrO 2 , TiO 2 , NiFe 2 O 4 and Ni 0,5 Zn 0,5 Fe 2 O 4 [START_REF] Meskin | Ultrasonically assisted hydrothermal synthesis of nanocrystalline ZrO 2 , TiO 2 , NiFe 2 O 4 and Ni 0,5 Zn 0,5 Fe 2 O 4 powders[END_REF], MoS 2 nanorods [START_REF] Zheng | Ultrasound-assisted cracking process to prepare MoS 2 nanorods[END_REF], InP [START_REF] Li | A novel method for the preparation of III-V semiconductors: sonochemical synthesis of InP nanocrystals[END_REF], spherical Ag 2 S/polyvinyl alcohol (PVA) and needle-like CuS/PVA nanocomposites [START_REF] Kumar | Sonochemical synthesis and characterization of Ag 2 S/PVA and CuS/PVA nanocomposite[END_REF] and various amorphous metals, alloys and metal oxides as new catalysts [START_REF] Kesavan | Nanostructured amorphous metals, alloys and metal oxides as new catalysts for oxidation[END_REF].

Dispersion and disruption of living cells

Dispersal of biological cells without damage to living organisms can be achieved (Rooney, 1988). On the other hand, biological decontamination of waste water and sludge treatment not only involves breakdown of bacterial clumps or the material to which bacteria adhere but also direct disruption of cell membranes, which kills all the biological material present [START_REF] Mason | Ultrasound in environmental protection -an overview[END_REF]2003;[START_REF] Roxburgh | Case studies of ultrasound application in wastewater treatment and digestion[END_REF]. Low frequency US disintegrates biosolids and decomposes bulking activated sludge flocs, predominantly by mechanical effects, to allow sedimentation. Firstly, the bacterial flocs are separated and single cells are produced; these cells, acting as nuclei for the formation of cavitation bubbles, can also be ruptured by the strong hydraulic shear stresses generated. The anaerobic digestion time of sonicated sludge then falls down to 8 days instead of 22 days [START_REF] Neis | Ultrasound in waste water and sludge treatment[END_REF]. In environmental and water technology; solid-liquid separation, by high-frequency (a few MHz) standing ultrasonic wave-induced coagulation/sedimentation of suspensions, turns to good account the formation of characteristic structures (bands, discs, columns of particles) to accelerate sedimentation drastically. Besides, under strictly laminar flow conditions, a promising concept of particle manipulation is cited: the suspension enters the resonant chamber, particles are driven into bands, then the flow, gradually expanded, leaves the bands of particles unaffected and a flow divider separates the highly particle enriched from the clarified media [START_REF] Spengler | Ultrasound assisted solid-liquid separation in environmental and water technologylarge scale processing performance enhancement and small scale analytical applications[END_REF].

Foams

As mentioned above ( § 2.2), reduced foaming is observed in US emulsification. In fact, although the transfer of airborne acoustic energy into a liquid is rather inefficient (power loss) [START_REF] Riera | Airborne ultrasound for the precipitation of smokes and powders and the destruction of foams[END_REF],. ultrasonic whistles have long been used for defoaming [START_REF] Ashley | Ultrasonics in chemical processing[END_REF] and high intensity US waves, with the vibrating tip located nearby in the surrounding air, allows to break foams in a clean way, i.e. without contact with the liquid (Sandor and [START_REF] Stein | Foam destruction by ultrasonic vibrations[END_REF]. US is not only efficient at destabilizing static (already formed) foams [START_REF] Dedhia | Static foam destruction; role of ultrasound[END_REF], but also at controlling dynamic ones, i.e. during their production: it is therefore suitable in processes requiring continuous defoaming. It is likely that US vibrations act by generating squeezing mode waves [START_REF] Morey | Foam destabilization by mechanical and ultrasonic vibrations[END_REF]. Even very low, audible frequencies control or suppress slag foaming, probably by accelerating film rupturing [START_REF] Komarov | Control of foam height by using sound waves[END_REF][START_REF] Komarov | Suppression of slag foaming by a sound wave[END_REF]. A highintensity ultrasonic defoamer, based on a stepped-plate transducer, has been successfully applied to foam control in automatic filling of bottles and cans. The main applications of US defoaming are related to preformulation operations, rather to formulation properly said: they have been developed to control foam in industrial reactors (even big ones) and fermenters [START_REF] Pugh | Foam breaking in aqueous systems[END_REF][START_REF] Mason | Sonochemistry and sonoprocessing: the link, the trends and (probably) the future[END_REF][START_REF] Riera | Airborne ultrasound for the precipitation of smokes and powders and the destruction of foams[END_REF]. For very viscous liquids (e.g. molasses in the sugar industry), power US at 20 kHz (100-200W, yielding acoustic levels of 150-170 dB) has little effect due to poor penetration and higher frequencies are probably needed [START_REF] Corral | [END_REF]. US also proved useful in combating foam, mainly stabilized by filamentous microorganisms (bulking sludge) in sewage treatment plants [START_REF] Dittrich | Formation and combating of foam in sewage treatment plants with nutrient salt removal[END_REF][START_REF] Wunsch | Combating bulking sludge with ultrasound[END_REF][START_REF] Roxburgh | Case studies of ultrasound application in wastewater treatment and digestion[END_REF]. A rather similar application of US is the degassing of liquids (e.g. milk), possibly associated with defoaming [START_REF] Ueno | Method and device for degassing of liquids using ultrasonic waves[END_REF][START_REF] Villamiel | Degassing of milk by high-intensity ultrasound[END_REF]. However, it cannot be concluded that US always acts as a defoamer. Let us first notice that simultaneous sonication and air blowing of various aqueous solutions produce turbidity due to the formation of a concentrated bubble cloud disappearing about 1 min after switching US off [START_REF] Louisnard | High bubble concentrations produced by ultrasounds in binary mixtures[END_REF]. In the presence of a surfactant, this observation is consistent with Sirotyuk's work (1970 a,b) but in K 2 SO 4 solutions, only a surface tension gradient could be invoked. Thanks to smaller bubble size and narrower bubble size distribution, what reduces Ostwald ripening, an US probe located near a pneumatic foam generator enhances foam stability (Lim and Barigou, 2005a,b). In addition, in the food and culinary domains, US improves the foaming properties of soybean protein isolate [START_REF] Liu | Henan Gongye Daxue Xuebao[END_REF] as well as those of egg-white, allowing both lighter and stiffer foam to be produced, thus making meringues crisper (Centre de Ressources et d'Informations en Agro-Alimentaire, 2003). But the foaming power of US is more commonly exploited in the polymer field, for example in relation to polyolefin extrusion [START_REF] Horiuchi | Extrusion of polyolefin foams[END_REF] or polyurethane foam production by applying US activation to the gas-supersaturated resin [START_REF] Park | Processing of cellular polyurethane by ultrasonic excitation[END_REF].

Aerosols

Liquid-gas systems

After the first description of US atomization, that is the generation of small liquid droplets from a liquid surface [START_REF] Wood | Physical and biological effects of high-frequency sound waves[END_REF], this phenomenon was found a very effective way to get fine droplets and was namely used in humidifiers, sprays and powder production [START_REF] Ashley | Ultrasonics in chemical processing[END_REF][START_REF] Mason | Sonochemistry: theory, applications and uses of ultrasound in chemistry[END_REF]Rooney, 1988;[START_REF] Mason | Chemistry with Ultrasound[END_REF]. Droplets are extracted from a thin liquid layer contacting a vibrating surface. Their diameter depends on US frequency and fluid properties (density, viscosity, surface tension): for instance, with water, it varies from 60-80mm at 20 kHz to 3 mm at 2 MHz. Rather monodisperse aerosols are obtained (standard deviation around 1.4) [START_REF] Sinaptec | [END_REF]. An interesting example is the concentration of surfactants from their aqueous solutions into collected mist droplets [START_REF] Takaya | Enrichment of surfactant from its aqueous solution using ultrasonic atomization[END_REF].

Solid-gas systems

As regards S/G aerosols, produced from a pure liquid phase (e.g. cooled molten glass, pure metal or alloy) or a solution, the advantages of an US process lie in the spherical shape of the particles, the control of their size as a function of US parameters and their narrow size distribution, e.g. for metal powders [START_REF] Ashley | Ultrasonics in chemical processing[END_REF][START_REF] Shoh | Ultrasound: its Chemical, Physical and Biological Effects[END_REF][START_REF] Mason | Chemistry with Ultrasound[END_REF], amorphous high-surface area SiO 2 particles [START_REF] Rudakoff | Fine silica for fillers[END_REF][START_REF] Bräuer | Manufacture, structures, and properties of silica with increased surface area[END_REF] or ceramic (alumina, mullite, cordierite) powders [START_REF] Jokanovic | Influence of aerosol formation mechanism by an ultrasonic field on particle size distribution of ceramic powders[END_REF]. For solutions, the particle diameter, d p , is a function of solute concentration, C, and droplet diameter before drying, d g (d p =d g .C 1/3 ) [START_REF] Sinaptec | [END_REF]. On the contrary, airborne powders, that are microscopically small particles, light enough to subsist as S/G aerosols for a long time, can be agglomerated by US vibrations, which leads to faster settling [START_REF] Riera | Airborne ultrasound for the precipitation of smokes and powders and the destruction of foams[END_REF].

Vesicles

Vesicles are microscopic spherical structures formed by bilayers of amphiphilic compounds: the internal and external media are similar, so that there are either "direct" (with internal and external aqueous phases) or "reverse" (internal and external organic phases). It has been known for some time that catanionic species could yield unilamellar vesicles spontaneously [START_REF] Kaler | Spontaneous vesicle formation in aqueous mixtures of single-tailed surfactants[END_REF]. More classically, in some conditions of concentration and temperature, phospholipids or synthetic double-chain surfactant monomers form lamellar liquid crystals (hydrated bilayers, L a phase) able to give "onion-like" multilamellar vesicles (MLV) under stirring and single-lamellar vesicles (SLV) when they are sonicated above the bilayer phase transition temperature [START_REF] Robinson | Vesicles[END_REF]. For instance, according to the concentration of its hydroalcoholic solutions, didodecyldimethylammonium bromide yields large or small MLV (LMLV, Ø ~ 300 nm; SMLV, Ø ~ 200 nm) that US transforms into SLV (Ø ~ 50 nm) [START_REF] Rupert | Role of membrane hydration and membrane fluidity in the mechanism of anion-induced fusion of didodecylammonium bromide vesicles[END_REF]. Metastable SLV coming from single amphiphilic species can be stabilized by various means [START_REF] Ringsdorf | Molecular architecture and function of polymeric oriented systems: models for the study of organization, surface recognition and dynamics of biomembranes[END_REF][START_REF] Kostarelos | The effect of monovalent and divalent cations on sterically stabilized phospholipid vesicles (liposomes)[END_REF]. Liposomes, prepared from phospholipids and "niosomes", obtained from nonionic surfactants, have found cosmetic and pharmaceutical uses [START_REF] Uchegbu | Non-ionic surfactant vesicles (niosomes): physical and pharmaceutical chemistry[END_REF]. "Synthetic" vesicles, also of the direct type, are useful in solar energy storage and in catalysis [START_REF] Carmona-Ribeiro | Synthetic amphiphile vesicles[END_REF]. Reverse vesicles can be obtained, for example, with sucrose mono-, di-or triesters [START_REF] Kunieda | Formation and structure of reverse vesicles[END_REF]. In addition, vesicletemplated hollow spheres can be made from inorganic material (e.g. CdSe from sodium dodecylsulfate vesicles) [START_REF] Zheng | Formation of vesicle-templated CdSe hollow spheres in an ultrasound-induced anionic surfactant solution[END_REF].

Other dispersed media

US irradiation increases the formation rate of highly-conducting, solution cast-films of polyaniline [START_REF] Atobe | Preparation of polyaniline colloids under ultrasonication[END_REF]. After drying, these colloids, applied to electrode modification, show a clear voltammetric response [START_REF] Chowdury | Studies on solution and solution-cast film of polyaniline colloids prepared in the absence and presence of ultrasonic irradiation[END_REF]. More complex systems as those used in oil-drilling muds, whose disposal from off-shore oil rigs can be solved by aggregate breaking under the effect of US [START_REF] Mason | Large scale sonochemical processing: aspiration and actuality[END_REF], or involved in ore flotation (a process including emulsification and dispersion of solids), may also benefit from US action [START_REF] Baichenko | Improvement of ultrasonic emulsification of flotation reagents[END_REF][START_REF] Letmathe | Intensification of froth flotation through the use of ultrasound[END_REF]. During solid processing, insonation can replace internal lubricants (mixed with the powder for flow control in moulds) [START_REF] Castel | Mise en forme de solides -Aspects théoriques[END_REF]. It also allows good control of the size of the drops (ca. 1 mm) falling from prilling towers [START_REF] Castel | Mise en forme de solides -Procédés et appareils[END_REF]. Other applications include powder separation, sieving, dispersion, aggregation,… With fine powders (Ø < 100 mm), the forces induced by US on a sieving gauze, more than 100 times higher compared with conventional mechanical ones, reduce clogging considerably. Thus, the flowrate of 45 mm silica particles can show a 9-fold increase [START_REF] Sinaptec | [END_REF]. Power US is also effective in membrane fouling control and cleaning but the main challenge facing this technology is the generation of a uniform acoustic field across the whole membrane surface [START_REF] Muthukumaran | Application of ultrasound in membrane separation processes: a review[END_REF]. The last example will be that of "smart dust": flat amphiphilic silicon chips, previously etched so that one side appears red and the other green, then treated to make the former hydrophilic and the latter hydrophobic (by oxidation), are broken into mm-size pieces (mirrors) able to cluster and orient themselves around a drop of oil in an aqueous medium. The oil drop glitters red: such a color change can therefore indicate the presence of unwanted (possibly toxic) substances in water or food [START_REF] Link | Smart dust: self-assembling self-oriented photonic crystals of porous Si[END_REF].

Table 1 .

 1 Dispersed systems with a gas or liquid continuous phase

	Continuous phase Dispersed phase	System
	Gas	Liquid or solid	Aerosol
	Liquid	Gas	Foam
		Liquid	Emulsion (O/W, W/O)
		Solid	Suspension
		Liquid and solid	Suspo-emulsion
		Gas, liquid and solid	Whipped suspo-emulsion


Aurélien Masseboeuf
email: aurelien.masseboeuf@cemes.fr

Alain Marty

Pascale Bayle-Guillemaud

Christophe Gatel

Etienne Snoeck

Quantitative observation of magnetic flux distribution in new magnetic films for future high density recording media

Off-axis electron holography was used to observe and quantify the magnetic microstructure of a perpendicular magnetic anisotropic (PMA) recording media. Thin foils of PMA materials exhibit an interesting Up and Down domain configuration. These domains are found to be very stable and were observed at the same time with their stray field, closing magnetic flux in the vacuum. The magnetic moment can thus be determined locally in a volume as small as few tens of nm 3 .

From the first desktop (5.25 inch diameter) hard disk drive (HDD) in the early 1980's, with 5 MegaBytes storage capacity, to the 1 TeraByte hard drive distributed now by Hitachi, fundamental improvments have been achieved in HDD technology. The main evolutionnary step in the data storage history happened certainly before the first HDD-integrated Personal desktop Computer (Seagate in 1980) when the perpendicular recording idea was first introduced in 1977. 1 Now, all improvements in new memory capacity are expected to be reached thanks to perpendicular recording. While important studies are published on new materials design, 2,3 studies on magnetic interaction between the recording head and the data bits are mostely concerned about the tip field leakage characterization. 4,5 Here we show that it is also possible to obtain nanometric and quantitative magnetic informations of stray field and magnetic induction at the same time on perpendicular magnetic anisotropic (PMA) materials.

Due to the strong stray fields perpendicular to their surface, PMA materials have been extensively studied by Magnetic Force Microscopy (MFM), 6,7 micro-magnetic simulations, 8 and other magnetic characterization techniques. 9,10 However these techniques were not suitable to study the inner magnetic configuration of materials and inner magnetization at the same time.

Thus, formation of stray field distribution with respect inner magnetic parameters of the material have not been yet confirmed by experimental results. [START_REF] Hubert | Magnetic Domains[END_REF] This could be of great importance to evaluate the ability of a reading head to flip a bit.

Electron Holography (EH) in a Transmission Electron Microscope (TEM) is a powerful

technique which enables observation of electrostatic and magnetic fields at the nanoscale by a electron wave phase retrieval process. [START_REF] Gabor | [END_REF] Through the so-called Aharonov-Bohm effect, 13 it is known that an electron wave is sensitive to the electric and magnetic potential. As a consequence, it can be used to investigate magnetic properties of materials. Electron holography has thus been used to study many magnetic materials, for example, the analysis of stray fields around a MFM tip, 14 or the study of the magnetic configuration of magnetic nanoparticles, 15 magnetic films, 16 magnetic tunnel junctions, 17 or even magnetite core of magnetotactic bacteria. 18 The phase shift of the exit electron wave, ∆φ , travelling along the z direction across the sample, which interacted with the electromagnetic field (electrostatic and magnetic potentials from the sample) can be expresses as :

∆φ (x, y) = C E V int (x, y, z)dz - e h A z (x, y, z)dz (1)
where V (x) int represents the electrostatic contribution to the phase shift (in the case of a material it is mainly its Mean Inner Potential or MIP), and A z is the z-component of the magnetic vector potential describing the magnetic induction distribution in a plane (for a given z) pendicular to the optic axis. A z is related to the magnetic induction B by means of the Maxwell's

equation : B = (B x , B y ) = (∇ y A z , -∇ x A z ). C E is an electron energy related constant.
The keypoint is the separation of the magnetic and electrostatic contributions in the reconstructed phase shift and is extensively discussed elsewhere. 19 We have used for our purpose a method consisting of recording two holograms before (∆φ + )

and after (∆φ -) removing and inverting the sample. The electrostatic contribution to the phase shift remains similar in the two holograms while the magnetic contribution changes in sign.

The magnetic contribution (∆φ magn. ) can thus be obtained by evaluating half of the difference of the two phase images calculated from the two holograms. The MIP contribution (∆φ MIP ) is then half of the sum. To account for the sample reversal, it is necessary to reverse one of the two phase images and align them.

∆φ magn. = 1 2 * [∆φ + -∆φ -] = e h [A z ∆t(x, y)] ∆φ M.I.P. = 1 2 * [∆φ + + ∆φ -] = C E V (x, y) int ∆t(x, y) (2)
EH has been used to study the magnetic configuration and measure the remanent magnetization of a FePd L1 0 /FePd disord. stack, grown on MgO(001), which exhibits a strong PMA. The tetragonal axis of the chemically ordered FePd L1 0 crystalline structure lies along the growth direction corresponding to an alternate stacking of pure Iron and Palladium planes. This chemical anisotropy along the z-axis induces an easy magnetic axis in the same direction which gives rise to the up and down magnetic domain configuration. 6 The main purpose of the second "soft" layer, having a vanishing anisotropy, is usually described as the main factor for increasing the recording efficiency. [START_REF] Khizroev | Perpendicular Magnetic Recording[END_REF] The expected magnetic configuration of the FePd L1 0 /FePd disord. bilayer is presented in Our purpose is to analyse in more detail the inner magnetic configuration with higher resolution. Between these vortices are areas where the magnetic flux is parallel or anti-parallel to the growth direction. These correspond to the "up" and "down" magnetic domains. Stray fields close the flux in the vacuum and inside the stack. However, the vortices appear to be flatter at the bottom (close to the FePd disordered layer) than at the top (close to the vaccum). This asymmetrical shape of the vortices is due to the disordered FePd layer which forces the magnetic induction to lie within the foil plane. From equation 2, quantitative values of magnetic induction can be extracted provided that the MIP or the thickness of the different layers are known:

∆t(x) = ∆φ M.I.P. C E V (x) int B ⊥ = h e•∆t(x) ∇[∆φ magn. ] (3)
Figure 3 shows in yellow the experimental profile of the electrostatic contribution to the phase extracted along the dashed line in Figure 2. To deduce the thickness profile (in blue), we have first calculated the mean inner potential values for the Pd, FePd L1 0 and FePd disord. layers.

According to Equation 3, this thickness profile is used to calculate the B x and B y inside the layer (Figure 4, A and B). Neglecting the demagnetizing field within the material, the measured magnetic induction is directly related to the magnetization in the material. The value of the magnetic induction modulus in the FePd L1 0 region (i.e. inside the domains) gives rise to a magnetic induction of 1.3 ± 0.1 T while same measurements performed on the FePd disord.

area (under domain walls) gives an aeeeveraged values of 1.2 ± 0.1 T. Values measured for the µ 0 M s of the FePd L1 0 layer are the same as those expected for FePd. 6 The variations observed in the different domains come from a variation in the evaluation of the local thickness, due to small deformation of the crystal, or amorphization during ion milling. The difference found between the two layers is negligible. It should comes from the smaller area of magnetization purely perpendicular to the electron path in the soft layer under the domain wall. This implies a variation of the magnetization direction due to the presence of the domain wall and the inplane magnetization under the domains. The measured value is then no longer a pure magnetic moment but a projection of it.

More accurate measurements of the FePd magnetic properties can be done performing micromagnetic simulations. We have used a code based on LLG temporal integration, GL_FFT, 8 to simulate the magnetic flux (i.e. the iso-phase contours) observed by EH. Calculated magnetic phase shift (using bulk values 6 ) and experimental data are compared in Figure 4, C and D. In both images, iso-phase lines represent 0.08 rad and the color scale used is described.

It is seen that the Bloch walls are much wider in the experimental data which could be explain by a slight decrease of the thickness due to the thinning process (see also Supporting Informations). The magnetic flux can be quantitatively measured, both in and outside the sample. The stray field can then be related to the magnetization moment in the domains. This is potentially of great interest for the design of a reader of this kind of material.

Electron holography was used to highlight the magnetic structure of FePd L1 0 -FePd disord.

magnetic bilayer exhibiting PMA, with a resolution closed to the nanometer and an accurate measurement of the local magnetic induction. The magnetic configuration was then successfully compared to micromagnetic calculations. Moreover, the quantitative informations given by the technique can be directly related to the stray field of the materials, which are the bit information for reading heads in hard drives.

  Figure1-A. Domains in the FePd L1 0 layer are separated by Bloch walls where the magnetization lies in the plane of the foil, surrounded by a Néel Cap in which the magnetization runs around the Bloch wall axis. The bottom FePd disord. layer gives rise to in-plane components allowing a flux closure within the bilayer, and enables the domains to be aligned in a parallel stripes configuration. This magnetic configuration is confirmed by studying the external stray field by MFM experiment as shown in Figure 1-B.

Figure 1 :

 1 Figure 1: A. Magnetic configuration expected for the foil. B. MFM view of the sample in its stripe configuration. Black contrast is down domains, bright contrast is up ones. The dashed area shows the geometry used for TEM sample preparation, across magnetic orientation. C. Fresnel view of the sample showing black and white contrast where the beam are overlaping.

Figure 1 -

 1 C shows a Fresnel TEM micrograph[START_REF] Chapman | [END_REF] of the sample. The thin foil used for TEM experiments has been prepared in cross-section in order to observe the magnetic structure by the side instead of the top in the MFM geometry. In this micrograph, domains are clearly defined, separated from one another by a bright or dark line localised at the position of domain walls. The domain periodicity is 100 nm which fully agree with MFM measurement. EH experiments were carried out on the same area of the PMA magnetic film.

Figure 2 -

 2 Figure2-A shows the deduced MIP contribution to the phase shift, and the magnetic contribution is shown in Figure2-B. The iso-phase contours displayed on both phase images directly

Figure 2 :

 2 Figure 2: Mean Inner Potential (A) and magnetic (B) contribution to the phase of the electron wave. The color scale used here is a temperature scale described near each picture. The contour lines are for equi-phase lines and represent 1 radian for MIP contribution and 1/4 radian for magnetic contribution.

Figure 3 :

 3 Figure 3: Mean Inner Potential contribution to the phase analysis. This is a plot profile along the dashed line in Figure 2-A. Right (yellow) label and solid line is the phase profile used to deduce the different layers in the foil. Dashed lines present linear interpolation variations for each different layers. Thickness profile is presented in the colored (blue) area and is labeled on the left.

Figure 4 :

 4 Figure 4: A and B are the X and Y components (left and right respectively) of the magnetic induction deduced from phase gradients. Profile for quantitative interpration is displayed on each figure. C is a zoomed view of Fig. 2-B compared with a micro-magnetic simulation in D.In both images, iso-phase lines represent 0.08 rad and the color scale used is described.

Acknowledgement

Thanks are due to Dr. P. Cherns for critics and usefull discussions about this manuscript.

Supporting Information Available

The sample was grown on MgO (001) substrate by Molecular Beam Epitaxy (MBE) according to the following sequence : Cr (2.5 nm) in order to initiate the epitaxial growth, Pd (48 nm), FePd (15 nm) co-deposited at room temperature, FePd (37 nm) co-deposited at 450 • C and a 1.5 nm caping of Pt was added to avoid oxydation.

The sample has been prepared for electron microscopy using mechanical polishing and ion milling. The layer is thus exhibiting a double wedge geometry along the observation plane.

The microscope used for the holography experiments is a FEI Tecnai F20 field-emission-gun TEM fitted with a Cs corrector (CEOS). A FEI Titan FEG TEM fitted with a dedicated Lorentz lens was used for Fresnel imaging. A Gatan Imaging Filter was also used for zero loss filtering for the Fresnel images.

Holograms are recorded using off-axis electron holography with a rotatable biprism located in the SA aperture. The biprism is aligned along the foil direction x. The fringe spacing is 1.8 nm, the fringe contrast is 12 %. For calculating the phase image we perform a Fourier transform of the hologram and apply a mask of 0.25 nm -1 on the side-band spot before calculating an inverse Fourier transform.

To separate the electrostatic and magnetic contributions to the phase shfit, two holograms were recorded before and after inverting the sample. Image calculations were then performed to aligned the two images. The phase images have been digitally flipped for accordance with the physical inversion of the sample. After data acquisition, an accurate correction of the drift, rotation and scaling between the two images has been performed using recently developped scripts.

Mean Inner Potentials have been calculated using the Doyle and Turner scattering amplitude corrected with the Ross and Stobbs equation (see chapter 12 of. [START_REF] Volkl | Introduction to Electron Holography[END_REF] ) We calculate :

Micro-magnetical simulation has been carried out using the bulk FePd following parameters : Exchange constant A = 6.910 -12 J.m -1 , Uniaxial Anisotropy K=1.03 10 6 J.m -3 , Saturated Magnetization µ 0 M S = 1.294 Tesla . The cells are 0.781 nm × 0.625 nm and infinite along the z direction (considered as invariant).