domain: math spi spi.tron

text:
Ibtissem Benguettat-El Mokhtari

Yves Roussigné

Traian Petrisor

Fatih Zighem

Fatiha Kail

Larbi Chahed

Victor Pierron

Laurence Méchin

Mohamed Belmeguenai
email: belmeguenai.mohamed@univ-paris13.fr

Spin pumping and magnetic anisotropy in La 2/3 Sr 1/3 MnO 3 /Pt systems

Keywords: numbers: 75.70.Cn, 75.30.Ds, 75.70.Ak, 75.70.Tj Interface effects, Perpendicular magnetic anisotropy, Brillouin light scattering, spin waves and magnetization dynamics

La 2/3 Sr 1/3 MnO 3 (LSMO) thin films of various thicknesses (6, 8, 10, 20 and 30 nm), capped by 7 nm thick Pt layer were grown by pulsed laser deposition on SrTiO 3 (001) substrates. Xray diffraction revealed that LSMO films are (001) oriented. Vibrating sample magnetometer was used to determine the magnetization at saturation and the magnetic dead layer thickness. This latter is around 3.4 nm, significantly thicker compared with the one induced at interfaces of Pt with ferromagnetic transition metals. Microstrip line ferromagnetic resonance (MS-FMR) was used to extract the gyromagnetic ratio, which was found to increase with LSMO thickness. MS-FMR revealed that the in-plane magnetic anisotropy is dominated by uniaxial contribution for the Pt capped film while the non-capped 10 nm thick LSMO layer shows fourfold anisotropy.

Furthermore, the thickness dependence of effective magnetization revealed that the existence of a second order perpendicular anisotropy term, which is thickness-dependent and of a weak uniaxial interface anisotropy. The Gilbert damping coefficient was found to vary linearly with the inverse of the effective LSMO thickness due to spin pumping leading to relatively low spin mixing conductance of LSMO/Pt interface.

I-Introduction

The emergence of spintronics, where both electron spin and charge are used as information vectors, has triggered the research of new materials where the spin polarization rendered by the spin asymmetry at the Fermi level is one of the most important of their properties. Therefore, spintronic application revives the interest in the perovskite manganites, generally referred to as colossal magnetoresistance materials. La 2/3 Sr 1/3 MnO 3 (LSMO) is an attracting perovskite manganite material because of its low saturation magnetization, its expected spin polarization close to 100% [1] and its low magnetic damping [2], ingredients for low critical current densities in spin transfer torque switching [3]. Damping parameter, governing the magnetization relaxation, is an important technological parameter since it also controls how fast the magnetization reverses. Indeed, depending on the desired application, the damping value should be fixed: low damping is essential for low current densities in spin torque magnetization switching [3] whereas, higher damping values are needed for conventional magnetic recording media such as CoCrPt [4].

Spin pumping [5,6] is an efficient method to generate a spin current and therefore to tune the magnetic damping in ferromagnetic (FM)/normal metal (NM) based-systems via the choice of FM and NM material and their thicknesses. Spin pumping can be considered as the reciprocal effect of the spin current action on the magnetization: spin current can induce magnetization precession due to the spin transfer torque [7,8]. Indeed, the magnetization precession (induced by ferromagnetic resonance for example) generates a spin current at the FM/NM interface by transferring the transverse component of spin angular momentum from the FM layer to NM. This spin current diffuses into the NM layer in the direction normal to the interface and is accompanied by spin angular momentum loss in the FM layer which is an additional damping source of magnetization dynamics. The spin pumping phenomenon is governed by the spin mixing conductance parameter

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This article is protected by copyright. All rights reserved determining the efficiency of this effect at the interface between materials: it determines how much spin current is passed through the interface. Spin pumping in LSMO/Pt system were addressed by Luo et al. [9] and Lee et al. [10]. However, although the LSMO thicknesses were varied by Luo et al., they used conventional ferromagnetic resonance in cavity (X-band), which is limited for the investigation of damping since measurements are made at fixed frequency. In contrast, Lee et al. used a broadband ferromagnetic resonance but the LSMO thickness was fixed to 30 nm and here again the accuracy on spin mixing conductance is questionable. For the precise characterization of spin pumping, broadband technique and variable thicknesses of ferromagnetic layer should be used. Therefore, in this paper, we address the thickness dependence of damping and perpendicular magnetic anisotropy in LSMO/Pt systems. For this, broadband ferromagnetic resonance with microstrip line (MS-FMR) [11], vibrating sample magnetometer (VSM) and x-ray diffraction techniques have been used. We show the presence of a thick magnetic dead layer and that the damping coefficient varies with the inverse of the effective thickness of LSMO films. Moreover, we demonstrated that in contrast to 3d materials in contact with Pt, the perpendicular magnetic anisotropy in LSMO/Pt remains negligible.

II-Samples and experimental techniques

All LSMO films of variable thicknesses (t LSMO = 6, 8, 10 ,20 and 30 nm) were grown on SrTiO 3 (STO) (001) substrates using an advanced Pulsed Laser Deposition (PLD) system with in situ RHEED from TSST company with a laser fluence of 1.70 J/cm². The deposition temperature was kept at 750 °C in a pressure of 0.2 mbar of oxygen with a shot frequency of 1 Hz. After deposition, LSMO films were cooled down to room temperature in a 500 mbar O 2 pressure. Next, the samples were transferred to a DC magnetron sputtering chamber without breaking the vacuum for the deposition of the 7 nm thick Pt layer at room temperature in Ar pressure of 4 × 10 -2 mbar and at a power of 17.6W. Platinum is used here as the normal metal for spin pumping measurements. It is a heavy metal, which is an efficient spin-sink with a short spin diffusion length. A 10 nm thick uncapped LSMO layer grown in the same conditions is used as a reference sample. X-ray

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This article is protected by copyright. All rights reserved diffraction (XRD) combined with vibrating sample magnetometer (VSM) have been used to study the structural and the static magnetic properties of the films. Home-made microstrip ferromagnetic resonance (MS-FMR) was used to probe the magnetization dynamics through the investigation of the magnetic damping, gyromagnetic factor and the magnetic anisotropy [11]. In MS-FMR experiment, the magnetic sample is mounted on a microstrip line which is connected to a Hewlett-Packard microwave generator (83752B), operating in the range 0.01-20 GHz and to a Schottky detector used to measure the transmitted power. For each driven microwave frequency, the sample is swept through the resonance by varying the applied external field (up 2.3 kOe for in-plane or up to 15 kOe for out-of-plane applied magnetic fields). In addition, the external magnetic field is modulated at 170 Hz by small (4 Oe) alternating magnetic field allowing lock-in detection via a lock-in amplifier (Stanford research system SR830). The in-plane and the out-of-plane angular dependences measurements are ensured by rotating the sample via motors: in the plane, the sample is mounted on a rod connected to a motor, which ensures vertical translation before and after each rotation to bring the sample far from and into contact with the fixed microstrip line, respectively.

For out-of-plane measurements, the sample is directly fixed on the microstrip line and the whole set is connected to a motor which ensures the rotation in the gap of the electromagnet. All the measurements presented below were carried out at room temperature.

III-Results and discussion

1-x-ray diffraction

Figure 1 shows the 2θ/ω x-ray diffraction patterns measured for 30 nm thick LSMO sample capped with Pt. The patterns indicate the presence of the (00l) type LSMO and STO substrate peaks. This confirms the expected epitaxial growth of the LSMO layer on the single crystal STO (001) substrate, which is (001)LSMO||(001)STO and [001]LSMO||[001]STO. The LSMO out-ofplane lattice parameter was evaluated to be 3.84 Å. The calculated lattice parameter, which is less than the pseudo-cubic LSMO unit cell parameter, a LSMO =3.867 Å [12], suggests the presence of an in-plane tensile strain in the LSMO thin film. This is because the STO lattice parameter is a STO =

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This article is protected by copyright. All rights reserved 3.905 Å and an in-plane matched growth of the film results in the in-plane elongation of the film lattice, which in turn produces an out-of-plane compression. Moreover, the x-ray pattern presents a Pt (111) peak, originating from the Pt capping layer. To better visualize this peak, we have also performed a 2θ/ω diffractogram, with an out-of-plane 0.4° angle offset (not shown here). This resulted in the reduction of the additional substrate artefact peaks induced by the fact that the Xrays used here are not monochromatic. The growth of LSMO capped with Pt is thus similar to that of the uncapped LSMO [13] and is in agreement with previous investigations [9,10].

2-Static magnetic properties

For the precise determination of perpendicular magnetic anisotropy (PMA) and spin mixing conductance, the magnetization at saturation (M s ) and the eventual magnetic dead layer thickness (t d ) should be first evaluated. These two quantities are straightforwardly determined by VSM technique from the thickness dependence of the areal magnetic moment. Figure 2 shows the LSMO thickness (t LSMO ) dependences of the saturation magnetic moment per unit area (M s ×t LSMO ) for LSMO/ Pt system. M s and t d are determined from the linear fits of these data: the slope gives M s and t d corresponds to the horizontal axis intercept. The magnetic dead layer of 3.4±0.2 nm remains within the range given by Angeloni et al. [14] and Huijben et al. [15] for LSMO/STO systems but it is thicker than the reported values in references [9] (1.7 nm) and [16] (1.4 nm). Although a thickness dependence of the areal magnetic moment is used in [9], t d was determined from the temperature dependence of 7.2 nm thick LSMO layer. The magnetization at saturation M s =323±19 emu/cm 3 is in good agreement to the reported one in [9,15] and is higher than the given one (265 emu/cm 3 ) in [10]. This magnetic dead layer refers to the region at the surface or at the interface with the substrate and capping layer where the ferromagnetic properties of the manganite are greatly diminished or modified. The origin of this magnetic dead layer is still unclear and under debate, making it the subject of several researches over the last few years. It has been suggested that the dead layer likely contains oxygen vacancies which disrupt exchange coupling and hence destroy

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This article is protected by copyright. All rights reserved long range order [17]. In the following, the obtained values of M s (M s =323 ±19 emu/cm 3 ) and t d (t d =3.4±0.2 nm) will be used to calculate all the magnetic parameters presented in this study.

3-Dynamic magnetic properties

In this section, we focus on the investigation of PMA and Gilbert damping versus LSMO thicknesses. In the following, we consider the effective thickness of LSMO (t eff =t LSMO -t d ) defined as the nominal LSMO thickness (t LSMO ) reduced by the magnetic dead layer thickness. In this study, MS-FMR under in-plane and perpendicular-to-plane applied magnetic fields were used. Typical MS-FMR spectra are shown in figure 3a for the LSMO (8 nm)/Pt under in-plane applied magnetic field, where the recorded signal is proportional to the field derivative of the absorbed power as a function of the applied magnetic field. By fitting these experimental spectra using equation ( 1) [18], one can obtain the resonance field and the FMR linewidth which will be exploited in this section.

(

Where P ab is the absorbed power, δ denotes the mixing angle between dispersive and dissipative components, A 0 is the amplitude, A 1 is an offset value, H R is the resonance field and H is the half linewidth at half maximum. Note that the fitted experimental data with equation (1) allows deducing H.

Besides M s and t d , the g value, which determines the gyromagnetic ratio (γ) is also needed, for the precise determination of the Gilbert damping, the spin mixing conductance and the magnetic anisotropy. For this purpose, we first investigated the frequency variations versus the amplitude of the perpendicular to the film applied magnetic field as shown in figure 3b for various LSMO thicknesses. The observed linear dependence of the frequency versus the perpendicular applied field ( H = M =0°) is consistent with equation ( 2) and allows to deduce the gyromagnetic ratio /(2) from the slope of the linear fit of the experimental data (figure 3b).

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(2)
Where f Out is the frequency of the uniform precession mode for out-of-plane applied magnetic field, the effective magnetization out-of-plane, K is the effective perpendicular uniaxial anisotropy constant and  M ( H ) refers to the out-of-plane angle defining the direction of the magnetization (the applied field) and the normal to the film plane:  H =0° corresponds to a magnetic field applied perpendicular to the film plane. We also introduce , where K 2 is second-order perpendicular magnetic anisotropy constant and its inclusion in equations will be justified below.

The inset in figure 3b shows the thickness dependence of the g-factor (Landé factor), determined from the obtained values of /(2) ( .=g×13.97 GHz/T). The g-factor decreases with decreasing thickness, consistent with previous studies in metallic thin films [19,20]. For the larger thickness, the value of the g-factor is 2.11±0.012, which gives an estimation of the bulk value. One should note the slow thickness variation of the g-factor for thick LSMO films (beyond 20 nm) confirming again that the g-factor value of bulk LSMO is around 2.11±0.012. Moreover, the gfactor decreases as the thickness decreases to reach 2.04±0.01, which is close to the free electron value indicating a small spin-orbit interaction [21]. It is worth mentioning that the g-factor is known to depend on composition and is strongly influenced by surface and interface effects, as it depends on the local symmetry [21]. Such interface effects may lead to strong enhancements of the ratio of the orbital to spin angular momentum, which is related to the g-factor. Similar trend was observed in Permalloy [22], which attributed to the orbital motion of electrons not being entirely quenched at surface and interface due to the broken inversion symmetry at interface.

In the following, we employ MS-FMR to investigate both the in-plane and the out-of-plane angular dependences of the resonance field. Figure 4a shows the out-of-plane angular dependence

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This article is protected by copyright. All rights reserved of the resonance field, at 7 GHz microwave driven frequency, of LSMO thin films of various thicknesses with or without Pt capping layer. During the measurements, the samples were rotated by 180° (with the step of 5°) from in-plane direction ( H =±90°) to the normal to the layer plane ( H =0°). Maxima and minima of the FMR resonant field clearly identify the hard and easy axes of magnetization. The angular behavior is dominated by a uniaxial perpendicular effective anisotropy with an in-plane easy axis and will be discussed below. Figure 4b shows the typical in-plane angular dependence of the resonance field, at 5 GHz microwave driven frequency, of LSMO(8 nm)/Pt and 10 nm thick uncapped LSMO layer (reference sample). The small variation of the resonance field as a function of the in-plane angle confirms that the anisotropy fields are small and can be neglected at high applied magnetic fields [out-of-plane FMR measurements, justifying the neglected in-plane anisotropy in equation ( 2)]. This in-plane angular dependence shows that the 10 nm thick uncapped LSMO films exhibits a clear predominant fourfold magnetic anisotropy along the substrate edges ([100] and [010] axes) besides a uniaxial anisotropy along one of the substrate axes. The anisotropy axes are straightforward deduced from minima of the resonance field. It is well known that LSMO films grown on STO(001) present a an in-plane fourfold anisotropy with the easy axes along the [110] and [1  1 0] (at 45° with respect to the substrate edges), which was correlated with the full in-plane epitaxy of LSMO and STO [13,23,24]. In this case, the fourfold anisotropy has been attributed to a magnetocrystalline origin. Moreover, Vila-Fungueirino et al. [25] reported that LSMO films grown on STO(001) present a fourfold magnetocrystalline anisotropy with an easy axis along [110] superimposed to an order of magnitude smaller uniaxial anisotropy. The monoclinically distorted unit cell of LSMO produces an important difference in the magnitude of the orbital overlap along the equivalent [110] easy axis directions, introducing the extra uniaxial anisotropy term [25]. It is worth to mention that the magnetic anisotropy of the LSMO thin films is very sensitive to the symmetry and morphology of the substrate and that lattice strains can induce an additional anisotropy along the direction of tensile strain [26]. The observed uniaxial anisotropy could also be induced by the substrate morphology, i.e., residual substrate steps

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This article is protected by copyright. All rights reserved due to miscut angle [27]. The in-plane angular dependence of the STO/LSMO(8 nm)/Pt system seems to be governed by a uniaxial anisotropy. However, a careful observation of this angular dependence allow us to observe that the behavior is not a pure sinusoidal suggesting the existence of other anisotropy terms. Therefore, for the precise determination of the various contributions to the in-plane anisotropy, the angular behavior of the resonance field has been fitted with equation (3).

(

) 3
Where, f // is the frequency of the uniform precession mode for in-plane applied magnetic field, H u is the uniaxial anisotropy,  M is the in-plane direction of the magnetization,  u and  4 refer respectively to the angle of easy axis direction for the uniaxial and the biaxial anisotropies with the substrate edges. The fourfold in-plane anisotropy field is defined as .

Indeed, the best fits revealed the angular dependence of STO/LSMO(8 nm)/Pt result from the contribution of a fourfold magnetic anisotropy field H 4 =7±0. were reported for LSMO/STO [9,10].

We will now focus on the perpendicular anisotropy. This can be determined through the investigation of the effective magnetization. Therefore, the uniform precession mode frequencies versus the perpendicular and the in-plane applied magnetic fields (figure 4c for a given  H

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This article is protected by copyright. All rights reserved mentioned in the caption) have been fitted using equations ( 2) and (3), respectively. The obtained values of the effective magnetization (M eff// and M eff for in-plane and perpendicular to the plane applied magnetic field, respectively) shown in figure 4d versus the reciprocal LSMO effective thickness are significantly different. This difference suggests the existence of a second-order anisotropy term characterized by the constant K 2 to be added to the conventional uniaxial perpendicular anisotropy term as indicated in equation ( 2). According to equation ( 2) and for inplane applied magnetic field ( H =90°), the contribution of K 2 to the frequency vanishes and the fit of experimental data (figure 4c) lead to 4M eff// . Therefore, measurements under in-plane magnetic field are independent of K 2 . For perpendicular applied magnetic field ( H =0°), the frequency varies linearly versus the applied magnetic field (figure 3b) and the horizontal axis intercept gives . Therefore, the uniaxial and the second order anisotropy perpendicular constants have been deduced by combining the MS-FMR measurements of the resonance frequency versus in-plane (figure 4c) and perpendicular applied field (figure 3b). The accuracy on these values of K 2 has been checked by fitting the out-of-plane angular dependence of the resonance field using equation ( 2) where a good agreement is observed (figure 4a). We should mention that the out-ofplane angular dependence of resonance field (shown in figure 4a) could not be fitted without the use of K 2 . The obtained values of K 2 (shown in the inset of figure 4d) increase (in absolute value)

with the increasing LSMO thickness, suggesting the existence of interface contributions. Note the negative sign of the second order term of the perpendicular anisotropy reinforcing the in-plane easy axis of the magnetization. This second order anisotropy term can arise due to peculiarities of atomic structure at the interface or as a result of non-uniform mechanical stresses existing at interfaces presenting a large crystallographic mismatch [28]. Moreover, B. Dieny et al. have shown analytically that spatial fluctuations of the film thickness can lead to a higher order term if the period of the fluctuations is lower than the exchange length of FM material [28,29]. Furthermore, both M eff// and M eff// decrease linearly with the reciprocal LSMO thickness, suggesting the existence

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This article is protected by copyright. All rights reserved of an interface contribution. We will now focus on the uniaxial perpendicular anisotropy constant K . This latter obeys the relation .Therefore, the linear fit of the thickness dependence of M eff// (shown in figure 4d and extracted from the experimental data of figure 4c: measurements under in-plane applied magnetic field) has been used for the determination of the perpendicular uniaxial surface K s and the volume anisotropy K v constants from the slope and the intercept with the vertical axis, respectively. The obtained value of K s =0.03±0.004 erg/cm 2 is negligible and very low, suggesting that the interface contribution to the perpendicular is insignificant, in contrast to the 3d transition metal/Pt systems (such as Pt/Co [27]). The perpendicular volume constant K v =(-0.57±0.07)×10 5 erg/cm 3 favors the in-plane magnetization and is significantly lower than the reported values [13,10]. Indeed, by taking into account only M eff// , the surface and the volume perpendicular anisotropy constant are underestimated, since the contribution of K 2 to both terms is neglected. We thus, used M eff and we deduced the effective surface (K seff =0.055±0.007 erg/cm 2 ) and volume [K veff =(-2.01±0.035)×10 5 erg/cm 3 ]. Therefore, while the surface perpendicular anisotropy remains very low compared to that of Co/Pt, the effective volume perpendicular constant is in agreement with the reported values (-2.5×10 5 erg/cm 3 [24] and -4×10 5 erg/cm 3 [10]). The main source of this perpendicular anisotropy is generally considered as proceeding from the interfacial strain through the magnetoelastic coupling [31].

We have also investigated the magnetic damping of the LSMO/Pt samples by measuring the FMR linewidth. Figure 5a shows the typical angular dependence of the half wide at half maximum FMR linewidth (H) of the STO/LSMO(10 nm) and STO/LSMO(8 nm)/Pt samples at 5 GHz microwave driven frequency. The pronounced anisotropy of the linewidth is in disagreement with the intrinsic isotropic Gilbert damping and it must be due to additional extrinsic damping mechanisms. In our samples, extrinsic contributions results mainly from two magnon scattering and mosaicity [32]. In fact, for the STO/LSMO(10 nm), H shows the same fourfold symmetry (four maxima) as it is observed for the angular dependence of the resonance field. In the case of the

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This article is protected by copyright. All rights reserved STO/LSMO(8 nm)/Pt, the angular dependence of H shows mainly uniaxial symmetry the with minima directions that correspond to the magnetic anisotropy hard axes. Such behaviour of anisotropy linewidth reflecting the resonance field symmetry is a characteristic of two magnon scattering. This two magnon scattering contribution has been confirmed by the out-of-plane angular dependence of the linewidth (not shown here for sake of simplicity since more theoretical considerations would be required to fit H). Indeed, as we move from in-plane to out-of-plane, H increases drastically before decreasing significantly in the vicinity of the normal direction and reaching its minimum at this perpendicular direction. Furthermore, in our MS-FMR set-up the signal to noise ratio is lower for measurements under perpendicular applied magnetic field, especially for the thinner LSMO films (below 10 nm). Although this ratio ensures the determination of resonance field, it does not warrant precise measurements of the linewidth. We thus limited the measurements with perpendicular applied field to the determination of g-factor and we deduced damping from the in-plane measurements of linewidth (along in-plane applied field direction where H is minimal) since they are more sensitive and faster. This two magnon scatting contribution is correlated to the presence of defects preferentially oriented along specific crystallographic directions, thus leading to an asymmetry. Therefore, angular dependence has been fitted by equation ( 4) [32] (fit parameters are summarized in table I):

where is given by [11]:

here f 0 =M eff , f is the driven frequency, H 0 is the inhomogeneous residual linewidth, α is the Gilbert damping constant,  0 ,  2 (and  2 ) and  4 (and  3 ) are Coefficient (direction) used to characterize the strength of the two magnon contribution. We should mention that the main aim of this paper is to study the LSMO thickness dependence of Gilbert damping and not different Accepted Article relaxation mechanisms. Therefore, for each sample the angular dependence of H has been used to determine the applied field direction giving the minimum of the linewidth ∆H, for which the frequency dependence of ∆H is then measured for the sake of the minimization of the extrinsic contributions. In these conditions, the deduced damping from the frequency dependence of ∆H, using equation ( 4) and the measured gyromagnetic ratio for each sample, is much closer to the Gilbert coefficient. The frequency dependences of ∆H for the various samples are shown in figure 5b for the applied field directions mentioned in the figure caption. The analysis of the angular and frequency dependences of the linewidth has been carried out to deduce the Gilbert damping constants shown in figure 5c versus 1/t eff . It reveals that α increases linearly with 1/t eff thus confirming the interfacial nature of spin pumping in LSMO/Pt. Thus we consider that the total damping is given by α=α LSMO +Δα, where α LSMO is the Gilbert damping constant of the bulk LSMO and Δα is the additional damping introduced by the spin pumping effect [33]. The linear fit of the experimental data of figure 5c allows determining α LSMO and Δα from the vertical intercept and the slope respectively, since [34], where µ B is the Bohr magneton and is the spin mixing conductance of LSMO/Pt interface. The intrinsic damping of bulk LSMO [α LSMO = (3.76±0.09) ×10 -3 ] is slightly higher than the reported one by Lee et al. (1.91×10 -3 ) [10] but it is in good agreement with the obtained value for the reference LSMO(10 nm) layer (3.62±0.08) ×10 -3 .

The obtained spin mixing conductance of the LSMO/Pt interface ( 4.25± 0.3 nm -2 ), using M s =323 emu/cm 3 and g=2.08 (the average value of all samples), is significantly lower than those of Py/Pt (30 nm -2 [35]) and Co/Pt (45 nm -2 [36]). However, it is comparable to those reported for Pt/YIG (6.9 nm -2 [37]) and LSMO/Pt (5.5 nm -2 [10]). For this latter, we should mention that the obtained value has been deduced from the measurements of single LSMO(30 nm)/Pt sample making it less accurate.

Conclusion

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The thickness dependences of the perpendicular magnetic anisotropy and Gilbert damping constant of LSMO/Pt systems have been investigated. The VSM measurements revealed a low magnetization at saturation with a relatively thick magnetic dead layer, probably due to vacancies which disrupt exchange coupling and hence destroy long range order. The FMR investigations showed the existence of second order perpendicular and weak surface anisotropy constants.

Moreover, Pt capping layer enhanced significantly the Gilbert damping leading to moderate spin mixing conductance. Finally we conclude that the low magnetization at saturation combined with the low Gilbert damping make LSMO/Pt good candidate for spintronic devices.

[

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This article is protected by copyright. All rights reserved GHz driving frequency for LSMO(8 nm)/Pt system and the reference 10 nm thick LSMO without Pt capping layer. Symbols refer to the experimental data and solid lines are linear fits using equation ( 4) and parameters of Table I. (b) Frequency dependence of the FMR linewidth (H) for the LSOM/Pt system with various LSMO thicknesses and for the reference 10 nm thick LSMO without Pt capping layer. The magnetic applied field directions are  H 90°, 40 and 60°, respectively for t LSMO =8, 10 and 20 nm. For the reference sample, the measurements were acquired for  H =0°.

Symbols refer to experimental data and solid lines are linear fits using equation ( 4). The applied magnetic field was applied in the direction where H is minimal. (c) Gilbert damping constant as a function of the reciprocal LSMO effective thickness for LSMO/Pt films. Symbols refer to the experimental data and solid line is linear fit.

This work deals with the investigation of perpendicular magnetic anisotropy and spin pumping induced damping in La 2/3 Sr 1/3 MnO 3 (LSMO)/Pt systems. The magnetic damping coefficient varies linearly with the reciprocal LSMO effective thickness leading to relatively low spin mixing conductance of LSMO/Pt interface. Therefore, the low magnetization at saturation combined with the low Gilbert damping make LSMO/Pt good candidate for spintronic devices.

5 1  1 0

 511 Oe [K 4 =(0.57±0.07)×10 3 erg/cm 3 ] with easy axes along[110] and [] superimposed to a higher a uniaxial field [H u =16.75±1 Oe, K u =(2.7±0.29) ×10 3 erg/cm 3 ] along one edge of the STO substrate. For the STO/LSMO (10 nm) reference sample (uncapped layer), the in-plane angular dependence has been fitted with H 4 =16±1 Oe [K 4 =(1.29±0.14)×10 3 erg/cm 3 ] with the easy axes along [100] and equivalent directions and uniaxial anisotropy field of 4±0.3 Oe [K u =(0.64±0.073) ×10 3 erg/cm 3 ]. We should mention that no significant fourfold anisotropy has been detected in all the studied LSMO/Pt and the in-plane anisotropy remains weak and below 16 Oe. Similar trends (uniaxial in-plane dominated anisotropy)

Fig. 1 :Fig. 2 :Fig. 3 :

 123 Fig. 1 : Belmeguenai et al.

Fig. 5 :

 5 Fig. 5 : Belmeguenai et al.

Figure 1 :

 1 Figure captions

Figure 2 :

 2 Figure 2: Thickness dependence of the saturation magnetic moment per unit area for the LSMO/Pt

Figure 3 :

 3 Figure 3: (a) Ferromagnetic resonance spectra (measured at different driven frequencies)

Figure 4 :

 4 Figure 4: (a) Variations of the resonance field versus the out-of-plane angle ( H ), measured at 7

Figure 5 :

 5 Figure 5: (a) Dependence of the FMR half width at half maximum linewidth (H) versus the

  36] L. Zhu, C. D. Ralph and R. A. Buhrman ,Phys. Rev. Lett. 123, 057203 (2019). [37] H. Wang, C. Du, P. C. Hammel and F. Yang, Appl. Phys. Lett. 110, 062402 (2017).

Acknowledgements

This work has been supported by the Conseil regional d'Île-de-France (convention 1763) through the DIM NanoK (BIDUL project).


Bruno Da

Silva Pinho

Stéphane Girardon

Frédéric Bazer-Bachi

Ghislain Bergeot

Samuel Marre

Cyril Aymonier
email: aymonier@icmcb-bordeaux.cnrs.fr

Bruno Pinho

A microfluidic approach for investigating multicomponent system thermodynamics at high pressures and temperatures

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The knowledge associated with phase behavior is of primary interest for various industrial applications. Getting to know whether a complex fluid mixture will be multiphasic or monophasic depending on the operating parameters and the reaction profile is crucial data required by chemical engineers to build up their process, in particular when considering highpressure/high-temperature systems. In relation to phase behaviors, supercritical fluids [1][2][3][4] (pressure and temperature conditions above the critical point -P c , T c ) have attracted much interest because of their unique properties: liquid-like densities and gas-like viscosities and diffusivities, 5 which can be continuously adjusted by small changes in temperature and pressure to fit the process needs.

Traditionally, phase diagrams and critical coordinates are obtained with chromatography techniques 6 and/or high pressure optical cell (HPOC) measurement methods 7,8 using isochoric or dynamic methods. 9 Although these methods can lead to very precise and reliable thermodynamic data, they are also highly time-consuming. These limitations are mainly due to the macroscale volumes that need to reach equilibria (temperature, composition, etc.) before any measurements can be performed. Performing phase behavior analysis in microfluidic devices is emerging and aims at replacing these traditional approaches. Nowadays, phase microscale behavior analyses at high pressures and temperatures are possible because the microfluidic knowledge has evolved towards the microfabrication of devices that can handle such harsh conditions, 10 the development of supercritical microfluidics, 11 the modeling of supercritical microflows 12 and the understanding of pressure drops in microscale networks. 13 All of this knowledge opened space for conducting on-chip phase behavior studies, which require advanced control of the operating parameters (pressure, temperature and composition). Compared to conventional techniques, microfluidic approaches can add huge benefits, providing low reagent consumption, fast screening, low operating times (97% time reduction compared to conventional pressure-volume-temperature (PVT) techniques 14 ) and the ability to implement in situ analysis techniques. 15,16 Furthermore, the ability to work in continuous flow mode allows solving hydro/mass transfer/thermodynamic problems simultaneously through on-chip μPIV, 17 mass transfer analysis 18,19 and PVT diagram construction. 14 The first published approach to study on-chip phase behavior was proposed by Mostowfi et al. 14 In their work, the authors successfully developed a microdevice equipped with fluidic restrictions inside the microchannel responsible for droplet nucleation, assuring a fast equilibrium state after fluid restriction. It was used to investigate binary phase PVT diagrams using integrated membrane-based optical sensors for in situ pressure measurements. Fluid phase behavior was analyzed at equilibrium using flow visualization to determine gas-liquid volume fractions depending on temperature and pressure. These data provide sufficient information to construct binary phase diagrams. More recently, Fisher et al. 19 used the same principle to investigate hydrocarbon equilibrium in oil. Good agreement was obtained when compared to pycnometer flash experiments. In both cases, the microsystem inside pressure was maintained due to high pressure drop channels at the microsystem outlet, meaning the pressure was flow rate dependent. To some extent, this could potentially affect the time to reach phase equilibrium, reducing the contact time between phases at high flow rates, although the authors proposed to use microchannels with a small hydraulic diameter allowing screening an acceptable range of pressure by small changes in flow rates.

In this general context, this paper presents a novel strategy to conduct on-chip multicomponent phase behavior studiesathighpressures(1< P (bar) < 200) and temperatures (300 < T (K) < 500). The developed method is primarily based on (i) bubble and dew point detection through optical characterization and (ii) the use of a so-called dynamic stopflow measurement mode for fast screening of the operating parameters.Theproofofconceptisdemonstrated through the determination of phase diagrams and critical locus curves for benchmark binary (alkanes + CO 2 )andternary (alkanes + CO 2 +H 2 ) mixtures. These mixtures were chosen as model mixtures for they are commonly used in industry, relatively stable under high pressure and high temperature conditions (no reactivity between components) and have existing literature data available for comparison.

The first section of the manuscript briefly discusses the microsystem design and set-up and the general strategy employed. We then expose the obtained experimental results, which are later compared with literature and numerical data calculated with the Peng-Robinson equation of state (PREOS).

Experimental section

Chemicals

Cyclohexane and pentane (both 99.5% purity) were purchased from Sigma-Aldrich® and used as received. Carbon dioxide, hydrogen and propylene were purchased from Air Liquide®.

Microsystem design

We developed a silicon/Pyrex microsystem fabricated using standard lithography/dry etching techniques. 10 The microsystem exhibits rectangular channels with dimensions of 200 μm and 100 μm for width and depth, respectively (i.e. hydraulic diameter of 133 μm). The microchannel has a totallengthof1mwithrounded edges. Theoverall design leads for instance to a pressure drop of 0.8 bar when considering pure cyclohexane flowing at 50 μLmin -1 ,whichwas roughly estimated using a Hagen-Poiseuille-based equation (note that the pressure drops calculated assuming laminar flow in the cylindrical configuration are in good agreement with the experimental measurements):

  P Q d L  128 4  H (1)
where ΔP is the pressure drop (Pa), L the channel length (m), μ the viscosity (Pa s), Q the volumetric flow rate (m 3 s -1 ) and d H the hydraulic diameter (m).

To guarantee mixing quality and fluid temperature, the microsystem has three different zones: 10 inlet/outlet zone, mixing zone and analysis zone (see Fig. 1).

The fluids enter at room temperature in the inlet/outlet zone and are later allowed to mix, while temperature increases from room temperature to the working temperature. Finally, the fluid mixture reaches the operating conditions before entering in the analysis zone, where it is characterized.

In order to study binary and ternary mixtures, the microsystem includes three inlets and two T-junction mixing points (see Fig. 2). The fluid inlets are kept with mitered 90°e lbows with sharp bends in order to enhance fluid mixture by generating vortex at the corners. To study binary mixtures, one of the inlets is plugged. This approach allows validation of the experimental set-up for binary and ternary mixtures separately.

The microsystem was connected to the external set-up using a compression module described elsewhere. 10

Experimental set-up

The general set-up is described in Fig. 2. Briefly, the flow rates are controlled by three high-pressure piston pumps (ISCO® 100DM), while a back pressure regulator (Equilibar® ULF) placed downstream is used to maintain a constant pressure in the microsystem. Finally, temperature is controlled within a ±0.1 K interval by a EuroTherm® 3200. The microdevice is heated with an electrical resistance of 200 W contacting its silicon backside to guarantee a constant sufficient heat transfer at temperatures above 380 K to compensate for convective heat losses. To monitor the temperature within the full set-up, two K-type thermocouples are used to give the temperatures of (i) the fluids in the piston pumps thermostated with a recirculating cooling fluid, (T 1 ), and (ii) the working part of the microsystem (being put in direct contact with the backside of the microsystem), T 2 .Tomonitor the pressure, two pressure transducers are placed upstream and downstream the microsystem. To guarantee the exact same measurement conditions, both pressure sensors were immersed in a constant temperature bath (50/50 wt% ethylene glycol/water) at 300 K. Analyses were conducted within a temperature range of 300 to 500 K and a pressure range of 60 to 150 bar.

A bypass line was also added to run the set-up in dynamic stop-flow mode (see details below).

To construct the thermodynamic diagrams, several movies were recorded with Phantom® Version 9.1 at different capturing velocities. For bubble formation and nucleation, movies were captured at 300 fps (1632 × 1200). For visualizing equilibrium evolutions, movies were captured between 4 and 40 fps (1632 × 1200). The high-speed camera is coupled with a Zeiss Axiovert 200 M inverted microscope equipped with a 5× objective.

Microfluidic strategy to build P-T phase diagrams

Principle: bubble and dew point detection

In order to construct a multicomponent P-T diagram, our approach is based on bubble and dew point on chip optical detection to access the phase envelope. Initially, a fluid mixture with a particular composition is introduced at equilibrium within the microsystem, displaying multiphasic liquid-vapor, i.e., droplets/bubbles are formed inside the microchannel and can be easily observed (see Fig. 3-a-C*). Starting from these initial conditions, the temperature was subjected to variations under isobaric conditions to find the bubble and dew points. This means that the mixture turns to a fully miscible mixture (Fig. 3-a-A*) by crossing the phase envelope in the (P,T) projection of the phase diagram. This process can be viewed as the transition from the 2-phase to the 1-phase region. The bubble point (Fig. 3-a-B*) corresponds to a temperature value (at a given pressure) where the first bubble starts forming in the fluid medium. Similarly, the dew point (Fig. 3-a-D*) corresponds to the temperature (at a given pressure) where the last bubble disappears. After finding these points, the pressure was increased and the process was repeated. This loop method was carried out until the maxcondenbar (MCB) point is found, which is located at the maximum pressure of a P-T envelope (Fig. 3-b). After finding the MCB, the loop method was carried out until the maxcondentherm (MCT) is found, which is the maximum temperature of a P-T envelope (Fig. 3-b). Then, to determine the critical point, it is worth noting that the critical pressure (see Fig. 3-b, left) is located between the MCT and the MCB pressures (P MCT < P c < P MCB ). 20 By definition, the critical point is the junction between bubble and dew curves (see Fig. 3-b). This means that if P < P c there is a dew point and a bubble point; however, for P > P c , we will have either two dew points (lower and upper) or two bubble points (lower and upper). The lower and upper bubble points and dew points are easy to spot in microfluidics, as exemplified in Fig. 4 (see also movies 1 to 4 in the ESI †).

Repeating the proposed method for various compositions and linking the critical points to one another allow determination of the mixture critical locus curve, as exemplified in Fig. 3-c for a binary CO 2 -cyclohexane (CYC) mixture.

The microsystem can be operated in stop-flow (isochoric) or continuous flow mode. In isochoric systems, the pressure is directly controlled by temperature variations, 21 e.g. increasing temperature leads to increasing fluid pressure. These approaches could lack of flexibility when targeting the fast screening of phase behavior. On the contrary, continuous microflow mode allows for fast screening of the operating conditions (temperature, pressure, composition), but leads to poor precision over the system operating pressure due to the induced pressure drop inside the microchannels.

To overcome these limitations, our strategy is to operate the microsystem in a so-called dynamic stop-flow mode (see the next section). In this regime, the fluid is completely stopped (no induced pressure drop), but is not confined to a specific volume, which means that P-T parameters can be changed independently. This ensures an improved precision over the operating conditions, making it ideal to do precise and fast phase behavior studies.

The dynamic stop-flow concept for fast screening

The dynamic stop-flow approach takes advantage of an open systemsimilar to the continuous flow regimebut the fluid is in a no-motion status. The concept developed behind this method is based on hydrodynamic resistance. Flowing through a small hydraulic diameter (d H ) tubing leads to higher hydrodynamic resistance than flowing through a tube with a larger hydraulic diameter (the fluidic resistance varies as d H 4 , see eqn (1)). Therefore, bypassing the microsystem with a large hydraulic diameter tubing (Fig. 5) forces the fluid to select the lower hydrodynamic resistance line. Indeed, by considering the Hagen-Poiseuille equation (eqn (1)) for equal tubing lengths, we can express the flow rate ratio as the inverse of the fluidic resistance ratio and as the hydraulic diameter ratio raised to the power of 4 (eqn (2)):

Q Q R R d d A B system Bypass H Bypass H system             4 (2)
where Q A and Q B are the volumetric flow rates in each section (bypass and microsystem), respectively, R is the fluidic resistance and d H is the hydraulic diameter.

Considering our set-up, the hydraulic diameters are 133 μm and 750 μm for the microsystem and the bypass, respectively. Thus, the flow rate ratio between the bypass and the microsystem line is ~10 3 . In other words, at the T-junction, the fluid will always largely flow preferentially into the bypass line (Q A ≫ Q B ), thus flowing around the microsystem without disturbing the fluid mixture inside it.

In a typical experiment, pure fluids first mix in the desired proportions while flowing through the microsystem until the upstream pressure is stable (Q Total = 50 μL min -1 , bypass valves are closed, meaning Q A = 0 μL min -1 ). This step lasts from 1 to 10 min depending on the overall compressibility of the considered fluid mixture. When the bypass valves are opened, the mixture inside the microsystem is kept in a quasi no-motion state (Q A ~50 μL min -1 , Q B ~0.05 μL min -1 ). In order to prove the concept, a flow of CO 2 + CYC was introduced into the microsystem at a velocity of ~8 × 10 -2 m s -1 . Then, the bypass valves were opened, which led to a small decrease of the upstream pressure sensor, since the pressure drop is smaller in the bypass line, and the fluid starts to be in a quasi no-motion state (ESI † movie 5).

To observe fluid flow inside the microchannel in continuous mode with some details, it is necessary to capture movies with a minimum of 300 fps (Fig. 6-a). On the contrary, in the case of quasi no-motion flow, 4 fps is enough and it is perfectly possible to visualize in detail the size of each bubble (Fig. 6-b).

Each isobaric set of experiments to cover the full range of temperatures typically takes less than 5 minutes (temperature variation is almost instantaneous). When temperature rises inside the system, the fluid will expand and the volume variation will be expelled by the system into the bypass line, avoiding pressure variations inside the microsystem. Changing the P-T conditions destabilizes the stationary regime of the system, inducing slight movements in the fluid Fig. 3 (a) General loop method for building a P-T phase diagram through bubble and dew point detection and phase envelope construction. The bubble and dew points were determined by increasing or decreasing the temperature from the initial conditions to isobaric conditions. The images were captured at 4 fps for a mixture of CO 2 + CYC. (b) Scheme of the critical point based on whether it is located before or after the maxcondenbar (c) General description of a binary mixture critical locus (CO 2 -CYC: type I-a mixture 24 ).

until a new stationary regime is reached. Between stationary states, the bubbles' appearance, growth and disappearance can be easily investigated (Fig. 6 and ESI † movie 6).

In the transition between stationary states the bubbles/ droplets increase or decrease in size, giving opportunity to extend the concept proposed in this paper to study PVT diagrams.

Results and discussion

Set-up reliability and P-T diagrams for binary CO 2 -alkane mixtures

To check the reliability of our approach, two benchmark binary mixtures were considered: CO 2 + cyclohexane and CO 2 + pentane, for which literature data are available, allowing estimation of the set-up performance. Furthermore, the critical locus curve in mixtures of hydrocarbons and CO 2 is commonly of type II from the Scott and Van Konynenburg classification system. 22,23 In other words, these binary fluid mixtures only have a heterogeneous liquid-gas equilibrium and a liquid-gas critical locus curve between the critical points of each pure component. Note, however, that it is also possible to have a liquid-liquid heterogeneous equilibrium. 20 To obtain a P-T phase envelope, the system was first placed under equilibrium conditions. Then, the procedure described previously was followed in order to determine the bubble and dew points. Fig. 7 presents P-T miscibility diagrams obtained for some selected CO 2 + pentane and CO 2 + cyclohexane compositions. The critical point for each composition can be found using the above-described procedure. The evolution of the critical pressure and temperature as a function of mixture composition was then determined based on the critical points obtained from the P-T diagrams; this curve was directly compared with literature and PREOScalculated data (Fig. 8). Details about the PREOS calculations of critical points can be found in the ESI. † Compared to the literature data, the obtained values have an average relative deviation of 2% and a maximum relative deviation of 4%. The main contribution to the deviation value is the composition variation at the system inlet, i.e., small pump flow rate variations change the overall system composition, making it possible to have slight deviations in the critical temperature and pressure. However, the excellent overall agreement of the results with the literature proves that our approach can be used to obtain critical data for binary mixtures.

From binary to ternary/quaternary diagrams

To verify that the concept can be used for more complex mixtures, a quaternary mixture, CO 2 + propylene + propane + H 2 , was chosen as a model mixture. Such a system is representative of a hydrogenation reaction of propylene by hydrogen in CO 2 generating propane. The addition of hydrogen dramatically complicates the system, since this compressible fluid can interact with all three other species.

To simplify the system, we fixed the propylene/propane molar ratio to 93 : 7. We first determine the CO 2 + propylene + propane ternary critical locus curve to obtain a pre-validation step before investigating the quaternary system including hydrogen. The obtained experimental results were later compared with the numerical results obtained through the Peng-Robinson cubic equation of state (Fig. 9).

In the case of CO 2 + propylene + propane, the results are in excellent agreement with the calculated data, having an average relative deviation of 2% for critical temperature and pressure.

Note, however, that the calculated data strongly depend on the binary interaction parameter k ij . In a ternary mixture, three k ij have to be considered; therefore, inaccurate binary interaction parameters strongly influence the PREOS modeling results. The PREOS can give incoherent results when using k ij that are not adapted to the working temperatures. Switching from PREOS to PPR78 can address this limitation by considering temperature-dependent k ij obtained through a group contribution method, as previously reported. 27,28 We later introduce H 2 into the mixture to investigate the quaternary phase diagram (CO 2 + propylene + propane + H 2 ). Using the same microfluidic approach, the P-T phase diagrams were plotted and the critical locus was determined as a function of the molar fraction of CO 2 (Fig. 10).

Given the complexity of such a mixture, modeling is not straightforward. However, our approach can be used to back calculate the various binary interaction parameters k ij (required for the EOS calculations).

Conclusions

A microfluidic-based approach was presented to determine the P-T phase diagrams of multicomponent mixtures. This method couples an on-chip optical detection of both bubble and dew points with a so-called dynamic stop-flow mode for fast screening of the operating parameters (temperature, pressure, composition). This regime was created using a bypass line, making it possible to maintain a constant pressure inside the system while tuning the temperature.

We demonstrate that this strategy can provide accurate thermodynamic data for multicomponent mixtures, which were compared to PREOS-calculated and literature data. Additionally, this microfluidic approach can work very efficiently (typically 5 times faster than conventional HPOC methods), taking advantage of (i) the fast heat and mass transfer, (ii) the easy control of all the set-up operating parameters and (iii) the use of a dynamic stop-flow mode, which can provide all of the advantages of a quasi no-motion fluid in an open system. This ease of operation allows for accessing data that can later feed EOS modeling, in particular through the back calculation of binary interaction coefficients (k ij ). Although the presented method was only used to investigate P-T diagrams, it is also possible to envision building PVT diagrams, taking into account the void fraction, as previously reported. 14 Fig. 10 Critical points for the quaternary CO 2 + propylene + propane + H 2 mixture with a constant 10% molar composition of hydrogen and constant propylene-to-propane molar ratio of 93 : 7. On-chip experimental results.

Fig. 1

 1 Fig. 1 Schematic of the microfluidic device with the three different zones. A, B and C inlets represent generic fluids.

Fig. 2

 2 Fig. 2 Schematic diagram of a multiphasic microfluidic installation with a bypass line.

Fig. 4

 4 Fig. 4 Schematic of the point detection of the different behavior of the medium at P > P C . The videos of each point can be found in the ESI. † The variable ρ represents the density of the fluid.

Fig. 5

 5 Fig. 5 System flow resistance circuit that one fluid undergoes in dynamic stop-flow mode.

Fig. 6

 6 Fig. 6 Volume variation of the bubbles during a temperature increase. The bubbles are trapped into the microsystem, having a molar composition of 24% cyclohexane and 76% CO 2 at P = 138 bar. (a) Images captured at 300 fps using continuous flow mode and (b) images captured at 4 fps using dynamic stop-flow mode.

Fig. 7 P 2 Fig. 8

 728 Fig. 7 P-T miscibility diagrams for binary CO 2 + cyclohexane and CO 2 + pentane mixtures obtained through the microfluidic approach. Bubble points; dew points; Maxcondenbar; Maxcondentherm; Mixture critical points.

Fig. 9

 9 Fig.9Critical points for the ternary CO 2 + propylene + propane mixture (propylene/propane molar ratio fixed at 93 : 7). On-chip experimental results; -PREOS results for CO 2 + propylene + propane (CO 2 + propylene k ij = 0.067; propylene + propane k ij = 0.00830; propane + CO 2 k ij = 0.1241).

Acknowledgements

The authors would like to thank the IFPEN, the ANR (through the SEED "CGSμLab" project and the international "Sustainable Manufacturing" project), the CNES and the Région Aquitaine for financial support.


Solange Vivès

Philippe Bellanger

Stéphane Gorsse

Wei ‡ Changdong

Qiaofu Zhang

Ji-Cheng Zhao

Combinatorial Approach Based on Interdiffusion Experiments for the Design of Thermoelectrics: Application to the Mg 2 (Si,Sn) Alloys

Thermoelectric materials are ranked by their ability to meet a criterion of excellence that is related to their conversion efficiency. This dimensionless figure of merit, zT = (σS 2 T)/κ, measures how well a candidate material can convert heat to electricity where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. This antagonist

combination of properties controlling the performance of thermoelectrics makes it difficult to design these materials. Notwithstanding, it has been established that functional properties are controlled by both composition and microstructural parameters and that refined microstructures promote better thermoelectric properties. (1) This has led to new approaches inspired by metallurgical concepts that rely on the use of thermal treatments to generate and control microstructures that give the desired transport properties. (1)(2)(3)(4)(5) The combination of alloy composition and time and temperature that can be applied in a thermal treatment is nearly infinite; thus, identification of the optimal composition and treatment is very time-consuming using one-composition-at-a-time synthesis and property testing.

Designers need the knowledge of thermodynamics and kinetics, as well as an understanding of the composition-microstructure-property relationships to better identify the most promising materials.

Instead of using the traditional trial and error methodology, our approach involves coupling combinatorial synthesis with localized property measurement techniques. Combinatorial synthesis allows preparation of large variations/libraries of compositions and can give access to a range of microstructural parameters for a systematic study from a single sample. (6) One such implementation is using a diffusion couple to generate a large library of compositions by thermal interdiffusion.

Variation of a given functional property across the composition range can then be obtained using micrometer-scale localized property measurements. (7) Moreover, by tracking the composition changes in one dimension across an interdiffusion zone, it is possible to extract both the equilibrium tie lines and the diffusivities.

To illustrate this combinatorial approach, we have selected the Mg 2 Si-Mg 2 Sn pseudobinary system because it forms extensive solid solutions with promising composition-dependent thermoelectric properties. (8)(9)(10) P a g e | 2

In this work, we sought to evaluate the influence of composition of Mg 2 (Si,Sn) solid solutions on thermal conductivity. The Mg 2 Si/Mg 2 Sn diffusion couple experiments were also conducted to determine the interdiffusion coefficient ( ) and the solubility limit of the solid solutions. Herein, we outline our experimental approach and the conclusions that can be drawn from examination of the composition-phase-properties 2D maps.

Figure 1 shows scanning electron microscopy (SEM) images of the diffusion couple after spark plasma sintering (SPS) bonding of the Mg 2 Si and Mg 2 Sn block halves and the interdiffusion zone developed after annealing at 600 °C for 120 h, along with the corresponding Si and Mg concentration maps.

Figure 1. Backscattered electron image of the diffusion couple after welding (a) and after annealing at 600 °C for 5 days (b). The interdiffusion zone is denoted by the red dotted line in (b). Corresponding Mg with composition profile superimposed (c) and Si (d) composition maps.

In order to avoid Mg loss, annealing is performed in Ta tubes sealed under partial pressure of Ar. The diffusion annealing at 600 °C gave rise to the formation of solid solutions exhibiting a concentration gradient in Si whereas the Mg content is constant across the diffusion couple. The average composition in planes parallel to the original interface throughout the interdiffusion zone is obtained from electron probe microanalysis (EPMA) measurements (Figure 2). The Sn and Si concentration profiles exhibit a step corresponding to the miscibility gap between the two terminal solid solutions, the Sn-rich and Si-rich Mg 2 (Si,Sn). 14)).

During the diffusion process at a constant temperature, the height of the concentration step remains unchanged; however, the width of the interdiffusion zone increases following a parabolic growth law, typical of a diffusion-controlled process (Figure 3a). Since the solubility limit of Si in Mg 2 Sn is relatively high, we calculated the composition-dependent interdiffusion coefficient using the forwardsimulation method. (15,16) Based upon the compositional diffusion coefficient obtained using the Sauer-Freise expression, (17) the composition profile was simulated and compared with experimental measurements to further adjust the diffusivity until a satisfied composition profile is obtained, as

shown in Figure 2a. The perfect overlap between these two composition profiles in Figure 2a demonstrates the validity of the interdiffusion coefficients obtained, as displayed in Figure 3b. The interdiffusion coefficient plotted in Figure 3b describes the diffusion behavior inside the Sn-rich Mg 2 (Si,Sn) solid solution. It can be seen that the change with composition is significant. The diffusivity increases dramatically by adding Si to the Mg 2 Sn phase which may be caused by the increasing density of defects. Then, for Si content above 2 atom %, the interdiffusion coefficient remains almost constant.

Figure 4 shows the thermal conductivity map across the interdiffusion zone, compared with the Si composition map at the same location. The thermal conductivity map was obtained using timedomain thermoreflectance (TDTR) with the delay time fixed at t = 300 ps. Measurement details could be referenced to our previous works in refs [18][19][20] . Two micrometer-sized indents were marked on the sample surface at the measurement location before mapping, to spatially correlate with the composition profiles. Specific heat (C) values of the solid solutions were estimated using Kopp-Neumann law. In order to interpret the large number of data provided all at once by the combinatorial experiment, we can describe the composition dependency of the measured thermal conductivity using a parametric model. In semiconductors, the heat is mainly carried by phonons. The treatment of the lattice thermal conductivity is based on the kinetic theory by considering frequency dependent phonon mean free path:

where C is the heat capacity, v is the group velocity, ω is the frequency, Λ eff is the effective mean free path, and λ is the polarization of the phonons. This expression, obtained from gas kinetic arguments,

shows the importance of the heat capacity of the heat carriers, their velocity, and their mean free path. The heat capacity is estimated using the Debye model. In this continuum approximation, the crystal behaves as a box containing a phonon gas.

Phonon scattering processes arise from different mechanisms and can be represented by mean free paths. We have considered two contributions: umklapp processes and alloy disordering. Assuming that this scattering processes are independent, the scattering probabilities are additive: Λ eff -1 = Λ um -1 + Λ ss -1 . Phonon mean free path for umklapp scattering is well described by Han: (31) Λ um -1 = A 1 Tω 2 e - A 2 /T /ν, where A 1 and A 2 are two adjustable parameters. The atomic defect scattering due to solute atoms can be accounted by the effective medium approach using a Rayleigh-like expression as described by Klemens: (32) where δ is the radius of the solute atom in the host lattice and Γ s characterizes the scattering cross section that is correlated to the mass defect scattering and strain scattering due to solute atoms: (33) where M denotes the molar weight of the Mg 2 (Si,Sn) solid solution, ΔM is the mass difference, Δδ is the radius difference between the mixing atoms, respectively, and ϵ is an adjustable parameter.

It can be seen Figure 5 that the calculated room temperature thermal conductivity fit well the experimental measurements. The values of the estimated parameters are summarized in Table 1. The important decrease of the thermal conductivity with small addition of solute, either Si in Mg 2 Sn or Sn in Mg 2 Si, denotes the strong effect of the scattering mechanisms due to solute atoms.

In conclusion, the use of a combinatorial method is an efficient way to evaluate the solubility limits, the diffusivities, and the influence of composition on the thermal conductivity of solid solutions. We were able to identify the Mg 2 Si 0.4 Sn 0.6 alloy composition as the optimal solute content corresponding for the minimum value of the thermal conductivity. A simple model, incorporating the scattering contribution of solute atoms (mass defect scattering and strain scattering), allows prediction of the room temperature thermal conductivity of Mg 2 (Si,Sn) thermoelectric materials as a function of composition. The present work merges several techniques and analysis to get key parameters required for the design of thermoelectrics and materials for thermal management. It represents an endeavor toward the development of general methodology based on an experimental high throughput approach that intends to help navigating through the immense realm of all possible material compositions. The combined optimization of both thermopower and thermal conductivity can be regarded as the next step. This requires diffusion length compatible with the spatial resolution of the Seebeck microprobe currently achievable (around 10 μm). If thermal treatments are not sufficient to produce the desired scale, thermomechanical treatments, such as rolling and/or forging, could be considered in order to expand the interface region.

Figure 2

 2 shows the relationship between the phase diagram and the concentration profiles. It is then possible to extract the single tie-line from the composition analysis: Mg 0.67 Si 0.02 Sn 0.31 and Mg 0.67 Si 0.15 Sn 0.18 . In other words, the solubility limits of the Mg 2 Si x Sn 1-x solid solutions are x = 0.06 and x = 0.45. By reproducing these experiments at different temperatures, one can deduce the miscibility gap in the pseudobinary system Mg 2 Sn-Mg 2 Si. These results confirm the experimental assessment by Muntyanu and co-workers.(11)

Figure 2 .

 2 Figure 2. (a) Composition profile measured across the interdiffusion zone in the diffusion couple Mg 2 Sn/Mg 2 Si after annealing at 600 °C for 5 days; circles for experimental value and solid line refers to simulation. (b) Pseudobinary Mg 2 Sn-Mg 2 Si phase diagram showing the miscibility gap assessed from the analysis of the composition profiles in comparison with assessment from previous works (Muntyanu et al., (11) Kozlov et al., (12) Jung et al., (13) Nikitin and Zaitsev (14)).

Figure 3 .

 3 Figure 3. (a) Thickness of the interdiffusion zone measured as a function of the annealing time at 600 °C. (b) Interdiffusion coefficient as a function of Si concentration in Mg 2 (Si,Sn).

P a g e | 4

 4

Figure 4 .Figure 5 .

 45 Figure 4. Si composition map and the corresponding thermal conductivity map of the region delimited by the white line in the interdiffusion zone of a Mg 2 Sn/Mg 2 Si couple (Figure3d). The composition profile and the thermal conductivity data were aligned and accurately referenced to the dead centers of the premarked micrometer-sized indents.

Figure 5 .

 5 Figure 5. Composition dependent thermal conductivity values of Mg 2 (Si,Sn) solid solutions obtained from the diffusion couple and calculated thermal conductivity (red line).

Acknowledgment

This research work was financially supported by the CEA (Grant No. C14123). The work at The Ohio State University was supported by the National Science Foundation under Grant NSF-DMR-1237577.

Author Information

 Corresponding Author :

- -The authors declare no competing financial interest.


Weber Marie-Laure
email: marie-laure.weber@umontpellier.fr

Fallery Bernard
email: bernard.fallery@umontpellier.fr

Enlightening the pre-implementation of a digital tool and anticipate individual change

Keywords:

    Enlightening the pre-implementation of a digital tool and anticipate individual change by analyzing the activity. An illus-tration through the activity of French teachers-researchers.

Introduction

The digital transformation of organizations is bringing changes both at the level of work and at the level of individuals. The activity of the actors is modified and becomes a source of potential learning, by its dynamic of encounter between an individual and a context [START_REF] Rabardel | Instrument, activité et développement du pouvoir d'agir[END_REF]. In a context of digital transformation of work, the actor is facing a redefinition of his activity. The great theories of learning [START_REF] Piaget | Réussir et comprendre[END_REF][START_REF] Vygotski | Pensée et langage[END_REF] allow to characterize the impact of a new tool on the individual learning of the actor and on the determinants of behavior [START_REF] Cooper | Paradigm shifts in designed instruction: From behaviorism to cognitivism to constructivism[END_REF]. to answer to a new situation mediated by an instrument, the individual will tend to an individual cognitive progress [START_REF] Piaget | Réussir et comprendre[END_REF] by joining a community of work (Engestrӧm, 1999) where social relations are particularly vectors of understanding the action [START_REF] Vygotski | Pensée et langage[END_REF]. The theory of activity [START_REF] Engestrӧm | Activity theory and the social construction of knowledge: A story of four umpires[END_REF] can shed light on understanding the interactions between actors and technology in a unified vision of technology and activity [START_REF] Karanasios | Toward a unified view of technology and activity: the contribution of activity theory to information systems research[END_REF]. Drawing on the psychology of development, cognitive ergonomics and professional didactics, activity analysis focuses on studying work activity for the personal and cognitive development of individuals.

In recent years, the digital transformation of work is part of the strategic impetus of the Ministry of Higher Education, Research and Innovation1, following the law of July 22, 2013, which places digital technology as the "lever of a university in motion". Four ambitions are put forward, the first of which positions digital technology to serve students' success and the second which distinguishes digital as a tool for the renovation of teaching practices.

Technopedagogical devices transform the conditions of exercise of the profession of teacherresearcher [START_REF] Peraya | Professionnalisation et développement professionnel des enseignants universitaires: une question d'actualité. Distances et médiations des savoirs[END_REF]. The use of new technologies to teach leads to changes in the activity of teacher-researchers and questions the resulting individual impact and commitment to institutional requirements.

The purpose of this paper is to examine the individual impact of a combination of policy changes and technological and digital breakthroughs. More specifically, it attempts to answer the following question: how to respond to ministerial and institutional impulses requirements in terms of digital strategy and the adaptation of pedagogical processes?

We propose to clarify, using the principles of activity analysis, the pre-implementation of a new digital tool and to anticipate individual change. The first part will first cross two visions of the digital transformation of work with regard to individual learning and then the determinants of behavior. In an activity analysis approach, the second part will define a conceptual framework. Finally, the third part will offer an illustration by the activity of teacherresearchers.

Crossing two visions of the digital transformation of work: individual learning and behavioral determinants

The evolution of activities related to digital is important and affects all forms of organizations, potentially leading to a transformation of actors. The impact is twofold: it affects individual learning (1.1) and the determinants of behavior (1.2).

Impact of digital work transformation on individual learning: ownership and adaptation competency

If we are interested in the effects of digital transformation on the actors of an organization, a first vision of the impact of this transformation can be approached from the point of view of individual learning and appropriation [START_REF] De Vaujany | Les grandes approches théoriques du système d'information[END_REF] and in the user-technology relationship, the individual impact can be understood from three points of view: technocentric, sociotechnical or sociomaterial [START_REF] Orlikowski | Sociomaterial practices: Exploring technology at work[END_REF][START_REF] Orlikowski | The sociomateriality of organisational life: considering technology in management research[END_REF]. The technocentric vision sees the technical object as structuring and modifying factor of the human behavior (Amalberti, 2001a[START_REF] Amalberti | La maîtrise des situations dynamiques[END_REF]. Ownership is then a process by which individuals incorporate advanced technologies into their work practice, an adaptive process from tool to use [START_REF] De Vaujany | Les grandes approches théoriques du système d'information[END_REF]. The socio-technical vision conceives an interdependence of technical and social systems as well as a joint optimization of the two dimensions and their relation [START_REF] Plane | Théorie et management des organisations-3e ed[END_REF]. The appropriation is then a taking possession of the technology by the user, an identification of the technology [START_REF] Barcenilla | L'acceptabilité des nouvelles technologies: quelles relations avec l'ergonomie, l'utilisabilité et l'expérience utilisateur?[END_REF][START_REF] Gaskin | Psychological ownership and the individual appropriation of technology[END_REF]. The sociomaterial vision privileges neither the human nor the material nor their relation, but takes into account a reciprocal entanglement [START_REF] Orlikowski | The sociomateriality of organisational life: considering technology in management research[END_REF]. Ownership is then a combination of managerial flexibility that changes behaviors supporting an idea of innovation [START_REF] Alter | L'innovation ordinaire[END_REF][START_REF] Daniellou | L'ergonomie dans la conduite de projets de conception de systèmes de travail[END_REF][START_REF] Daniellou | Activity-oriented approaches to ergonomics: some traditions and communities[END_REF] ; and instrumental flexibility that turns the technical tool [START_REF] Akrich | Les utilisateurs, acteurs de l'innovation[END_REF][START_REF] Bourmaud | De l'analyse des usages à la conception des artefacts: Le développement des instruments[END_REF].

By placing ourselves in this sociomaterial vision, we can conceive the appropriation of a new digital tool for the user such as the transition from an artifact to an instrument [START_REF] Rabardel | Les hommes et les technologies; approche cognitive des instruments contemporains[END_REF].

The change of technology is then interpreted as a moment of learning by instrumental genesis and affordance [START_REF] Gibson | The ecological approach to visual perception[END_REF] on the condition of taking the instrument-operator or actorobject relationship as an entanglement where each contributes to changing the other, transforming itself [START_REF] Bourmaud | De l'analyse des usages à la conception des artefacts: Le développement des instruments[END_REF][START_REF] Rabardel | Les hommes et les technologies; approche cognitive des instruments contemporains[END_REF]. The importance of reflexivity comes into play in this dynamic of learning and skills.

The implementation of a new digital tool in the professional environment then took place potential development situations [START_REF] Mayen | Des situations potentielles de dévelopment[END_REF], professional situations where the operator transforms his repertoire of skills to adapt to a new professional condition. Moreover, for [START_REF] Pastré | Dynamique et métamorphose des compétences professionnelles[END_REF], The dynamics and metamorphosis of professional skills follow the evolution of work and we see a shift from an execution skill to an adaptive skill. The skill lies in the ability to adapt [START_REF] Pastré | La didactique professionnelle[END_REF]. By always going further and keeping the idea of sociomatériel entanglement, we can also analyze the skills necessary to know how to adapt and this even beyond the individual, to favor an organization capable [START_REF] Falzon | Pour une ergonomie constructive[END_REF][START_REF] Mollo | Pratiques réflexives et développement des individus, des collectifs et des organisations[END_REF].

Impact of digital work transformation on the determinants of behavior: from intention to capability

The impact of the digital transformation of work can also be approached according to a second vision, that of behavior and especially the intention to behave. In cognitive psychology, we consider that mental representations and perceptions influence the intention to behave and then the behavior.

So the theory of reasoned action [START_REF] Ajzen | The theory of planned behavior[END_REF] characterizes the intention as preceding the behavior. This intention is determined by factors internal to the individual (expectation and perceived effects) coupled with external factors (influence of social norms). For [START_REF] Vroom | Work and motivation. new york: John willey \& sons. Inc. VroomWork and Motivation[END_REF] or [START_REF] Bergeron | Un cadre théorique pour l'étude de la relation entre la participation et la motivation au travail[END_REF], the individual is motivated by his expectations and the chances of attaining them. Expectations theory determines a level of motivation by the equation VIE (Valencia Instrumentality Expectation) = (Am I able?) x (What am I going to get?) x (What value for the benefits obtained?).

In a more sociocognitive approach, Bandura & Locke (1977[START_REF] Rogalski | Y at-il un pilote dans la classe?: une analyse de l'activité de l'enseignant comme gestion d'un environnement dynamique ouvert[END_REF] emphasize the importance of individual factors of intent that lead to behavior. The behavior towards a new technology is then no longer guided solely by the expected results but also by the individual perception of its capacity. The sense of self-efficacy then acts as a determinant of commitment.

This representation of competence for the individual as a vector of action can be deepened by the capacity-based approach. The central point of this theory rests on the idea of the ability of individuals to be able to do something -the power to be and the power to do [START_REF] Sen | The idea of justice[END_REF]. This theory links the notions of freedom and autonomy with the capacity to act. Capability then highlights the act in situation and mobilizable skills of the individual, as well as the constraints and opportunities they have to act [START_REF] Corteel | Capacités et développement professionnel. Formation emploi[END_REF] and thus slip into the notion of enabling environments to implement behaviors expected by the organization [START_REF] Falzon | Pour une ergonomie constructive[END_REF].

It is in this rich and dense theoretical context that we propose to synthesize a conceptual framework through the analysis of the activity.

An approach by analyzing the activity: definition of a conceptual framework

From the work of Vygotski (1934[START_REF] Vygotski | Pensée et langage[END_REF], the theory of activity [START_REF] Engeström | Activity theory and individual and social transformation[END_REF](Engeström, , 2000(Engeström, , 2011) ) focuses on characterizing a conceptual framework for studying human practices as a process of development. It presents a set of tools to report on the activity in its complexity. It establishes a heuristic model that makes intelligible the components of human activity mediated by artifacts in a defined context and the conduct of joint collective activities. Human activity is modeled as a system that associates individual and community because "he mobilization of the activity makes it possible to define at a time T the components of the activity (the elements of the systemic model), to grasp its interrelations as well as the emerging contradictions, with a view to the development by expansion of the committed systems. [...] It invites to a study of the activity being done" (Durand & Barbier, 2017, p. 499). In this system, it is therefore important to analyze the interrelationships between the activity system itself and the system of activity included in its organization, the theory of activity suggesting that the interaction results from technical and psychological tools. that actors use to interact with their environment [START_REF] Engestrӧm | Activity theory and the social construction of knowledge: A story of four umpires[END_REF].

In this general theoretical framework and by integrating the concepts of Information Systems (IS) and Human Machine Interface (IHM), the analysis of the activity can then be defined as "a coherent system of internal mental processes, external behaviors and motivational processes that are combined and directed to achieve conscious goals" (Bourguin et al., 2005, p. 7). The activity is approached as an intermediate concept between the study of the social system (context) and the behavior (individual) to study the mediation by the tool. The approach of the activity can also be understood from different angles depending on the field of practice, including professional didactics, constructive ergonomics, or enaction. These fields of practice are neither totally similar nor fully diversified. The basic conceptual bases overlap partially, under different semantics, the notions intertwine and complement each other. From the action and based on the theory of activity, these visions are differentiated only by their polarization but all "interested in joint transformations of activities, topics in and through their activities, and organizations" (Durand & Barbier, 2017, p. 3). For exemple:

• Professional didactics [START_REF] Pastré | La didactique professionnelle[END_REF] focuses on the analysis of the work for the personal and cognitive development of the agents, with a view of the development established from the point of view of the activity (in this case the professional activity)

and from the point of view of the development subject (more precisely the development of his skills).

• Constructive ergonomics [START_REF] Falzon | Pour une ergonomie constructive[END_REF] Its objective is the development of individuals and organizations through the construction of the operator in interaction with the world and action on the world to understand or transform it. The activity of the subjects is the engine of transformation, the competence becomes a vector of health and performance.

• The enaction [START_REF] Weick | Enacted sensemaking in crisis situations [1[END_REF] addresses the activity as self-referential where the individual is an actor structurally coupled to the environment, possessing a prereflexive consciousness. For [START_REF] Durand | The challenges of activity analysis for training objectives[END_REF], world and actor are a "fact-emerged", responding to the principle of autopoiesis, that is to say the property of a system to selfproduce (adapt) to its environment to maintain a state of equilibrium .

We propose to rely on the model of the double regulation of the activity [START_REF] Leplat | Regards sur l'activité en situation de travail: contribution à la psychologie ergonomique[END_REF] where "one speaks of regulation, in general, when the recovery A 'of an action A is modified by the results of this one, thus of a effect in return of the results of A on its new unfolding A' " (Piaget, 1975, p. 23). The activity is then defined as a coupling between the situation and the subject, which produces results on the situation and effects on the subject which modify themselves the same situation and initial subject. The activity is then analyzed as a perpetual loop where the subjects transform the activity and transform themselves because of the activity (Figure 1).

Illustration by analyzing the activity of research professors

A qualitative study was conducted by semi-structured interviews with 11 lecturers-researchers from French universities (Table 1). The number of interviews was stopped according to the principle of data saturation [START_REF] Thiétart | Méthodes de recherche en management-4ème édition[END_REF]. Saturation was reached at the 8th interview. After transcription, the content analysis [START_REF] Bardin | L'analyse de contenu[END_REF] is thematic and lexical [START_REF] Fallery | Quatre approches pour l'analyse de données textuelles: lexicale, linguistique, cognitive, thématique[END_REF]. The main objective was to highlight the factors of the activity which influence the actors in the realization of this one. The profession of teacher-researcher (EC) is synthesized into two main categories of missions: teaching and research. We will focus here only on the teaching activity.

Table 1 -Characteristics of the sample

In a first step and in order to obtain a more general characterization of the speech, we realized a lexical analysis with the Alceste 2017 software. This analysis distinguishes 4 classes of speech for 480 textual units of analysis and 72% of the explained variance. The 4 classes update a first representation of the concerns of the teacher-researchers and extract several oppositions (figure 3). Class 1 has a student-centered discourse, Class 2 a discourse centered on pedagogical transformation, Class 3 a discourse centered on the method and vision of teaching, and Class 4 a discourse centered on skills and traits. of personality. We propose the following main results (Table 2) that are illuminated by the literature.

Determinants of activity

The effects of a transformation of activity How to anticipate the digital transformation of work?

The teaching activity is a source of individual LEARNING.

Transformation perceived positively when the teacherresearcher is VOLUNTARY in the transformation.

Taking into account the importance of the subject actor in the digital transformation of work, the teacher-researcher must be ACTOR of the transformation. [START_REF] Falzon | Construire l'activité. Performances Humaines \& Techniques[END_REF] The subject's compelling SKILLS and personality traits influence the vision of the activity and its implementation.

Transformation perceived positively when the skills mobilized for the new activity have already been acquired: the feeling of AUTOEFFICIENCY is necessary.

Taking into account the internal and external vision of the competency that influences behavior, the teacherresearcher must feel CAPABLE in the digital transformation of work. [START_REF] Bandura | Self-efficacy: toward a unifying theory of behavioral change[END_REF][START_REF] Nagels | Construire l'auto-efficacité par l'analyse de l'activité en formation des cadres et dirigeants de la santé publique[END_REF][START_REF] Sen | The idea of justice[END_REF] Social and institutional SUPPORT has a sustained influence on the activity.

Transformation positively perceived by a COLLECTIVE PEDAGOGICAL INNOVATION.

Taking into account the importance of social and institutional influence and an empowering organization, the teacherresearcher must feel ACCOMPANIED. [START_REF] Falzon | Ergonomics, knowledge development and the design of enabling environments[END_REF][START_REF] Falzon | Pour une ergonomie constructive[END_REF] The activity plays a role in the WELL-BEING at work and the perceived PERFORMANCE of teacher-researchers.

Transformation perceived positively when it is a vector of individual LEARNING.

Taking into account the close link between health and cognition, the teacherresearcher must perceive his EVOLUTION. [START_REF] Montmollin | Compétences, charge mentale, stress: peut-on parler de santé" cognitive[END_REF]

Table 2 -Determinants of activity: transformation and anticipation

Based on the determinants of activity defined by the teacher-researchers, it is thus possible to understand the perceived effects of the transformation of the activity and to anticipate the impacts of the digital transformation of work. Ultimately, the teacher-researcher in his teaching activity sees the digital transformation of work as positive when he perceives himself as a capable actor, accompanied in his individual learning. As conceived by the principle of double regulation, the subject transforms activity and is transformed by activity.

In pre-implementation of a new digital pedagogy tool, a strong reflection on these important determinants could positively influence the transformation of the activity. Through the analysis of the activity, we proposed to highlight a conceptual framework integrating the individual learning and the determinants of the behavior.

Conclusion

In this research, we have tried to understand how the analysis of activity can be used as a resource to anticipate the impact of work transformation. The study of the literature on the effects of digital mutations makes it possible to highlight two visions. The first focuses on individual learning, from the concept of ownership to adaptation. The second vision is more oriented towards the determinants of behavior, integrating the notions of intention and capability. With the aim of crossing these two visions of the digital transformation of work, we presented the contributions of the analysis of the activity and proposed a conceptual framework.

From a study of the analysis of the activity of teacher-researchers we defined the determinants inherent to their work situation. These highlighted factors are then those on which any transformation of the activity would have an effect. Thus, a transformation of the activity of the teacher-researchers would have an individual impact on the development of skills, on the feeling of well-being at work and performance. The digital transformation would be associated with a personal desire for pedagogical innovation and change of teaching method, accompanied by a necessary institutional and collective support. In this illustration, we wanted to think about how taking into account the key determinants of an activity, defined by the actors themselves, could make it possible to anticipate the digital transformation of the work from the preimplementation of a new tool.

Our research has limits that form as many perspectives. This qualitative analysis is an entry point for the analysis of teacher-researcher activity and the individual impact of the implementation of a new digital tool. These first results are intended to enrich a process model for a quantitative study and a study of the intra-individual subjectivity of the transformation of skills. However, the richness of the speech of the teacher-researchers engages a first reflection.

It reinforces the conceptual framework and enriches the literature by proposing to cross the determinants of behavior and individual learning through the analysis of the activity; the preimplementation can thus be considered in a complex analysis. In French higher education, ministerial and institutional strategies are incentives and analyzing the impact of their impulses in the pre-implementation phase could help to characterize the factors of individual commitment.

Figure 1 -

 1 Figure 1 -Model of the double regulation of the activity[START_REF] Leplat | Regards sur l'activité en situation de travail: contribution à la psychologie ergonomique[END_REF][START_REF] Rogalski | Y at-il un pilote dans la classe?: une analyse de l'activité de l'enseignant comme gestion d'un environnement dynamique ouvert[END_REF]

Figure 2 -

 2 Figure 2 -Conceptual framework of activity analysis

Figure 3 -

 3 Figure 3 -Lexical analysis: 4 classes of speech

Figure 2 -

 2 Figure 2 -Thematic mapping of EC teaching activity

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