domain: chim sde

text:
Mohamed K Guediri

Derradji Chebli
email: derradji_chebli@yahoo.fr

Abdallah Bouguettoucha
email: abdallah.bouguettoucha@univ-setif.dz

Riadh Bourzami
email: riadh_bourzami@hotmail.com

Abdeltif Amrane
email: abdeltif.amrane@univ-rennes1.fr

Novel Fe 2 TiO 5 /reduced graphene oxide heterojunction photocatalyst with improved adsorption capacity and visible light photoactivity: experimental and DFT approach

Keywords: Fe 2 TiO 5, reduced graphene oxide, adsorption, photocatalysis, DFT, pollutants elimination

The design of high efficiency materials is a major challenge for the degradation of organic pollutants. In this work, type II p-n heterojunction photocatalyst Fe 2 TiO 5 /rGO, with enhanced performance, was successfully prepared through simple process. The Fe 2 TiO 5 /rGO composites were prepared by hosting several amounts of reduced Graphene Oxide (rGO) into pseudobrookite nanocrystals (Fe 2 TiO 5 ) which were priorly synthesized by a solid-state reaction. The morphology and the properties of the as-prepared A c c e p t e d M a n u s c r i p t composites were characterized trough different techniques. The fixation of rGO sheets on Fe 2 TiO 5 was proved using the X-Ray Diffraction analysis (XRD). The results for the Scanning Electron Microscope (SEM) analysis showed a good mixing of rGO with Fe 2 TiO 5 . The X-Ray Fluorescence (XRF) confirmed the purity of the pristine Fe 2 TiO 5 .

The Dynamic Light Scattering (DLS) illustrated a strong tendency to aggregation.

Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) analysis was performed to characterize the electronic aspect as the gap and the Urbach energies.

Finally, computational Density Functional Theory DFT calculations were carried out to confirm the experimental results. The adsorptive and photoactivity of Fe 2 TiO 5 /rGO heterojunction photocatalysts were evaluated by methylene blue (MB) degradation under visible light irradiation. The highest MB degradation rate was achieved for Fe 2 TiO 5 /rGO 10% photocatalyst with the highest value of the elimination rate.

Introduction

Water pollution is caused by various categories of toxic chemical and biological materials, such as organic dyes which are recognized as common pollutants that threaten human health and the environment [START_REF] Chen | Synthesis of Ag-loaded Sb2WO6 microsphere with enhanced photocatalytic ability for organic dyes degradations under different light irradiations[END_REF]. Often, several techniques are used to treat several types of pollutants on the same chain; depending on the quality of water, each method requires a distinct reuse or disposal action plan.

Biological, chemical and physical techniques of water remediation are frequently implemented [START_REF] Ambashta | Water purification using magnetic assistance: a review[END_REF]. The adsorption is the most applied method for the elimination of coloring agents due to its efficiency and ease of use.

However, the adsorption process has certain limitations, such as the inability to remove certain elements and the production of secondary wastes which require an additional treatment [START_REF] Mekhalif | Effect of acid and alkali treatments of a forest waste, Pinus brutia cones, on adsorption efficiency of methyl green[END_REF]). To avoid these drawbacks, the advanced oxidation processes (AOP) have been rapidly expanded and widely considered. Photocatalysis is one of the most effective AOPs in the degradation of organic compounds, the reduction of metal cations or the mixture of the two, which is characterized by low consumption of energy, moderate cost, and strong efficiency of pollutant degradation and mineralization [START_REF] Elhalil | Enhanced photocatalytic degradation of caffeine as a model pharmaceutical pollutant by Ag-ZnO-Al2O3 nanocomposite[END_REF]. Adsorption accompanied with the photocatalytic process seems therefore the most appropriate technique for the treatment of water, air and soil [START_REF] Alves | Nanophotocatalysts for Fuel Production[END_REF][START_REF] Inamuddin | Nanophotocatalysis and environmental applications: energy conversion and chemical transformations Jassby D[END_REF]). The photocatalysis is based on the production of non-selective Reactive Oxidizing Species (ROS) such as hydroxyl OH • , A c c e p t e d M a n u s c r i p t superoxide O 2

•-and H 2 O 2 , that allow the oxidation of many organic pollutants. The implementation of Semiconductor materials is of great interest in this process. When the semiconductor is activated by photons, electron-hole pairs are generated by exciting the electrons from the Valence Band (VB) to the Conductive Band (CB), and the migration of the photogenerated electron-holes to the semiconductor surface produces ROS through redox water reactions (Tong et al. 2016). However, the rapid recombination of the photogenerated electron-hole pairs limits the applications of photocatalysis systems in industry, and improving the life time are the major deals of the scientific research [START_REF] Li | Graphene-based heterojunction photocatalysts[END_REF]. Moreover, for implementation in industry, a photocatalyst must fulfill the following conditions: wide absorption in the visible solar spectrum, high charge transport properties; it should also exhibits a high surface/volume ratio, a structural stability (thermal and aqueous) and should be nontoxic, abundant, and cheap (Bassi et al. 2014).

The pseudobrookite Fe 2 TiO 5 material is an n-type semiconductor (Bassi et al. 2014;Waqas et al. 2017a) and has attracted a growing interest owing to its notable many prospective applications [START_REF] Lv | Fe2TiO5-incorporated hematite with surface P-modification for high-efficiency solar water splitting[END_REF]). It has an atomic structure matching that of TiO 2 and a band gap energy of about 2.0 to 2.2 eV (depending on the synthesis method). Moreover, the minimal energy of CB of Fe 2 TiO 5 is around -0.7 eV and that of TiO 2 is around -0.5 eV; those values lead to a quick charge transfer under light illumination (Waqas et al. 2017a). So, it was investigated as a photocatalyst for use in the visible domain (Bassi et al. 2016;Waqas et al. 2017b;[START_REF] Lv | Fe2TiO5-incorporated hematite with surface P-modification for high-efficiency solar water splitting[END_REF]. In parallel, Fe 2 TiO 5 also drew attentions due to its unique magnetic properties [START_REF] Enhessari | Structural, optical and magnetic properties of the Fe2TiO5 nanopowders[END_REF], anisotropic spin-glass behavior at low temperature (Tholence et al. 1986) and conductive properties [START_REF] Gao | Characterisation of Fe2TiO5 nanocrystallites synthesised via homogeneous precipitation[END_REF]. Thus, an alternative potential approach for enhancement of water oxidation consists to develop heterojunctions. In this way,

A c c e p t e d M a n u s c r i p t heterojunction between Fe 2 TiO 5 and other semiconductor was studied previously [START_REF] Li | Fabrication of FeVO4/Fe2TiO5 composite catalyst and photocatalytic removal of norfloxacin[END_REF][START_REF] Lin | Electrodeposited Fe2TiO5 nanostructures for photoelectrochemical oxidation of water[END_REF]Waqas et al. 2017a).

In concrete terms, graphene is used thermodynamically in the building of heterojunction photocatalysts susceptible to wide spectrum or in the doping of wide bandgap semiconductors (Zhang et al. 2011;Xie et al. 2013;[START_REF] Li | Graphene-based heterojunction photocatalysts[END_REF]. Owing to high efficiency charge transfer and high-quality heterojunction interfaces and extremely reactive catalyst surfaces (Zhang et al. 2011), graphene is also used kinetically to enhance the efficiency of photocatalysis owing to the high adsorption capacity of organic / inorganic contaminants [START_REF] Pu | Interfacial charge carrier dynamics of cuprous oxide-reduced graphene oxide (Cu2O-rGO) nanoheterostructures and their related visible-light-driven photocatalysis[END_REF]. For those arguments, the combination of graphene with various semiconductors was suggested as a promising research route. For example, the formation of p-n heterojunctions between KTaO 3 nanocubes and rGO (reduced Graphene Oxide) sheets leads to KTaO 3 /rGO composites, which have shown a significant improvement in the photocatalytic performance of phenol degradation under visible light irradiation that can be attributed to the role of graphene photosensitizer (Bajorowicz et al. 2015). A second example can be given by LaFeO 3 /rGO nanocomposites, which showed to be an effective photocatalyst for the degradation of methyl orange dye under visible light irradiation [START_REF] Kumar | Facile Preparation of LaFeO 3/rGO Nanocomposites with Enhanced Visible Light Photocatalytic Activity[END_REF]). Methylene blue (MB) (Sigma Aldrich) was used as target molecule for the photocatalytic studies. All chemicals were of analytical reagent level and were used as obtained without further purification.

Characterization techniques

X-Ray Diffraction XRD patterns were carried out in the range from 2θ = 4° to 85°, with an angular step about 0.0167° and 6.985 s per step, using a Siemens D5000 Patrica LA960, equipped with capillary cell. The photocatalytic studies performed using the Shimadzu 1700 UV-Vis spectrophotometer to determine the concentration of MB; the spectra were recorded between 200 and 800 nm and the kinetic curves were plotted for absorption at λ max = 664 nm of the MB.

Preparation methods

Preparation of Graphene Oxide (GO)

Graphene Oxide (GO) was synthesized according to the modified Hummers method (Dhinesh [START_REF] Kumar | Facile Preparation of LaFeO 3/rGO Nanocomposites with Enhanced Visible Light Photocatalytic Activity[END_REF][START_REF] Atout | Enhanced photocatalytic degradation of methylene blue: Preparation of TiO2/reduced graphene oxide nanocomposites by direct sol-gel and hydrothermal methods[END_REF]. In a typical synthesis, 5 g of powder graphite and 2.5 g of potassium nitrate K(NO 3 ) were added to 115 mL of 98% sulfuric acid H 2 SO 4 and mixed with constant stirring; the mixture temperature was kept below 10 °C using an ice bath. 15 g of potassium permanganate KMnO 4 was then slowly added to the suspension and the stirring persisted for 2 hours. At the end, the temperature was increased to 35 °C and the mixture was kept at this temperature for another 45 minutes. Additionally, 230 mL of distilled water was added to the paste, the temperature was increased to 98 °C and the reaction was retained for 45 minutes.

Finally, the resulted suspension was further diluted with addition of 350 mL of warm hours at 120 °C. The resulting product was washed four times with distilled water and ethanol using a centrifuge (6000 rpm for 5 minutes) and dried in an oven at 60 °C for 24 hours.

Kinetics of adsorption

The adsorption kinetics represent an important step in the study of an adsorption process, allowing to determine the time needed to achieve the adsorption equilibrium (Chebli et al. 2015). The procedure was carried out in the dark and at room temperature as follows: 0.25 g/l of Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15%) were dispersed in a 200 ml solution of MB at a concentration of 10 mg/L, the mixture was rigorously stirred. To follow the evolution of the adsorbed quantity (Q ads ), samples were taken at time

× 100

Eq.1

Where R is the percentage of MB fixed on the adsorbent, C 0 is the initial concentration of the MB solution (mg/L) and Ce is the residual concentration of the MB solution at equilibrium (mg/L).

Equilibrium modeling

The adsorption dynamics allow to determine the residence time of the adsorbed molecules on the interface of the solid solution. Among the several kinetic models implemented for this purpose (Chebli et al. 2015;Bouguettoucha et al. 2016;[START_REF] Guediri | Molecular dynamic simulation and DFT computational studies on the adsorption performances of methylene blue in aqueous solutions by orange peel-modified phosphoric acid[END_REF], three models were considered in this work: pseudo-first-order, pseudosecond order and pseudo n th order. The correlation coefficient R 2 allows to estimate the conformity of the experimental outcomes with those of the modeling.

Pseudo First Order model (PFO)

The PFO kinetic model can be related to the adsorption at the solid/liquid interface;

this model is based on the adsorption capacity (Chebli et al. 2015;Bouguettoucha et al. 2016;[START_REF] Guediri | Molecular dynamic simulation and DFT computational studies on the adsorption performances of methylene blue in aqueous solutions by orange peel-modified phosphoric acid[END_REF]) and described by the Eq.2:

𝑑𝑑𝑄𝑄 𝑡𝑡 𝑑𝑑𝑑𝑑 = 𝑘𝑘 1 (𝑄𝑄 𝑒𝑒 -𝑄𝑄 𝑑𝑑 ) Eq.2
Where Q e (mg/g) and Q t (mg/g) refer to the quantity of dye adsorbed at equilibrium and at t (min) respectively, k 1 (min -1 ) is the constant of equilibrium rate.

A c c e p t e d M a n u s c r i p t

The solution of the differential equation Eq.2 which took into consideration the boundary conditions t = 0 to t = t and Q t = 0 to Q t gives Eq.3:

𝑄𝑄 𝑑𝑑 = 𝑄𝑄 𝑒𝑒 (1 -𝑒𝑒 -𝐾𝐾 1 𝑑𝑑 ) Eq.3

Pseudo Second Order model (PSO)

The PSO model is applicable to a wider time interval, usually the entire adsorption process (Chebli et al. 2015;Bouguettoucha et al. 2016;[START_REF] Guediri | Molecular dynamic simulation and DFT computational studies on the adsorption performances of methylene blue in aqueous solutions by orange peel-modified phosphoric acid[END_REF]); this model is governed by Eq.4:

𝑄𝑄 𝑑𝑑 = � 𝑘𝑘 2 𝑄𝑄 𝑒𝑒 2 𝑑𝑑 𝑘𝑘 2 𝑄𝑄 𝑒𝑒 𝑑𝑑+1 � Eq.4
Where k 2 (g.mg -1 min -1 ) is the equilibrium rate constant of the PSO model.

Pseudo n th Order (Pn th O)

The general equation without pre-set order to a fixed value can be written as Eq.5 (Chebli et al. 2015;Bouguettoucha et al. 2016;[START_REF] Guediri | Molecular dynamic simulation and DFT computational studies on the adsorption performances of methylene blue in aqueous solutions by orange peel-modified phosphoric acid[END_REF]:

𝑑𝑑𝑄𝑄 𝑡𝑡 𝑑𝑑𝑑𝑑 = 𝐾𝐾 𝑛𝑛 (𝑄𝑄 𝑒𝑒 -𝑄𝑄 𝑑𝑑 ) 𝑛𝑛 Eq.5
The solution of the differential equation Eq.2 which took into consideration the boundary conditions gives Eq.6:

𝑄𝑄 𝑑𝑑 = 𝑄𝑄 𝑒𝑒 (1 - 1 �1+(𝑛𝑛-1)𝐾𝐾 𝑛𝑛 𝑑𝑑𝑄𝑄 𝑒𝑒 (𝑛𝑛-1) � 1 𝑛𝑛-1 )
Eq.6

Adsorption mechanism

Boyd and Webber Model (pore-diffusion model)

A c c e p t e d M a n u s c r i p t

Currently, the models of Boyd and Webber are widely used to study the adsorption mechanism [START_REF] Hameed | Malachite green adsorption by rattan sawdust: Isotherm, kinetic and mechanism modeling[END_REF]Viegas et al. 2014). The Boyd's model determines if the primary resistance to mass transfer is in the thin film (boundary layer)

surrounding the adsorbent particle or in the diffusion resistance within the pores. This model is described by the following relation:

𝐹𝐹 = 𝑄𝑄 𝑡𝑡 𝑄𝑄 𝑒𝑒 = 1 - 6 𝜋𝜋 2 � 1 𝑛𝑛 2 𝑒𝑒𝑒𝑒𝑒𝑒(-𝑛𝑛 2 𝐵𝐵𝐵𝐵) ∞ 𝑛𝑛=1 Eq.7
Where F is a fractional equilibrium at different times, t, and Bt is a function of F.

It is not possible to calculate B values for each adsorbed fraction from Eq.7. The following approximations were obtained by implementing the Fourier transformation and then integration:

for F > 0.85 𝐵𝐵𝐵𝐵 = -0,4977 -𝐿𝐿𝑛𝑛(1 -𝐹𝐹) Eq.8

for F < 0.85 𝐵𝐵𝐵𝐵 = (√𝜋𝜋 -�𝜋𝜋 -(

𝜋𝜋 2 𝐹𝐹 3
) ) 2 Eq.9

They could be used to forecast the mechanism of adsorption using Eq.6 and Eq.7. This can be accomplished by plotting Bt against time. If the plot is linear and goes through the origin, then the rate of mass transfer is governed by the pore diffusion [START_REF] Dural | Methylene blue adsorption on activated carbon prepared from Posidonia oceanica (L.) dead leaves: Kinetics and equilibrium studies[END_REF]. If the plot is nonlinear or linear but does not pass through the origin, the adsorption rate is controlled by film-diffusion or chemical reaction.

On the other hand, the pore-diffusion model of Webber is obtained from the second law of diffusion of Fick. The parameter for pore diffusion, k i (mg/g min 0.5 ) is described by: 𝑄𝑄 𝑑𝑑 = 𝑘𝑘 𝑖𝑖 . 𝐵𝐵 0,5 Eq.12 A c c e p t e d M a n u s c r i p t

Where Q t is the adsorbed quantity (mg/g) at a given time t. From Eq.12, if porediffusion is the rate-limiting step, then a Q t plot against t 0.5 must offer a straight line with a slope equal to k i and an intercept equivalent to zero.

Photocatalytic tests

The photocatalytic experiments were performed using a batch photoreactor at room temperature. A visible LED spotlight (LED Projector 50W Ultra-fin SMD -IP66) with luminous flow = 4000 lm, placed at 30 mm above an open-air double-walled beaker of 200 ml and a cooling water jacket which maintains the reaction temperature at (20 ±

2) C with exposed surface to LED light of 32.17 cm 2 . Due to its simple accessibility and its wide industrial use, MB was selected as the model molecule. The procedure was performed as follows: 0.25 g/l mg of Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15%) were dispersed in 200 ml of aqueous solution of MB (10 mg/L). Firstly, the solution was stirred in the dark for 120 min to achieve the adsorption-desorption equilibrium. After 120 minutes, the LED lamp was switched on and each 20 minutes, 1 ml of the suspension was removed (dilutions ×5) and centrifuged instantly to separate the catalyst from the solution, and then, the solution was analyzed by UV-Vis spectroscopy. All photocatalytic experiments were conducted in duplicate to guarantee photocatalyst photostability.

Computational Details

The aim of the DFT calculation was to explore the impact of electronic corrections on the electronic structure characteristics of the pseudobrookite Fe 2 TiO 5 , and then compare the theoretical results with the experimental ones. The calculations are performed within the framework of the local density approximation (GGA) and the CA-PZ [START_REF] Jin | Electronic structures and optic properties of Fe2TiO5 using LSDA+ U approach[END_REF]. In this work, the same parameters were also considered for the GGA method: plane wave-based cut-off energy was set at 1000 eV and k-points grid sampling were set at 7*7*3 for the Brillouin zone. For the parameters of convergence, maximum force tolerance was set at 0.03 eV/nm, maximum stress component was set at 0.05 GPa and total energy tolerance at 1.0*10-5 eV/atom.

Results and discussion

Structural analysis

Figure 1 shows the XRD patterns of Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15% (96-200-2303) as respectively (0 2 0), (1 1 0), (0 2 3), (1 3 0), (1 1 3), (0 2 4), (0 4 2), (0 4 3), (2 0 0), (0 0 6), (1 5 2) and (1 5 3)

Bragg's planes of the Fe 2 TiO 5 pseudobrookite structure, in an orthorhombic system. It can be seen that more x is important more the pattern appears noisy, indicating the presence of amorphous phase in the rGO sheets. Moreover, no new peak is observed for Fe 2 TiO 5 /rGO 5% in comparison with Fe 2 TiO 5 , proving that for small ratio of graphene the crystalline structure is conserved. Above x = 5%, some new peaks appear and are more intense when x increases, showing a partial transition of the Furthermore, the diffraction peaks shift to the small angles when x increases, which means that the crystal is stressed when the rGO sheets are inserted on the surface of Fe 2 TiO 5 . The characteristic crystalline parameters (a, b and c) are illustrated in Table 1, the average grain seize (D), micro-constraint (𝜀𝜀%) and dislocation density (𝛿𝛿), are calculated using the relations (i), (ii) [START_REF] Langford | Scherrer after sixty years: A survey and some new results in the determination of crystallite size[END_REF]Sugapriya et al. 2013), (iii) and (iv) (Yousaf et al. 2019) respectively:

(i) 2d hkl sinθ = nλand d ℎ𝑘𝑘𝑘𝑘 = 1 � h 2 a 2 + k 2 𝑏𝑏 2 + 𝑙𝑙 2 𝑐𝑐 2 (ii) D = Kλ β cos θ (iii) 𝜀𝜀% = β 4 tan θ (iv) 𝛿𝛿 = 1 𝐷𝐷 2
Where: d hkl is the interreticular spacing, K = 0.94, λ = 1.5406 Å is Kα1 X-ray wavelength, β is the width at half-height and θ is the angle of diffraction.

It is clearly seen that the fixation of graphene sheets on Fe 2 TiO 5 at the microscopic scale increases the lattice parameters, evidently accompanied with an increase of the micro-constraints and at the macroscopic scale reduces significantly the grain seize and induces strong structural defects estimated by an increase of the dislocation density. In comparison with XRD results, for a high ratio x, the defects reduce partially the crystalline structure of Fe 2 TiO 5 .

XRF analysis

A c c e p t e d M a n u s c r i p t

In view of confirming the purity of the synthetized Fe 2 TiO 5 , optical spectroscopy was investigated; XRF analysis proves the presence of O, Fe and Ti elements with different weight percent 37.95%, 43.2% and 18.85% (wt.%), respectively (Figure 2). These results are close to the theoretical weight percent given by an ideal unit cell of pseudobrokite Fe 2 TiO 5 , O (33.40 %), Fe (46.62 %) and Ti (19.98%), confirming the high purity of the compound. The empiric formula can be written as Fe 1.85 Ti 0.95 O 5.66 , and is considered thereafter.

SEM images

Figure 3 shows SEM images of Fe 2 TiO 5 /rGO x composite; the top images are captured at strong zoom (x20000) and illustrate that the nanocrystals have different sizes with elongated rhombohedric forms. Bottom images are carried out with weak zoom (x500)

and prove that the Fe 2 TiO 5 nanocrystals are uniformly scattered on the graphene sheets. These observations confirm the formation of the heterojunction of Fe 2 TiO 5 /rGO x .

DLS analysis

The adsorption and the photocatalytic activities are influenced by the external surface of the nanoparticules and their surface state; and the effective surface exposed to the external medium is the surface of the aggregated particles. Furthermore, the aggregation of transition metal oxides in aqueous solution is well known and wellstudied (Jassby et al. 2012;[START_REF] Melcher | Influence of TiO2 agglomerate and aggregate sizes on photocatalytic activity[END_REF]. For this purpose, the sample Fe 2 TiO 5 was characterized by DLS. Figure 4 shows the hydrodynamic diameter of the aggregated nanoparticles in distilled water suspension, and the experimental data was fitted with a Gaussian function.

A c c e p t e d M a n u s c r i p t

The fitted curve indicates the formation of different size of aggregated particles; the average seize value of hydrodynamic diameter is about 752.4 μm and the width at half height of the fitted curve is about 333.4 μm, showing a strong tendency to aggregate of the Fe 2 TiO 5 particles. The experimental data also shows a shoulder at low values of the hydrodynamic diameter, which can be related to unaggregated particles.

Electronic structure and optical properties

Band structure

For the theoretical calculation of the band structure, both LSDA and GGA were used and the results of the band gaps are compared to the experimental values (Table 2).

The figure 5 4) is about 1.81eV [START_REF] Jin | Electronic structures and optic properties of Fe2TiO5 using LSDA+ U approach[END_REF]; while the gap value calculated by GGA is about 1.88 eV, which is the closest to the experimental value.

UV-Vis-DRS

For the experimental quantification of the electronic properties, namely the gap energy and the Urbach energy, UV-Vis-DRS spectroscopy is a suitable tool. The spectra shown in Figure 6 (b) correspond to Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15%). The lines prove that the composites are transparent in the visible range beyond the wavelength λ = 600 nm, and show also a strong and large absorption band in the UV and in the visible near the UV regions, most likely due to transitions from the VB to the CB. This band is confirmed by the theoretical calculation using GGA calculation and the obtained theoretical spectrum is shown in Figure 6 (a). The bump around 550 nm can be attributed to the spin-flip d-d transition [START_REF] Marusak | Optical absorption spectrum of hematite, αFe2O3 near IR to UV[END_REF]Sherman and Waite 1985); this weak band is observed as a shoulder on the theoretical spectrum. To quantify E g , the absorption coefficients 'F(R)' must be deduced from the reflectance curves 'R', according to the Kubelka-Munk formula (Simmons 1975;Burgeth and Kisch 2002;[START_REF] Kortüm | Reflectance Spectroscopy: Principles, Methods, Applications[END_REF] :

𝐹𝐹(R) = (1-R) 2 2R
Eq.13

The band gap 'E g ' is estimated through Tauc plot on the linear part of (α.hυ) n ∝ (hυ-E g ), where n = 1/2 for indirect band gap (insert of Figure 6), and n = 2 for direct gap;

these optical transitions are respectively due to transitions from O2p to Fe3d orbitals and from O2p to Ti3d orbitals. The evaluated band gap energies are summarized in table 2.

A c c e p t e d M a n u s c r i p t

The Urbach energy E u is calculated by the relation Eq.14 ([CSL STYLE ERROR:

reference with no printed form.]), where E 0 and α 0 are constants relative to the studied compound:

F(R) = α 0 exp( hυ-E 0 E u )
Eq.14 'E U ' is estimated through the plot of ln[F(R)]∝hυ and it is evaluated from the slope of the linear part: ln[F(R)]∝ (hυ-E 0 ) (insert of Figure 6). Table 2 sums up the values.

E U characterizes the change in the electronic band structure caused by different origins (vacant or interstitial sites, lattice strain, dislocation…). These various origins generate additional states within the band gap close to HOMO and LUMO levels, and can be seen as tails of CB and VB, E U is roughly equal to the average width of tails [START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles[END_REF]. From the table 2, the deposit of rGO sheets on Fe 2 TiO 5 induces an increase of the Urbach energy. Both XRD and UV-Vis-DRS analyses confirm therefore that the deposit of rGO induces stress and defects within the Fe 2 TiO 5 nanocrystals.

Moreover, the decrease of E g by adding rGO sheets shifts the Fermi level. The shift of Fermi level strengthens the migration of photogenerated electrons and represses the electron-hole recombination, which let expect an amelioration of the photocatalytic activity.

Kinetic of adsorption

Figure 7 shows time-courses of experimental and fitted kinetic curves of MB adsorption. It is well seen that the equilibrium time was about 120 minutes.

Furthermore, due to the high specific surface area of rGO sheets, the adsorption of MB The constant rate K, the theoretical maximum adsorbed quantities Q e and the correlation coefficients R 2 of fit, given by modeling using PFO, PSO and Pn th O models are included in the Table 3.

From In an attempt to determine the rate controlling step involved in the adsorption of MB on the synthesized composites, the kinetic data were further analyzed using the Boyd and Webber models. Figure 8 (a) Shows Boyd plots for the first 70 min for MB adsorption. The plots indicate a linear section, which do not cross the origin at the beginning of adsorption. This implies that adsorption does not follow Eq.8 and Eq.9 and therefore does not regulates the adsorption rate during the initial period. This demonstrates that during this period, chemical reaction or film diffusion regulates the adsorption rate. is therefore verified that the first linear segment represents film-diffusion (or chemical reaction).

Photocatalytic activity study

The overall elimination of BM is shown in figure 9. We see that BM elimination increases with increasing rGO content for Fe 2 TiO 5 /rGO x composite (with x = 0, 5, 10 and 15 wt.%) by weight with a yield of R = 31.39% to 42.19% respectively. This elimination is due to the strong adsorption of BM by the rGO following the increase of the interfacial surface developed by the increase of the rGO content in the composite Fe 2 TiO 5 /rGO x . This can be clarified in two ways: (i) in dark, by the adsorption phenomena principally of MB on rGO sheets due to its height specific surface, (ii) after saturation of adsorption and under visible light, electron-hole pairs generated within Fe 2 TiO 5 can oxidize MB via photocatalysis and also, tend to be transferred to graphene sheets and works as the receiver, extending the lifetime of the charge carrier. It is worth noting that the concentration of MB shows no noticeable difference in the absence of Fe2TiO5 / rGOx, indicating that MB cannot be degraded by photolysis.

Therefore, any change in the MB concentration can be attributed to the photocatalytic process. The photocatalytic part of the kinetic curves was fitted with an exponential function, leading to the following values of the rate constants K*10 -4 = 1.61, 5.61, 15.10, and 4.91 min -1 for the Fe 2 TiO 5 /rGO x composite (with x = 0, 5, 10 and 15 wt.%) respectively. The heterojunction composite having the ratio x = 10% presents a weak modification of the crystalline structure and it has the highest value of the rate constant. This result can be explained by lower access to the photocatalytic sites of Fe 2 TiO 5 owing to the over-crowding of rGO sheets when the composite heterojunction has more than 10% of x [START_REF] Atout | Enhanced photocatalytic degradation of methylene blue: Preparation of TiO2/reduced graphene oxide nanocomposites by direct sol-gel and hydrothermal methods[END_REF]). In addition, other factors are also known as the fundamental operating parameters in heterogeneous photocatalysis such as the initial pollutant concentration, catalyst load, pH of the solution, and the intensity of light, which can affect the effectiveness of the photocatalytic process, and it's been discussed in detail by [START_REF] Favier | REMOVAL OF AN ORGANIC REFRACTORY COMPOUND BY PHOTOCATALYSIS IN BATCH REACTOR -KINETIC STUDIES[END_REF][START_REF] Favier | Advanced oxidation process for the removal of chlorinated phenols in aqueous suspensions[END_REF]Ounnar et al. 2016b, a). The effect of these factors on the photocatalytic degradation rate of most organic compounds is evident according to last works.

Proposed mechanism

A preliminary mechanism for the photocatalytic process is proposed based on the above results and illustrated in figure 10. Upon LED light excitation, electron-hole pairs generate within the semiconductor Fe 2 TiO 5 , these photogenerated electrons tend to be transferred to graphene sheets, graphene accepts the electrons generated during photocatalysis and has worked beneficially a the receiver, This extends the lifetime of the charge carrier and is then scavenged by dissolved oxygen, thus facilitating the separation of the hole-electron. A c c e p t e d M a n u s c r i p t

Conclusions

The Fe 2 TiO 5 nanocrystals were successfully synthesized and were used with rGO to form heterojunction composites. The XRD analysis proved that the Fe 2 TiO 5 nanocrystals have a pseudobrookite crystalline structure; this structure is conserved after the deposition of rGO, although the crystalline seize decreases. Furthermore, SEM images proved the formation of the heterojunction and the good mixing of rGO with Fe 2 TiO 5 . The experimental gap value was given by the UV-Vis-DRS spectroscopy and was found to be about 2.06 eV, while the gap value given by DFT was about 1.81 eV and 1.88 using LSDA+U and GGA calculations respectively; the gap value was reduced when the quantity of rGO increased.

Moreover, the adsorption kinetic curves showed that the amount of MB adsorbed increased with the increase of the rGO amount. The fit of the kinetic curves proved that the pseudo n th order model is adequate, and the modeling of the experimental results

  A c c e p t e d M a n u s c r i p t (LDA+U) by using the Cambridge Serial Total Energy Package (CASTEP) based on the plane-wave pseudopotential DFT methods with the Perdew-Burke-Ernzerhof (PBE) approximation for the exchange correlation functional. A previous paper used the local spin density approximation Hubbard U (LSDA++U (U = 4)) approach with

  s c r i p t pseudobrokite crystalline phase; these new peaks are related to the reduction of Fe 2 TiO 5 during the fixation of rGO sheets, leading to precursors of Fe 2 O 3 and TiO 2 .

  shows the band structures calculated by GGA of the Fe 2 TiO 5 compounds at the equilibrium lattice parameters along the highest symmetry direction in the Brillouin zone. The separation of the bands exists, and the Fermi level can be located at the band gap, which indicates the semi-conductivity character of Fe 2 TiO 5 . The maximum VB and minimum CB are not located at the same Brillouin area point, indicating that the gap is indirect. Moreover, the Fe 2 TiO 5 compound with pseudobrookite crystalline phase is a paramagnet semiconductor; its magnetic moment is principally centered on the Fe (III) ions, with less contribution of the Ti and O ions that are magnetized via super-exchange interactions. The effective magnetic moment of Fe 2 TiO 5 is determined and found to be 3.80 μ B /Fe[START_REF] Jin | Electronic structures and optic properties of Fe2TiO5 using LSDA+ U approach[END_REF]. This magnetic moment of the semiconductor split the levels of the band structure, forming two possible band structures (i) alpha electronic stat channel and (ii) beta electronic stat channel. The value of the band gap for the alpha channel calculated by LSDA (

  (from Q ads = 2.68 mg/g to Q ads = 16.26 mg/g) for increasing values of x.

Figure 8

 8 Figure 8 (b) shows a pore diffusion plots of MB adsorption obtained by the Webber

Furthermore

  photoexcited electrons on the rGO surface to promote the formation of • O 2 -and H 2 O 2 for MB

  oxide is a mixture of hybridized carbon atoms Sp2 and Sp3 with very high oxygen bonding on hybrid carbon Sp3. Oxygen atoms have a higher electronegativity than carbon atoms and thus graphene oxide becomes a p-type semiconductor material where its conduction band is the orbital antibonding π* and its valence band is basically the orbital Op2. By removing most of the oxygen-containing functional groups from the GO surface using the hydrothermal route, the reduced graphene oxide was obtained. However, the p-type behavior of the rGO was caused by oxygen remaining on the carbon surface. A p-n junction composite photocatalyst can be obtained when combining rGO sheets with n-type Fe 2 TiO 5 semiconductors. There are some advantages of forming p-n heterostructures, such as (a) more effective charge separation; (b) fast charge transfer to the catalyst; (c) longer charge carrier life; and (d) separation of locally incompatible nano-space reduction and oxidation reactions.As a result, the enhancement of the photocatalytic activity of Fe 2 TiO 5 /rGO composites under visible light could be ascribed to the formation of p-n heterojunctions between Fe 2 TiO 5 and rGO sheets. It should be noted that the electronic interactions and charge balance between graphene and semiconductor result in a shift in the Fermi level and decrease the semiconductor's CB potential (Figure10) E gap = 2.06 eV to E gap = 1.95 eV. Thus, the negative shift in Fe 2 TiO 5 /rGO Fermi level and the high migration of photoinduced electrons efficiency effectively suppress the recombination of the charge, resulting in increased photocatalytic activity.

Figure 1 :Figure 2 :

 12 Figures CaptionFigure1: XRD patterns of Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15%), normalized by the highest

Figure 3 :

 3 Figure 3: SEM images, top images strong zoom x20000 and bottom images weak zoom x500

Figure 4 :

 4 Figure 4: Hydrodynamic diameter of Fe 2 TiO 5 in H 2 O suspension.

Figure 5 :

 5 Figure 5: Energy band structures of alpha (blue line) and beta spin channel (red line) of

Figure 6 :

 6 Figure 6: UV-Visible spectra of Fe 2 TiO 5 /rGO x (x=0, 5, 10 and 15%). The insert figure:

Figure 7 :

 7 Figure 7: Adsorption kinetic curves of BM on Fe 2 TiO 5 /rGO x composites (C catalyst = 0.25 g/l

Figure 8 :

 8 Figure 8: Adsorption of MB on Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15%) (a) Boyd plots (b)

Figure 9 :Figure 10 :

 910 Figure 9: Photodegradation of BM dye (C catalyst = 0.25 g/l, C MB = 10 mg/l, and Vol solution =

.3.2. Preparation of Fe 2 TiO 5 nanocrystal using solid state reaction

	M 2.1Equimolar quantities of Fe 2 O 3 and TiO 2 were placed as a lubricant in 50 ml of acetone. The solution was put for 1 h in an ultrasonic bath. The powder was dried at 80 °C and placed in a crucible and air-heated at 900 °C for 24 hours (Huerta-Flores et al. 2015). a n u s c r i p t 2.1
	distilled water, and then treated with 15 mL of H 2 O 2 (30%) solution to reduce the residual permanganate; the addition of H 2 O 2 resulted in yellow color, indicating high level of oxidation and the manganese dioxide swing to colorless soluble manganese sulfate. The suspension was washed five times with 5% HCl aqueous solution using a centrifuge (6000 rpm for 5 minutes) until the complete removal of SO 4 -2 . The removal of SO 4 -2 was detected by addition of barium chloride, where the presence of sulfate ion induced a white precipitate when barium chloride was added to the supernatant (Emiru A c c e p t e d
	and Ayele 2017). The mixture was then repeatedly centrifuged and washed with de-

.3.3. Preparation of Fe 2 TiO 5 /rGO x photocatalysts

	The preparation of Fe 2 TiO 5 /rGO x photocatalysts with different weight ratios 5, 10 and
	15 wt.% of rGO to Fe 2 TiO 5 were prepared via hydrothermal method (Bajorowicz et
	al.). 200 mg of Fe 2 TiO 5 powder and appropriate mass percent of GO were dispersed in
	solution of H 2 O (40 mL) and ethanol (20 mL); the obtained mixture was treated for 2
	hours in an ultrasonic bath and stirred for another 2 hours to obtain homogeneous
	suspension. The suspension was transferred to Teflon sealed autoclave and kept for 12

  Table 3, the adsorbed quantities Q ads calculated by the Pseudo-First Order (PFO) model were fairly far from the experimental values; while the Pseudo-Second Order (PSO) model led to closer Q ads values. To assess for the accuracy of the fit, the correlation coefficients R 2 is the most appropriate factor; it was found between [0.875-0.959] and [0.935-0.990] for PFO, PSO models; while the Pn th O model led to better results, with R 2 values in the range [0.973-0.998], and the order for this last model was between 0.119 and 0.425.

Table 1 :

 1 Lattice parameters, grain seizes, microconstraints and dislocation densities of Fe 2 TiO 5 /rGO x (x = 0, 5, 10 and 15%)

		Fe 2 TiO 5 /rGO x	0%	5%	10%	15%
		experimental	3.726 3.729 3.730 3.730
	a
		LSDA 3.715
	(Å)	theoretical
		GGA 3.737
		experimental	9.767 9.780 9.799 9.797
	b
		LSDA 9. 797
	(Å)	theoretical
		GGA 9.992
		experimental	9.968 9.978 9.985 9.999
	c
		LSDA 9.879
	(Å)	theoretical
		GGA 10.125
		D (Å)	984.28 818.84 654.65 542.97
		ε %	0.126 0.157 0.206 0.235
		𝛿𝛿 (nm) -1 10 -3	0.103 0.149 0.233 0.339

Table 4 :

 4 Boyd and Webber (pore-diffusion) plots fit parameters

				Model
			Webber model		Boyd plot
	Fe 2 TiO 5	k I D 1	R 2	k I D 1	R 2	Intercept	R 2
	x = 0%	0.464	0.909	0.172	0.833	-0.206	0.970
	x = 5%	1.656	0.881	0.804	0.984	0.022	0.959
	x = 10%	2.426	0.953	0.310	0.983	0.672	0.913
	x = 15%	3.613	0.881	0.503	0.982	0.649	0.959


Dr Quentin Remy

Prof Junta S Igarashi

Dr Satoshi Iihama

Prof Grégory M Malinowski

Michel Hehn

Dr Jon Gorchon

Julius Hohlfeld

Shunsuke Fukami

Hideo Ohno

Stéphane Mangin
email: stephane.mangin@univ-lorraine.fr

Dr J Hehn

Prof. S Hohlfeld

Mangin

Prof H Fukami

Prof H Ohno

Prof S Fukami

Prof S Fukami

Prof S Fukami

Prof S Fukami

Energy Efficient Control of Ultrafast Spin Current to Induce Single Femtosecond Pulse Switching of a Ferromagnet

Keywords: femtosecond laser, magnetism, single shot all optical switching, spintronics

New methods to induce magnetization switching in a thin ferromagnetic material using femtosecond laser pulses without the assistance of an applied external magnetic field have recently attracted a lot of interest. It has been shown that by optically triggering the reversal of the magnetization in a GdFeCo layer, the magnetization of a nearby ferromagnetic thin film can also be reversed via spin currents originating in the GdFeCo layer. Here, using a similar structure, it is shown that the magnetization reversal of the GdFeCo is not required in order to reverse the magnetization of the ferromagnetic thin film. This switching is attributed to the ultrafast spin current and can be generated by the GdFeCo demagnetization. A larger energy efficiency of the ferromagnetic layer single pulse switching is obtained for a GdFeCo with a larger Gd concentration. Those ultrafast and energy efficient switchings observed in such spintronic devices open a new path toward ultrafast and energy efficient magnetic memories.

Introduction

All optical switching (AOS) of magnetization using ultrashort laser pulses, whether it is helicity dependent [START_REF] Stanciu | [END_REF][2][3][4][5][6] or helicity independent (AO-HIS), [4,[6][7][8][9][10][11][12] exhibits a very interesting out of equilibrium physics as well as promising practical implications. [13] Many attempts to unveil the detailed mechanism of those phenomena have been carried out both theoretically [8,[14][15][16][17][18][19][20][21][22][23][24] and DOI: 10.1002/advs.202001996 experimentally. [START_REF] Stanciu | [END_REF][2][3][4][5][6][7][8][9][10][11][12][25][26][27][28] AO-HIS is observed after a single femtosecond laser pulse and most of the materials showing this effect are gadolinium-based rare earth (RE)/transition metals (TMs) ferrimagnets. [24] AO-HIS has been extensively studied in amorphous GdFeCo alloy. [10] It was shown that AO-HIS could only be observed for a limited range of alloy concentration, laser fluence and pulse duration. [10,29,30] The first models able to reproduce such behaviors considered two antiferromagnetically exchange coupled magnetization sublattices and different relaxation times for each sublattice. [8,[14][15][16][17]24] However it was recently shown by Iihama et al. [27] that AO-HIS can be achieved in an uncoupled ferromagnetic layer within a spin valve structure. Their experiment consisted in shooting a single laser pulse on a spin valve structure with a Gd 23.3 (FeCo) 76.7 layer and a Co/Pt multilayer separated by 10 nm of Cu such that both magnetic layers are magnetically decoupled. It was shown that the Co/Pt multilayer magnetization could be reversed with a single pulse provided that the GdFeCo magnetization is switched (AO-HIS) and that the Gd magnetization sublattice is initially oriented in the direction opposite to the magnetization of the Co/Pt multilayer. The ferromagnetic layer reversal is explained by the generation of a spin current due to the GdFeCo switching.

In the present paper, we first show that we can reproduce the Co/Pt layer switching with two different GdFeCo layer compositions. By changing the composition of the GdFeCo layer, we aim at tuning the polarization of the spin current and affect the single pulse switching of the ferromagnetic Co/Pt multilayer. We then demonstrate that the reversal of the ferromagnetic layer can be obtained without switching the GdFeCo magnetization. A partial demagnetization is sufficient. Furthermore, in that case the GdFeCo layer does not even exhibit AO-HIS by itself because its Gd concentration is too high. [10] Moreover, we show that the fluence threshold required to observe the ferromagnetic multilayer magnetization reversal is reduced by increasing the Gd concentration in the GdFeCo alloy.

Experimental Section

Two spin valves consisting of a ferrimagnetic GdFeCo layer and a ferromagnetic Co/Pt multilayer separated by a 10 nm thick Cu spacer were studied. In the GdFeCo amorphous alloy, the magnetization of the RE sublattice is exchange coupled antiferromagnetically with the magnetization of the TM sublattice. Depending on the relative concentrations the net magnetization of the alloy is aligned along the RE magnetization sublattice (RE dominant) or the TM magnetization sublattice (TM dominant). The samples are, namely, glass/Ta(5)/Pt(4)/[Co(0,4)/Pt(1)] x2 / Co(0,4)/Cu(10)/Gd x (FeCo) 1-x (5)/Cu(5)/Ta(5), with x = 27.3% (TM dominant) or 33% (RE dominant). Numbers in parenthesis are the layer thicknesses in nanometers. They were grown by DC/RF magnetron sputtering as described by Iihama et al. [27] In addition, two glass/Ta(5)/Cu(5)/Gd x (FeCo) 1-x (5)/Cu(5)/Ta(5) samples of same concentrations were grown under the same conditions, as a reference. Note that the GdFeCo alloys have the same interfaces in all the samples. In Figure 1b,d, hysteresis loops were measured by polar magneto optical Kerr effect (MOKE) using a continuous He-Ne laser with a wavelength of 633 nm. For both samples, both magnetic layers exhibit perpendicular magnetic anisotropy. Linearly polarized femtosecond laser pulses of around 35 fs and with a central wavelength of 800 nm (1.55 eV) were used. For all measurements, the gaussian beam diameter (evaluated with the 1/e² convention) was around 115 µm. Samples were observed with a table top differential polar MOKE microscope using an LED (center wavelength around 630 nm) as a light source. As depicted in Figure 1, laser pulses were sent through the substrate, i.e., bottom side, and the magnetic domains were imaged subsequently from the other side, i.e., top side. In the opposite configuration, where the laser pulses were sent on the top side, samples exhibited the same qualitative behavior (see Section S8, (equation) #(S1)). All measurements were performed at room temperature.

Results

In spin valve structures, we will define the four possible magnetic configurations as parallel (P+ and P-) or antiparallel (AP+ and AP-) if the magnetizations of the TM sublattices (FeCo sublattice in the GdFeCo alloy and the Co/Pt multilayer) are parallel or antiparallel, respectively. Magnetic configurations are called positive (P+ or AP+) or negative (P-or AP-) if the Co/Pt magnetization is along the positive field direction or the negative field direction, respectively. Figure 1a,c shows a sketch of both samples in the P+ configuration. For all the figures, the green arrows represent the Gd magnetization sublattice while black arrows represent the FeCo magnetization sublattice (at the top) or the Co/Pt multilayer (at the bottom). In Figure 1a, the green arrow of Gd is bigger than the black one of FeCo to emphasize the difference in magnetization for both sublattices in this case.

We first consider the magnetic properties of the GdFeCo single layers only. They both show perpendicular magnetic anisotropy. Because the magneto-optical Kerr effect for the wavelength used is mainly sensitive to the TM magnetization and weakly sensitive to the RE magnetization, [31] we expect two different hysteresis loops depending on whether the ferrimagnetic alloy is RE dominant or TM dominant (see Section S5, Supporting Information). When x = 33%, the RE dominant case, the hysteresis loop is thus inverted compared to what is usually observed for a ferromagnet.

The major hysteresis loops for the spin valve structures are presented in Figure 1b,d for x = 33% and x = 27.3%, respectively. In the spin valve structure with x = 33%, we can then identify the inverted hysteresis loop (with a smaller coercive field in Figure 1b) caused by the GdFeCo response while the other hysteresis loop (with a higher coercive field) corresponds to the response of the Co/Pt multilayer. In the spin valve structure with x = 27.3%, the response of the Co/Pt multilayer can be easily identified since it is the same as for the previous spin valve (Figure 1d). The hysteresis loop with the smaller coercive field in Figure 1d then corresponds to the GdFeCo response. We observe that, for both spin valves, the Kerr signal due to the GdFeCo response is greater than the Co/Pt multilayer response. This is because the 10 nm of Cu attenuate the Kerr signal of the Co/Pt. A color code, shown in Figure 1b,d, is assigned to each magnetic configuration (P+, AP+, AP-, and P-) and will be the same for MOKE images. One important point is that we could conclude that the coupling between the two layers, deduced from the shift of the minor loops, is less than the resolution of the Hall probe (0.1 mT) used to measure magnetic fields. We consider in the following that the coupling is zero. We could even show (see Section S7, Supporting Information) that, at high fluence when a multidomain state is reached, P and AP magnetic configurations seem to be observed with equal probability. Whereas if a coupling was present, it should favor one type of magnetic configuration (P or AP).

We now look at the response of all samples to a single laser pulse. The GdFeCo single layer sample with x = 27.3% exhibits AO-HIS as observed by several groups, [START_REF] Stanciu | [END_REF]4,7,8] however with x = 33% the GdFeCo single layer sample does not show AO-HIS (see Section S5, Supporting Information). In that latter case, only multidomain patterns are observed after exciting the sample with a single 35 fs laser pulse. Xu et al. [10] showed that if there is no magnetization reversal on a long time scale, it will also not happen on a short time scale.

Let us now concentrate on the spin valve structure. Figure 2 shows the result of irradiating single femtosecond laser pulses with an energy of 0.23 µJ on the spin valve sample with x = 33%. We first initialize the sample in a P+ configuration using an external magnetic field. After the first pulse, a light-red color appears, corresponding to the reversal of the Co/Pt multilayer's magnetization. Exciting the sample with more pulses does not change the Kerr contrast. Performing the same experiment, with the same pulse energy, but starting in the AP-configuration, leaves the magnetic configuration unchanged (not shown). Simplified sketches of the spin valve magnetic configuration, showing only the magnetizations of the TM sublattice of each magnetic layer are displayed under each MOKE microscope image. Thus, we see that a single femtosecond laser pulse can induce the switching of the Co/Pt multilayer without affecting the GdFeCo orientation.

Figure 3 presents the results obtained for both samples while using pulses of higher energy. For the sample with x = 33% (Figure 3a,b) we observe the switching of both GdFeCo and Co/Pt magnetization after each pulse such that transitions P+ to Pand P-to P+ are observed but only in the central part of the laser spot. The clear red ring around this region corresponds to the reversal of the Co/Pt multilayer's magnetization, which only appears after the first pulse. This P+ to AP-transition is identical to the one observed at low fluence. In Figure 3b, one can see that no ring appears because the sample is already in the APconfiguration. Small AP+ domains at the rim appear after the second pulse that is attributed to the dipolar field that is stronger close to domain boundaries.

Figure 3c,d shows results obtained for the sample with x = 27.3%. Starting from a P+ configuration, we find two distinct behaviors. At a lower energy, i.e. what is observed further away from the center of the beam in Figure 3, only the GdFeCo layer reverses its magnetization. At higher energies, both the GdFeCo and the Co/Pt multilayer's magnetization reverse. When sending another laser pulse on the same spot, both magnetic domains disappear. We note that a tiny magnetic domain remains after the second pulse. In this region, the Co/Pt multilayer's magnetization has not been switched back, which can be explained by small deviations of the laser beam's position. Starting from an AP-configuration, only the GdFeCo layer's magnetization reverses after the first pulse while for the second pulse, the GdFeCo layer's magnetization is switched back and the Co/Pt multilayer's magnetization reverses in a smaller region.

Figures 2 and3 demonstrate the types of transitions which can be observed. To be more quantitative, we wish to determine the pulse energy required to observe one given transition. Because the laser beam has a gaussian profile, a smaller pulse energy implies that the created magnetic domain is smaller. However due to micromagnetic interactions, small reversed domains need to have a critical size to be stable on a long-time scale and cannot be observed with our microscope setup. [32] The typical minimum size we can observe in these samples is around 20 µm. In order to determine the threshold energy for a given transition, the size of the reversed magnetic domains as a function of the pulse energy is plotted as in Figure 4. By fitting the data with a function parametrized by the threshold energy and the laser beam size, one can determine these threshold energies (see Section S4 of the Supporting Information for more details). Only the cases with a P+ configuration are shown. No difference in threshold fluences is found with the antiparallel case for a given sample, within uncertainties. All the threshold fluences are reported in Table 1. No difference was found when starting from a P-or a P+ configuration, as expected from time reversal symmetry (not shown). In Figure 4, simplified stack sketches are shown to summarize what happens in each region. In both samples, nothing happens below the first threshold fluence (TF1) and both layers reverse their magnetization (i.e., P+ to P-transitions are always observed) above the second threshold fluence (TF2). However between TF1 and TF2, only the magnetization of the Co/Pt multilayer switches when x = 33% while when x = 27.3% only the GdFeCo layer's magnetization is reversed.

Discussion

To explain those behaviors we have already excluded the possibility of an exchange coupling between both magnetic layers. We could also expect a magnetization switching induced by the dipolar field generated by one magnetic layer when it is demagnetized due to the local heating of the laser. In our structures with two magnetic layers, each layer generates a dipolar field which can impact its own magnetization (in that case we speak about demagnetizing field-case 1) or the other layer's magnetization (in that case we speak about stray field-case 2). Case 1 cannot explain the Co/Pt multilayers switching because it happens only starting from a P configuration. It is also ruled out for GdFeCo layers because AO-HIS in single GdFeCo layers samples is not due to a dipolar field. The effect in case 2 is more subtle because GdFeCo may cross its compensation temperature when x = 33% (the compensation temperature is above room temperature when the sample is Gd dominant). Indeed, when crossing the compensation temperature the sign of the stray field can be inverted. However, we note that the dipolar field, if it has an effect, should be strongly dependent on the size of the magnetic domains as well as the way the sample is heated up. We notice the same reversal behavior for a wide range of fluences. In particular, the Co/Pt reversal in the sample with x = 33% is seen from 1.7 to 5.8 mJ cm -² (see Section S7 of the Supporting Information for the latter fluence), corresponding to various domain sizes and shapes. In addition, the same reversal behavior is observed when Table 1. Threshold fluences for the magnetization reversal of both magnetic layers in each sample. P and AP refer to the magnetic configuration before sending the first laser pulse. exciting from the top side whereas the light absorption profiles and therefore heating is quite different (see Section S6, Supporting Information). Thus, we believe that dipolar fields cannot explain the observed behavior. However, it can explain the multidomain patterns observed at high fluences together with thermal activation [20] and it could partially explain the domains at the rim (where the dipolar field is the strongest) of the inner domains in Figure 3a,b. Our explanation for the observed phenomena is based on i) the generation of a spin current due to the fast magnetization change of the Gd sublattice [33] and ii) the possibility of this spin current to interact with the Co/Pt multilayer when it is sufficiently demagnetized as proposed by Iihama et al. [27] Before diving into the details, we first summarize what is known regarding points (i) and (ii).

There is a growing awareness that nonlocal transfer of angular momentum is a key feature of AO-HIS, [25] which is related to the coupling between atomic spins of the ferrimagnet with conduction electrons. [19,21,22] Therefore, conduction electrons can get spin polarized with a spin generation rate following the demagnetization rate. [33,34] This can be seen in models including an s-d interaction [21,[35][36][37] where both magnetization sublattices contribute to the spin generation rate through their own demagnetization. This can lead to a spin current leaving the magnetic layer if it is sharing an interface with a metallic layer. [38][39][40][41] Choi and Min observed this spin current in RE/TM ferrimagnetic layers. [33] The typical signal observed in these experiments shows two peaks of opposite signs at two different time delays. The first peak is mostly due to the ultrafast demagnetization of the TM sublattice which gives rise to a primary spin current whose polarization is given by the TM magnetization sublattice. The second peak is due to the continuing ultrafast demagnetization of the RE sublattice plus some spin dependent Seebeck effect. [33] Thus, the secondary spin current has a polarization given by the RE magnetization sublattice. When the ferrimagnetic material contains Gd, Choi and Min [33] showed that it is possible to increase the spin polarization of the current by increasing the amount of Gd. This can be understood by the fact that the total magnetic moment in the Gd sublattice increases and thus the Gd demagnetization rate will increase if the demagnetization time remains the same. This explains point (i).

Regarding the ferromagnetic layer, one needs to consider two different time scales. At short time scales (below ≈1 ps) the laser heating plus the heating due to hot carriers coming from other layers [42] will cause the demagnetization of the Co/Pt multilayer. The primary spin current previously mentioned may also participate to the demagnetization. [37,43] In a second wave, the secondary spin current whose polarization is set by the Gd magnetization sublattice interacts with the ferromagnetic layer [33] when the Co/Pt multilayer is demagnetized. We note that in the work of Bergeard et al., [42] the Co/Pt multilayer starts remagnetizing after 0.5 ps. However, increasing the fluence of the laser can extend the time range at which the ferromagnetic layer reaches its minimum magnetization. [44,45] Being demagnetized in the presence of a spin current, the ferromagnetic layer can remagnetize in the direction set by the polarization of the spin current. This is indeed consistent with the fact that, in Figure 2 when x = 33%, the Co/Pt multilayer's magnetization only switches when the Gd magnetization sublattice has the opposite direction. When x = 27.3%, the GdFeCo layer's magnetization reversal is assigned to AO-HIS and the Co/Pt multilayer's magnetization reversal is due to its interaction with the spin current generated by the Gd magnetization sublattice. In Figure 4, we represent this spin current only when it is able to induce the reversal of the Co/Pt multilayer's magnetization. We can then see that the sample with x = 27.3% reproduces well the results of Iihama et al. [27] The fact that the Co/Pt multilayer's magnetization can be reversed without observing the reversal of the GdFeCo layer's magnetization in the sample with x = 33% can be explained by previous works. [21,33,34] Indeed, the key ingredient here is the spin current and the spin generation rate is proportional to the magnetization decay (-dM/dt). The reversal process of the GdFeCo layer's magnetization (when it exhibits AO-HIS) is an ultrafast demagnetization followed by a slower reversal. [7] The fastest dynamics happens during the demagnetization and thus no full magnetization reversal is needed to generate the spin current. We note that Choi and Min [33] indeed measured the spin generation rate without reversing the GdFeCo magnetization. To observe the reversal of the ferromagnetic layer without switching the ferrimagnetic layer, it is then required to have a threshold fluence for AO-HIS higher than a threshold fluence for a sufficient demagnetization of the Co/Pt multilayer. We demonstrate here that this can be done by increasing the Gd concentration, which not only increases the spin polarization of the current but also increases the threshold fluence required to observe AO-HIS or even makes it impossible to observe anymore as in the case of x = 33% here. [10] Something less expected is the significant increase in the energy efficiency of the ferromagnetic layer switching just by changing the Gd concentration, i.e., the different threshold fluences observed in both samples as can be seen in Table 1. Indeed, increasing the Gd concentration by 5.7% implies a threshold fluence reduction for the Co/Pt multilayer of 21.8% (from 2.11 to 1.65 mJ cm -²). We also note that in both cases, these threshold fluences are much smaller than the value of 11.8 mJ cm -2 found by Iihama et al. [27] In the work by Choi and Min, [33] it can be seen that increasing the Gd concentration does not change much the primary spin current. Therefore, we conclude that the Co/Pt multilayer demagnetizes the same way in both samples. We could not find a difference in the complex optical indices of both GdFeCo alloys by ellipsometry measurements, thus the laser excitation process in both samples should be the same. Consequently, we attribute this difference in threshold fluences to the secondary spin current generated by the Gd sublattice demagnetization. Because this spin current carries more angular momentum in the sample with x = 33%, less demagnetization is required to reverse the ferromagnetic layer.

Another interesting point is the reversal of the GdFeCo layer's magnetization in the sample with x = 33% with one femtosecond laser pulse. We remind the reader that under the same conditions the single GdFeCo layer does not switch. There is a simple hypothesis to understand this switching if the initial magnetic configuration is P. Assuming that conduction electrons mediate exchange scattering in AO-HIS as in s-d models, the Co/Pt multilayer could help the magnetization reversal of the GdFeCo layer by acting as a supplementary reservoir of angular momentum. However, this explanation fails when the initial magnetic configuration is AP. It could be that the presence of the Co/Pt increases the composition range to see AOS, thanks to additional (polarized or not) hot electrons coming from this layer compared to samples without the Co/Pt multilayer. The spin current coming from the spin dependent Seebeck effect in Co/Pt multilayers [46] could also play a role but this would require more careful studies and is beyond the scope of this paper. However, we note that these effects should be dependent on temperature gradients, which will be different when exciting from the top side or the bottom side, as expected from the calculated absorption profiles (see Section S6, Supporting Information).

Finally, we note that in a recent work, van Hees et al. [28] used the spin current coming from a ferromagnetic layer in order to assist or hinder the AOS of a GdFeCo layer. This is a different but somewhat complementary work, which supports our conclusions. In their case, the spin current is coming from the ferromagnetic layer and changes the threshold fluence for the AOS of a Gd/Co bilayer. In our case, all threshold fluences for GdFeCo are the same, but the polarization of the spin current coming from the GdFeCo determines whether the ferromagnetic layer's magnetization switches or not. The same spin current generated by the ferromagnetic layer most likely also exists in our sample but we cannot observe its effect.

Conclusion

To conclude, we confirmed that, in a spin valve structure GdFeCo/Cu/[Co/Pt], a single femtosecond laser pulse can induce a fast change of the Gd magnetization sublattice, which generates a spin current sufficient to switch the ferromagnetic layer if the ferromagnetic layer is sufficiently demagnetized. The switching of the ferromagnetic layer is observed when the femtosecond laser induces a full magnetization switching (AO-HIS) but also when it only induces a partial demagnetization of the GdFeCo. Indeed, we demonstrated that with a GdFeCo layer not showing AO-HIS we could still switch the ferromagnetic layer with one single laser pulse. Moreover, we have shown that in this case the ferromagnetic layer switching requires less energy, i.e., a lower laser fluence.

This work should motivate more dynamical measurements as well as more theoretical work in order to fully understand the transfer of energy and angular momentum that seems to be the cause of this behavior and the unexpected GdFeCo single pulse switching at high Gd concentration.

Figure 1 .

 1 Figure 1. a) Simplified stack of the sample with x = 33% and b) its corresponding major hysteresis loop. c) Simplified stack of the sample with x = 27.3% and d) its corresponding major hysteresis loop. 𝜃 Κ is the normalized Kerr rotation in arbitrary units. Green arrows indicate the Gd magnetization sublattice while black arrows indicate transition metals magnetizations. The red cone indicates where MOKE observation of magnetization (with a continuous laser or a MOKE microscope) is made. Black arrows on the hysteresis loops correspond to the black arrows in the simplified stacks.

Figure 2 .

 2 Figure 2. Results of sending a femtosecond laser pulse on the sample with x = 33% in a P+ configuration as observed using a MOKE microscope. The pulse energy is 0.23 µJ. The magnetic configuration is summarized below each image where only the transition metals magnetizations have been represented. The scale bar in the last image is 20 µm long.

Figure 3 .

 3 Figure 3. Results of sending a femtosecond laser pulse observed using a MOKE microscope. The magnetic configuration is summarized below each image where only the transition metals magnetizations have been represented. a,b) For the sample with x = 33% with a P+ and an AP-initial configuration, respectively. The pulse energy is 0.43 µJ. c,d) For the sample with x = 27.3% with a P+ and an AP-initial configuration, respectively. The pulse energy is 0.31 µJ. The scale bars in the last image of each figure are 30 µm long.

Figure 4 .

 4 Figure 4. Domain size versus pulse energy diagrams for samples with a) x = 33% and b) x = 27.3% starting from the P+ configuration. TF1 and TF2 are the first and second threshold fluences, respectively. Sketches indicate what happens in each region of the diagram after the first laser pulse is sent. The turquoise arrow is meant to represent the spin current generated from the Gd demagnetization. Its polarization (the black arrows) is given by the initial Gd magnetization sublattice.

Adv. Sci. 2020, 2001996 © 2020 The Authors. Published by Wiley-VCH GmbH

Adv. Sci. 2020, 2001996 © 2020 The Authors. Published by Wiley-VCH GmbH 2001996 (8 of 8)

Acknowledgements

The authors thank Thomas Hauet, Maarten Beens, and Bert Koopmans for valuable discussions. The authors also thank Thomas Easwarakhanthan

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Conflict of Interest

The authors declare no conflict of interest.