[Show abstract][Hide abstract] ABSTRACT: Nanostructures with controlled size, morphology, and composition represent a main challenge in materials science because controlling these parameters is fundamental to optimizing the subsequent functional properties. Aggregated nanostructures, combining both individual and collective properties of nanocrystals, offer interesting perspectives to design new magnetic nanomaterials. In that context, original porous raspberry shaped nanostructures consisting of oriented aggregates of ferrite nanocrystals have been synthesized by an one-pot polyol solvothermal method. Synthesis conditions have been optimized to obtain nanostructures featured by similar sizes of about 250 nm and nanocrystal sizes modulated from 5 to 60 nm, leading to porous structures with tunable specific surface area. Structural and magnetic studies of nanostructures as a function of the nanocrystal size evidenced their low oxidation state and enhanced magnetic properties. Indeed, the oriented aggregation of nanocrystals leads to high interface between nanograins reducing significantly their surface oxidation and enhancing their saturation magnetization in comparison to individual nanoparticles of similar sizes. Magnetic moments of each grain are also consequently strongly coupled by dipolar interactions which led to superspin glass effects.
Full-text · Article · Oct 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The structural and chemical order are the most important parameters governing the physical properties of the Heusler compounds. Here, we give a comprehensive overview of the correlations between structural and chemical order, electronic transport (longitudinal and transverse) and magnetic (static and dynamic) properties of Co2FeAl0.5Si0.5 Heusler alloy epitaxial thin films grown on MgO(001) single-crystal substrates. X-ray diffraction measurements indicated that depending on the annealing temperature the films show B2 or L21 chemical ordering. Longitudinal magnetoresistivity experiments revealed that for the best L21 ordered film, at temperatures bellow 125K, the magnon assisted electronic scattering is quenched indicating the appearance of half-metallicity. The presence of quantum correction in resistivity, whose strength is dependent on the structural ordering, was evidenced at low temperatures. Anomalous Hall experiments indicated that the intrinsic band structure contribution has an opposite sign and that is dominant over the extrinsic skew scattering mechanism. The presence of a small uniaxial magnetic anisotropy contribution superimposed on a larger biaxial one was evidenced via ferromagnetic resonance microstrip line measurements. The biaxial term is well correlated with chemical ordering, having a minimum value for the optimum L21 ordered film. The damping parameter was evaluated from ferromagnetic resonance linewidth measurements, and a coefficient as low as 1.9×10-3 was found for the L21 phase.
Full-text · Article · Aug 2015 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Bimetallic PdxPt1−x/TiO2-P25 photocatalysts have been proven to maintain high activity for CO photocatalytic oxidation even under 50% relative humidity for 0.3 < x < 0.5 molar compositions, whereas humidity traces are highly detrimental when the reaction was performed over TiO2 supported monometallic particles. The catalysts were characterized by XPS, TEM, HAADF-STEM, TPR, CO-TPD and FTIR-CO adsorption characterization. The results suggested a random distribution of Pd and Pt atoms within the bimetallic nanoparticles, and in particular at their surface. It was proposed that the synergy observed in bimetallic PdxPt1−x/TiO2 may be explained by their modified electronic structure, playing a key role in overcoming the negative effects of humidity on the photocatalytic CO conversion. The behavior of PdxPt1−x/TiO2 under humid air was assumed to result from a preferential oxygen and water adsorption on surface Pt sites, leaving Pd sites free for CO adsorption, Pd being more resistant to oxidation than Pt in Pt-rich PtPd particles.
[Show abstract][Hide abstract] ABSTRACT: The properties at different scales of Ti1 -xAlxN films deposited by reactive magnetron sputtering from TiAl sintered (S) targets produced by powder metallurgy are compared with those of a set of films previously deposited in the same conditions from mosaic targets (M) made of pure Ti and Al metals. For compositions close to the hcp/fcc transition (around x = 0.6), the friction behaviour, growth directions and organization of crystallized domains are found to be sensitive to the type of target used. The resistance to crack creation is higher for Ti0.54Al0.46N (S) and Ti0.38Al0.62N (S) than for Ti0.50Al0.50N (M) and Ti0.32Al0.68N (M). From the measurement of mechanical properties, toughness, wear volumes and from the observation of wear tracks, it is found that films prepared from sintered targets exhibit a better wear resistance. Grazing Incidence X-ray Diffraction and Electron Energy Loss Spectroscopy in Transmission Electronic Microscopy are used to investigate the long- and short-range orders within the films. The morphology of Ti0.54Al0.46N (S) film can be considered as an array of crystalline domains having reciprocal-space vectors 111 and 200 directed along the meridian but with random in-plane orientation. Ti0.38Al0.62N (S) Al-rich film presents a random orientation of the crystalline domains whereas Ti0.32Al0.68N (M) deposited from composite targets exhibits a well-oriented fibrillar structure. The N K-edge Electron Energy Loss Near Edge Spectra are discussed with previous results of Extended X- ray Absorption Fine Structure Spectroscopy, which has evidenced different values of Al-N and Ti-N bond lengths, either octahedral (cubic-like) or tetrahedral (hexagonal-like) within Ti0.50Al0.50N (M) and Ti0.32Al0.68N (M) films. For similar compositions, films deposited from sintered alloys contain more nitrogen atoms in octahedral cubic-like environment than coatings made from mosaic targets, which could explain their better resistance to cracking, higher hardness and better wear resistance.
[Show abstract][Hide abstract] ABSTRACT: Undoped and Mo-doped ZnO (2% Mo) films about 1 μm thick were deposited by radio-frequency magnetron sputtering on Si(100) and glass substrates at 30 and 300 °C. X-ray diffraction patterns show that all films exhibit the hexagonal wurtzite crystal structure with a preferred orientation of the crystallites along the  direction. Plane view and cross-section transmission electron microscopy observations showed that the films present a columnar growth. Rutherford backscattering spectrometry indicates that Mo is homogeneously distributed inside the films. Scanning electron microscopy and atomic force microscopy show that Mo doping leads to a reduction of the grain size and surface roughness. According to X-ray photoelectron spectroscopy measurements, the valence of the Mo ions in the ZnO matrix is + 5 and + 6. Optical measurements in the UV–Visible range show a transmittance increasing from about 60 to 80% when increasing the wavelength from 400 to 800 nm. A sharp absorption onset is observed at about 375 nm corresponding to the fundamental absorption edge of ZnO at 3.26 eV. This gap value remains unchanged upon Mo doping. The Hall effect measurements carried out at room temperature show that both undoped and Mo-doped ZnO films present an n-type conduction. The 2% Mo doping increases the carrier concentration and decreases the resistivity measured in pure ZnO by about three orders of magnitude. A comparison with 2% Al-doped ZnO films grown in the same conditions underlines the important role of the preparation conditions on the transport properties of ZnO based transparent conductive oxides.
[Show abstract][Hide abstract] ABSTRACT: The current challenge in the field of nano-medicine is the design of multifunctional nano-objects effective both for the diagnosis and treatment of diseases. Here, dendronized FeO1-x@Fe3-xO4 nanoparticles with spherical, cubic, and octopode shapes and oxidized Fe3-xO4 nanocubes have been synthesized and structurally and magnetically characterized. Strong exchange bias properties are highlighted in core shell nanoparticles (NPs) due to magnetic interactions between their antiferromagnetic core and ferrimagnetic shell. Both in vitro relaxivity measurements and nuclear magnetic resonance (NMR) distribution profiles have confirmed the very good in vitro magnetic resonance imaging (Mm) properties of core shell and cubic shape NPs, especially at low concentration. This might be related to the supplementary anisotropy introduced by the exchange bias properties and the cubic shape. The high heating values of core shell NPs and oxidized nanocubes at low concentration are attributed to dipolar interactions inducing different clustering states, as a function of concentration. In vivo MRI studies have also evidenced a clustering effect at the injection point, depending on the concentration, and confirmed the very good in vivo MRI properties of core shell NPs and oxidized nanocubes in particular at low concentration. These results show that these core shell and cubic shape dendronized nano-objects are very suitable to combine MRI and hyperthermia properties at low injected doses.
Full-text · Article · Aug 2014 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: Spherical core−shell Co x Fe 1−x O@Co y Fe 3‑y O 4 nanoparticles (NPs) as well as spherical and cubic shaped CoFe 2 O 4 NPs were synthesized through a thermal decomposition method by adjusting parameters such as the nature of precursors and ligands. The use of metal (iron and/or cobalt) oleates and stearates as precursors in the presence of oleic acid as ligand leads to core−shell NPs, due to the reducing environment provided by oleate groups from the oleic acid and precursors. By contrast, the use of oleylamine as ligand favored the decomposition of precursors and less reducing medium, which allows obtaining NPs with homogeneous composition. In addition, cobalt ferrite cubic-shaped NPs were synthesized using mixed oleate formed in situ from metal iron chloride and cobalt chloride in the presence of sodium oleate. The as-synthesized NPs were carefully characterized by combining several techniques including TEM, XRD, 57 Fe Mö ssbauer spectrometry, STEM-EELS, and XMCD. The correlation between the crystalline structure and the magnetic properties was investigated by carrying out magnetic measurements as a function of an applied field and of temperature. The CoFe 2 O 4 NPs were found to display high coercivity due to their homogeneous composition, while the core−shell NPs show higher blocking temperature and exchange bias properties originating from the interaction between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) layer at the surface.
Full-text · Article · Aug 2014 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: We report on the changes on the microstructural, hardness, and corrosion properties induced by carbo-chromization of 316L stainless steel prepared by Spark Plasma Sintering technique. The thermo-chemical treatments have been performed using pack cementation. The carburizing and chromization were carried out between 1153 K (880 °C)/4 h to 1253 K (980 °C)/12 h and 1223 K (950 °C)/6 h to 1273 K (1000 °C)/12 h in a solid powder mixture of charcoal/BaCO3 and ferrochromium/alumina/NH4Cl, respectively. The obtained layers were investigated using X-ray and electron diffraction, optical and scanning electron microscopies, Vickers micro-hardness, and potentiodynamic measurements. The thickness of the carbo-chromized layer ranges between 300 and 500 μm. Besides the host γ-phase, the layers are mainly constituted of carbides (Fe7C3, Cr23C6, Cr7C3, and Fe3C) and traces of α′-martensite. The average hardness values decrease smoothly from 650 HV at the sample surface down to 200 HV at the center of the sample. The potentiodynamic tests revealed that the carbo-chromized samples have smaller corrosion resistance with respect to the untreated material. For strong chromization regimes, the corrosion rate is increased by a factor of four with respect to that of the untreated material, while the micro-hardness of the layer is three times larger. Such materials are suited to be used in environments where good corrosion resistance and wear properties are required.
Full-text · Article · Jun 2014 · Metallurgical and Materials Transactions A
[Show abstract][Hide abstract] ABSTRACT: Nanometric size Zn-doped SnO2 particles with Zn concentration varying from 1 to 6 % were prepared using the co-precipitation method. X-ray diffraction patterns show for all samples a typical rutile-type tetragonal structure of SnO2 without any additional peaks from spurious phases. These results together with transmission electron microscopy analyses have shown that the size of the nanoparticles decreases with Zn doping down to 4 nm. According to UV–visible absorption measurements this decrease of particle size is accompanied by a decrease of the band gap value from 3.34 eV for SnO2 down to 3.28 eV for 6 % Zn doping. The electrical conductivity of the system has been investigated between 473 and 718 K, in the 200 Hz–5 MHz frequency range, by means of impedance spectroscopy. The temperature dependence of the bulk conductivity was found to obey the Arrhenius law with activation energies of 0.74 eV for SnO2 and 0.69 eV for 6 % Zn doping.
Full-text · Article · May 2014 · Journal of Materials Science Materials in Electronics
[Show abstract][Hide abstract] ABSTRACT: Sub-10 nm CoPt nanoparticles were slowly grown at 400 °C in epitaxy on a NaCl substrate. Their faceted shape was analyzed using state-of-the-art TEM techniques: aberration-corrected imaging, electron tomography, and probe-aberration-corrected scanning transmission electron microscopy. These nanoparticles consist in truncated octahedrons with a chemically disordered face-centered cubic (FCC) structure. We evidenced slight variations of the truncation of these nano-octahedrons depending on their size: the largest particles are less truncated than the smallest particles. We also highlighted the up–down symmetry of the NPs, suggesting that the adhesion energy of FCC-CoPt on NaCl is negligible. Energy descriptions of these NPs were made by using quenched molecular dynamics in the framework of the second moment approximation of the tight-binding formalism, while taking into account the random distribution of Co and Pt atoms. In a general manner, this original energy approach for studying faceting in chemically disordered nanoalloys is consistent with experimental results, particularly for small-size clusters. However, as the experimentally observed size-effect on the NPs truncation was not theoretically predicted, this phenomenon could originate from kinetic effects inherent to nanocrystal growth.
[Show abstract][Hide abstract] ABSTRACT: The control of the magnetization of a material with an electric field would make the design and the integration of novel electronic devices possible. This explains the renewed interest in multiferroic materials. Progress in this field is currently hampered by the scarcity of the materials available and the smallness of the magnetoelectric effects. Here we present a proof-of-principle experiment showing that engineering large strains through nanoscale size reduction is an efficient route for increasing magnetoelectric coefficients by orders of magnitude. The archetype magnetoelectric material, Cr2O3, in the form of epitaxial clusters, exhibits an unprecedented 600% change in magnetization magnitude under 1 V. Furthermore, a multiferroic phase, with both magnetic and electric spontaneous polarizations, is found in the clusters, while absent in the bulk.
Full-text · Article · Jan 2014 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: In this study, silicon rich silicon oxynitride layers containing more than 15% nitrogen were deposited by electron cyclotron resonance assisted plasma enhanced vapor deposition in order to form silicon nanoparticles after a high temperature thermal annealing. The effect of the flows of the precursor gases on the composition and the structural properties of the layers was assessed by Rutherford backscattering spectroscopy, elastic recoil detection analysis, and infrared spectroscopic measurements. The morphological and crystallinity properties were investigated by energy filtered transmission electron microscopy and Raman spectroscopy. We show that the excess of silicon in the silicon oxynitride layer controls the silicon nanoparticles size. On the other hand, the crystalline fraction of particles is found to be strongly correlated to the nanoparticle size. Finally, the photoluminescence measurements show that it is also possible to tune the photoluminescence peak position between 400 and 800 nm and its intensity by changing the silicon excess in the silicon rich silicon oxynitride matrix.
No preview · Article · Jul 2013 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We report on the epitaxial growth of 100 nm thick triclinic γ-CoV2O6 thin films deposited by pulsed laser deposition on TiO2(100) substrate. The layers were grown in narrow experimental conditions, at 600 °C and 0.1 millibar oxygen pressure. X-ray diffraction and transmission electron microscopy evidenced the presence of two variants and the following epitaxial relation between the layers and the substrate: TiO2(100)∥[0±10]γ-CoV2O6(100). Besides the magnetization steps expected in γ-CoV2O6, low temperature magnetic measurements performed along different crystalline axes show the existence of a strong anisotropy compatible with that expected from a one dimensional system, with the easy magnetization axis lying along the b direction (i.e., the Co chains).
Full-text · Article · May 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Core–shell nanoparticles (NPs), which consist in a ferrimagnetic (FIM)/antiferromagnetic (AFM) interface and result in exchange bias coupling, became recently of primary importance in the field of magnetic nanoparticles. The enhancement of some applications such as hyperthermia or magnetic storage media based on the miniaturization of devices is correlated to the size reduction of NPs, which results in the decrease of the magnetocrystalline anisotropy and of the blocking temperature. We present here the synthesis of Fe3−δO4@CoO core–shell NPs by a one-pot seed-mediated growth process based on the thermal decomposition of metal complexes at high temperature. A 2 nm thick CoO shell was grown homogeneously from the starting Fe3−δO4 core surface. The Fe3−δO4@CoO core–shell NP structure has been deeply investigated by performing XRD and advanced techniques based on TEM such as EELS and EFTEM. The high quality of the core–shell interface resulted in the large exchange bias coupling at 5 K (HE ≈ 4.1 kOe) between the FIM and the AFM components. In comparison to starting Fe3−δO4 NPs, the dramatic enhancement of the magnetic properties such as a high coercive field (at 5 K, HC ≈ 15 kOe) were measured. Furthermore, the core–shell structure resulted in the enhancement of the magnetocrystalline anisotropy and the increase of the blocking temperature to 293 K.
Full-text · Article · May 2013 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The L10-MPt alloys (M = Co, Fe, FeNi) are interesting due to their high magnetic anisotropy as arrays of nanoparticles or nanostructures may be used in high density storage media devices. CoPt, FePt and NiPt alloy layers have been prepared by molecular beam epitaxy or cathodic sputtering, epitaxied on MgO(100) substrates. The layers have the L10 tetragonal structure, ordered in the growth direction with an easy magnetization direction perpendicular to the layer plane. L10-ordered FeNiPt2(001) thin films could be prepared by the interdiffusion of L10-ordered FePt(001) and NiPt(001) layers. The long-range order is conserved in the FeNiPt2 layers after interdiffusion at 900 K, contrary to what is expected from a simple vacancy migration process. Quenched molecular dynamics simulations were performed to determine the most probable atomic processes.
The CoPt layers have been nanostructured using two routes after electron beam lithography: either ion etching or ion irradiation. The first route produces hard ferromagnetic dots separated by vacuum whereas the second route produces hard ferromagnetic dots separated by soft ferromagnetic dots. The magnetic behaviour of both types of nanostructures is compared. Hysteresis curves of single dots were measured using magnetic force microscopy.
The superparamagnetic limit occurs in nanoparticles when the thermal energy equals the magnetic anisotropy energy. Therefore it is hoped that the limit of super-paramagnetic limit may be postponed by using highly anisotropic magnetic materials. CoPt nanoparticles have been grown and ordered on different substrates.
[Show abstract][Hide abstract] ABSTRACT: Doping wide band gap semiconductors, such as ZnO, with trivalent rare earth (RE) ions is well known to enhance their optical activity. The present paper shows that high quality ZnO:Yb layers can be obtained by RF magnetron sputtering and that an efficient electronic transfer from ZnO to Yb +3 ions can be achieved. It is also shown that the rare earth is optically active at any deposition temperature and that its contribution to the photoluminescence (PL) is important even at very low concentrations. In particular, for samples deposited at low temperatures, the rare earth strongly enhances the photon conversion, yielding a total PL up to three times more intense. On the other hand, if the layers are exposed to temperatures above 200 1C, either during deposition or upon post-deposition annealing, the presence of Yb quenches the total PL. Thermal annealing of the films at 700 1C under Ar or O 2 gas flow highly improves the PL of both ZnO and ZnO:Yb.
Full-text · Article · Jan 2013 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: The aim of the present study is to check the influence of annealing under vacuum and a mixture of N(2)-H(2) atmosphere on the magnetic properties of polycrystalline Co-doped CeO(2) diluted magnetic oxides (DMOs) with Co concentrations of 5 at% synthesized using the coprecipitation technique. X-Ray diffraction (XRD) patterns and transmission electron microscopy (TEM) showed for all samples the expected CeO(2) cubic fluorite-type structure and that Co ions are uniformly distributed inside the samples. Room-temperature Raman and photoluminescence (PL) spectroscopies indicate an increase in the concentration of oxygen vacancies upon Co doping and further annealing. Field dependent magnetization measurements revealed a paramagnetic behavior for as-prepared Co-doped CeO(2), while a ferromagnetic behavior appears when the same samples are annealed under vacuum or N(2)-H(2) atmosphere. Temperature dependent magnetization measurements suggest that the observed ferromagnetism is due to the presence of metallic Co clusters with nanometric size and broad size distribution. These results are supported by electron paramagnetic resonance studies.
[Show abstract][Hide abstract] ABSTRACT: We report on the effect of the film thickness on the magnetic properties of Ca₃Co₂O₆films with an emphasis on the magnetization steps usually observed in the M-H curves below 10 K. Films with thicknesses between 35 and 200 nm all present two magnetic transitions at about T(C₁) = 22 K and T(C₂) = 10 K, corresponding to a 3D long range ferrimagnetic order and the transition to the formation of a frozen spin state, respectively. The magnetization curves at 10 K exhibit the expected stepped variation. However, by decreasing the thickness below a critical value of about 60 nm, no magnetization plateau is observed when the M-H curve is recorded at 2 K. Moreover, an additional transition in the susceptibility curve is observed at 45 K. These changes can be attributed to the reduced coherence length of the propagation vector along and perpendicular to the chains, and are supported by the magnetization relaxation measurements which indicate a reduction of the relaxation time. These results are helpful for understanding the origin of the magnetization steps in the one-dimensional Ca₃Co₂O₆ cobaltite and confront the theoretical models aimed at explaining the magnetic properties in this system.
[Show abstract][Hide abstract] ABSTRACT: We have studied the Si initial growth mechanisms on LaAlO3(001), a crystalline oxide with a high dielectric constant (high-κ material). The clean LaAlO3(001) substrate exhibits a c(2×2) reconstruction that can be attributed to surface O vacancies.Si deposit by molecular beam epitaxy was studied as a function of both deposition temperature and thickness. Epitaxy was obtained only above 550°C. In this case, a Volmer–Weber mode is observed. The associated nanodots are relaxed and formed by pure Si as ascertained by the Si2s XPS peak, which remains for 1 and 10ML at the binding energy corresponding to Si–Si bonds. Moreover the islands have an abrupt interface with the LaAlO3(001) substrate without the formation of silicate or silica. A unique epitaxial relationship between LaAlO3 and the crystallized Si islands is pointed out by RHEED and confirmed by HRTEM, where the Si(001) planes are parallel to the LaAlO3(001) ones, but rotated by 45° in the  direction. This orientation leads to mismatch and strain minimization of the Si film.
No preview · Article · May 2011 · Journal of Crystal Growth
[Show abstract][Hide abstract] ABSTRACT: To test whether the interface between an Fe-alloy electrode and a SrTiO3 (STO) tunnel barrier constitutes or not a good spin injector, we have studied the transport and magnetic properties of CoFe2 (7.5nm)/SrTiO3 (3nm)/CoFe2 (1 nm)/NiFe (8 nm) stacks prepared by pulsed-laser deposition on STO(001) substrates and varied the STO barrier deposition temperature. While this parameter does not strongly influence the magnetic properties of the two electrodes, the resulting barrier height at the bottom CoFe2/STO interface is lowered if the STO barrier is deposited at 300°C rather than 80°C and an already low-tunnel-magnetoresistance ratio is suppressed. We discuss our findings in terms of the oxide stability of the CoFe2/STO interface.