Véronique Dupuis - Academia.edu (original) (raw)
Papers by Véronique Dupuis
Thin Solid Films, 1997
Silicon clusters prepared by low energy cluster beam deposition exhibit visible luminescence simi... more Silicon clusters prepared by low energy cluster beam deposition exhibit visible luminescence similar to that observed in porous silicon. The structure and properties are characterized by different complementary techniques with a special emphasis on Raman spectroscopy. The comparison with silicon clathrates structures is promising. The validity of the confinement model in a size range down to a few ångströms is discussed. q 1997 Elsevier Science S.A.
Physical Review B, 2011
We discuss the possibility of L1 0 chemical order parameter quantification for an individual part... more We discuss the possibility of L1 0 chemical order parameter quantification for an individual particle of CoPt, using transmission electron microscopy. While "usual" approaches are found to be unapplicable for small particles (less than 4 nm in diameter), we present a method based on the comparison between an experimental high-resolution image and simulated ones with various degrees of chemical order.
Physical Review B, 2008
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Physical Review B, 2013
We propose a theoretical framework enabling the simulation of isothermal remanence magnetization ... more We propose a theoretical framework enabling the simulation of isothermal remanence magnetization (IRM) curves, based on the Stoner-Wohlfarth model combined with the Néel macrospin relaxation time description. We show how low temperature IRM curves, which have many advantages compared to hysteresis loops, can be efficiently computed for realistic assemblies of magnetic particles with both a size and anisotropy constant distribution, and a biaxial anisotropy. The IRM curves, which probe the irreversible switching provoked by an applied field, are shown to be complementary to other usual measurements (in particular low-field susceptibility curves where a thermal switching is involved). As an application, the experimental IRM curve of Co clusters embedded in a carbon matrix is analyzed. We demonstrate how powerful such an analysis can be, which in the present case allows us to put into evidence an anisotropy constant dispersion among the Co nanoparticles.
Physical Review B, 2010
The magnetic anisotropy energy distribution of a diluted assembly of CoPt nanoparticles, size-sel... more The magnetic anisotropy energy distribution of a diluted assembly of CoPt nanoparticles, size-selected around a diameter of 3 nm, is characterized by superconducting quantum interference device magnetometry. By comparison with unselected CoPt clusters and size-selected Co clusters, we show experimental evidence of a significant anisotropy constant dispersion, which is a nanoalloy specificity.
Physical Review B, 2006
In this present work, we report a structural and magnetic study of mixed Co 58 Pt 42 clusters. Mg... more In this present work, we report a structural and magnetic study of mixed Co 58 Pt 42 clusters. MgO, Nb and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission Electron Microscopy (TEM) observations show that Co 58 Pt 42 supported isolated clusters are about 2 nm in diameter and crystallized in the A1 fcc chemically disordered phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties which were investigated with conventional magnetometry and X-Ray Magnetic Circular Dichroïsm (XMCD). Magnetization and blocking temperature are weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in MgO is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic CoPt crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium which have a blocking temperature 3 times smaller than clusters embedded in MgO.
Physical Review B, 2013
We examine the effect of the applied magnetic field amplitude on zero field-cooled/field-cooled (... more We examine the effect of the applied magnetic field amplitude on zero field-cooled/field-cooled (ZFC/FC) curves, through the nonlinear susceptibility of superparamagnetic particles (i.e., at thermondynamic equilibrium, but taking into account the magnetic anisotropy). This nonlinear effect is shown to be the first to manifest itself when going away from the linear response regime (i.e., when the magnetic moment is simply proportional to the applied field), largely before the modification of the macrospin switching energy due to the external field. We demonstrate that it has a significant impact on ZFC/FC curves, especially for the low-temperature behavior of the FC curve, even in usual experimental conditions. We then show how this nonlinearity can be taken into account, in an easy way, to obtain a better modeling of the susceptibility curves and consequently a more reliable determination of the nanoparticles' magnetic properties. The theoretical considerations are confronted in a series of experimental measurements on Co nanoparticles.
Physical Review B, 2010
We report on the magnetic properties of Co clusters embedded in four different matrices ͑Ag, Au, ... more We report on the magnetic properties of Co clusters embedded in four different matrices ͑Ag, Au, Si, and amorphous carbon͒. The recently developed "triple fit" method for treating conventional magnetometry data allows, together with micro-superconducting quantum interference device ͑-SQUID͒ investigations, the detailed study of the influence of the surrounding matrix on the magnetic volume and the magnetic anisotropy of Co nanoparticles. While interdiffusion between matrix and Co atoms cannot be excluded in Si and amorphous C matrices, the structure of clusters embedded in the metallic matrices remains intact. Ag and Au matrices increase significantly the magnetic anisotropy energy of the Co clusters.-SQUID experiments indicate that the magnetic anisotropy of embedded clusters is not affected by a magnetically dead layer and that an anisotropy dispersion must be taken into account for size-selected nanoparticles.
Physical Review B, 2013
An annealing-driven transition from a chemically disordered A1-like structure to a chemically ord... more An annealing-driven transition from a chemically disordered A1-like structure to a chemically ordered L1 0-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios c/a are inferred. This ratio is found to be remarkably different depending on the approach used (c/a > 1 from x-ray absorption at the Co K edge and c/a 0.94 from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2003
In a previous work on 3 nm diameter Co clusters embedded in a Nb matrix, we suggested a core-shel... more In a previous work on 3 nm diameter Co clusters embedded in a Nb matrix, we suggested a core-shell model to account for the cluster structure. In this model, clusters are made up of a pure face-centered-cubic Co core surrounded with a “magnetically dead” CoNb alloyed interface. In the present work, we focus on the structure of this interface using different x-ray based analysis techniques to correlate structure and magnetism in such a nanogranular (Co)Nb system. As a model for the cluster/matrix interface, in situ measurements are also performed on a Co/Nb bilayer grown under an ultrahigh vacuum. No epitaxial growth of Nb on Co is achieved at room temperature. Therefore, we can conclude that magnetoelastic effects are too weak to explain the large magnetic anisotropy previously obtained with the microsuperconducting quantum interference device technique on a single Co cluster embedded in a Nb matrix. On the other hand, interface anisotropy, estimated from the Néel model, is in good ...
The Journal of Physical Chemistry Letters, 2011
The Journal of Chemical Physics, 1998
Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been pro... more Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2–3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination...
Journal of Applied Physics, 1990
The thermal behavior of X/C multilayers (nanometer-thick layers made of tungsten, nickel, or SiWS... more The thermal behavior of X/C multilayers (nanometer-thick layers made of tungsten, nickel, or SiWSi alternating with carbide or pure carbon) was studied. Two types of annealing were performed: the pulsed laser annealing in air and the classical thermal annealing in a vacuum furnace. Depending on the composition and the structure of the layered materials, thermal stability or diffusion mechanisms were observed and further analyzed by small-angle x-ray scattering, transmission electron microscopy, and Auger electron spectroscopy. The results show that the period expansion and the reflectivity evolution, that were observed in some cases after treatment, are caused both by structural changes into the layers and by exchange of matter between layers. These changes always induce a partial graphitization of the amorphous carbon and, in the case of W/C multilayers, the formation of a W2C compound.
Journal of Applied Physics, 1994
Clusters of iron, cobalt, and nickel are produced in a laser vaporization source. The size distri... more Clusters of iron, cobalt, and nickel are produced in a laser vaporization source. The size distributions of the incident clusters are checked by time-of-flight mass spectrometry before deposition at low energy. Studying the near threshold photoionization, Con and Nin clusters exhibit an icosahedral structure while for iron, no clear structure emerges. Neutral clusters were deposited on different substrates at room temperature with thicknesses up to 100 nm in view to determine their structure and magnetic properties. A limited coalescence of the clusters is observed from high-resolution transmission electron microscopy. No icosahedron has been observed but cuboctahedron and interface twins between adjacent particles have been clearly identified in Ni films. Grazing incidence x-ray diffraction experiments reveal a classical phase with grain size around 6 and 4 nm for Fe and Ni films, respectively but an anomalous fcc phase for Co films and a very low grain size of 2 nm. The density of...
Journal of Applied Physics, 1995
Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization rever... more Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization reversal of about 104 μB corresponding to a sensitivity of about 10−16 emu. The detector is a niobium micro-bridge DC superconducting quantum interference device (SQUID), fabricated using electron-beam lithography. It is operational in the temperature range of 0–7 K. Furthermore, we present a method to deposit on the SQUID loop a small number of Co clusters of about 2–5 nm in diameter. The first results obtained on these samples show that there is still a ferromagnetic coupling between the clusters and the magnetization reversal takes place by small avalanches.
Journal of Applied Physics, 2002
We study the temperature dependence of the magnetization switching fields of single nanoparticles... more We study the temperature dependence of the magnetization switching fields of single nanoparticles with an improved micro-superconducting quantum interference device (SQUID) technique, that allows us to perform measurements independently of the critical superconducting temperature of the SQUID. A simplified Néel–Brown model that describes the magnetization reversal by thermal activation over a single anisotropy barrier is compared to measurements on a 3 nm cobalt nanoparticle embedded in a niobium matrix.
Journal of Applied Physics, 1990
Multilayers consisting of alternating thin bilayers of W and Si (period: 1.5<d<9 nm) have b... more Multilayers consisting of alternating thin bilayers of W and Si (period: 1.5<d<9 nm) have been analyzed by x-ray scattering (absolute reflectivity, period, mosaicity, interface roughness, crystallinity, and density) and by cross-sectional transmission electron microscopy observations (periodicity, crystalline phase, and damaged area). Our purpose was to determine the thermal properties of the multilayers with respect to the period value under pulsed laser heating (with a nanosecond Nd-YAG laser at different energy densities up to 1 J/cm2 and at a wavelength λ=0.53 μm) and by furnace annealing (250<T<1000 °C under 10−7 Torr pressure). We propose that two distinct diffusion mechanisms are involved in annealings: first, interdiffusion in the amorphous phase and then crystallization into WSi2, the latter related to a period contraction of about 5–10%. The diffusion coefficients and the crystallization temperature depend drastically on the period value. Simulations of small-a...
Journal of Applied Physics, 2012
Experimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performe... more Experimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performed on samples prepared by physical and chemical pathways. By combining the triple fit method and a careful analysis of ac-experimental curves, we demonstrate an unambiguous and consistent determination method of both the magnetic particle size distribution and anisotropy for diluted granular nanostructures of magnetic clusters. Specifically, we highlight the importance of the size distribution in the determination of the magnetic anisotropy constant as well as the low relevance of the deduced parameters by considering alternative measurements alone.
Journal of Applied Physics, 2011
Thin films (submonolayer) obtained by deposition of size-selected CoPt clusters on graphite surfa... more Thin films (submonolayer) obtained by deposition of size-selected CoPt clusters on graphite surface have been synthesized. The preformed clusters can easily diffuse on the surface and gather to form “islands” or “bunches” of clusters. By changing the cluster size (in the range of 1.8 to 5.8 nm in diameter), very different morphologies can be obtained, going from large ramified islands to bunches of noncontacting clusters having the size of the initially deposited particles.
Journal of Applied Physics, 2011
We have synthesized diluted samples of CoPt clusters and investigated by various techniques the e... more We have synthesized diluted samples of CoPt clusters and investigated by various techniques the effect of a 2 h anneal at 750 K. Transmission electron microscopy observations have put into evidence L10 chemical ordering without any detectable coalescence upon annealing. Magnetic measurements on CoPt clusters embedded in amorphous carbon have been used to provide additional and stronger evidence of the absence of coalescence. We show how the analysis of normalized ZFC curves can be used as a convenient and powerful technique to detect subtle variations in the cluster size distribution.
Thin Solid Films, 1997
Silicon clusters prepared by low energy cluster beam deposition exhibit visible luminescence simi... more Silicon clusters prepared by low energy cluster beam deposition exhibit visible luminescence similar to that observed in porous silicon. The structure and properties are characterized by different complementary techniques with a special emphasis on Raman spectroscopy. The comparison with silicon clathrates structures is promising. The validity of the confinement model in a size range down to a few ångströms is discussed. q 1997 Elsevier Science S.A.
Physical Review B, 2011
We discuss the possibility of L1 0 chemical order parameter quantification for an individual part... more We discuss the possibility of L1 0 chemical order parameter quantification for an individual particle of CoPt, using transmission electron microscopy. While "usual" approaches are found to be unapplicable for small particles (less than 4 nm in diameter), we present a method based on the comparison between an experimental high-resolution image and simulated ones with various degrees of chemical order.
Physical Review B, 2008
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Physical Review B, 2013
We propose a theoretical framework enabling the simulation of isothermal remanence magnetization ... more We propose a theoretical framework enabling the simulation of isothermal remanence magnetization (IRM) curves, based on the Stoner-Wohlfarth model combined with the Néel macrospin relaxation time description. We show how low temperature IRM curves, which have many advantages compared to hysteresis loops, can be efficiently computed for realistic assemblies of magnetic particles with both a size and anisotropy constant distribution, and a biaxial anisotropy. The IRM curves, which probe the irreversible switching provoked by an applied field, are shown to be complementary to other usual measurements (in particular low-field susceptibility curves where a thermal switching is involved). As an application, the experimental IRM curve of Co clusters embedded in a carbon matrix is analyzed. We demonstrate how powerful such an analysis can be, which in the present case allows us to put into evidence an anisotropy constant dispersion among the Co nanoparticles.
Physical Review B, 2010
The magnetic anisotropy energy distribution of a diluted assembly of CoPt nanoparticles, size-sel... more The magnetic anisotropy energy distribution of a diluted assembly of CoPt nanoparticles, size-selected around a diameter of 3 nm, is characterized by superconducting quantum interference device magnetometry. By comparison with unselected CoPt clusters and size-selected Co clusters, we show experimental evidence of a significant anisotropy constant dispersion, which is a nanoalloy specificity.
Physical Review B, 2006
In this present work, we report a structural and magnetic study of mixed Co 58 Pt 42 clusters. Mg... more In this present work, we report a structural and magnetic study of mixed Co 58 Pt 42 clusters. MgO, Nb and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission Electron Microscopy (TEM) observations show that Co 58 Pt 42 supported isolated clusters are about 2 nm in diameter and crystallized in the A1 fcc chemically disordered phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties which were investigated with conventional magnetometry and X-Ray Magnetic Circular Dichroïsm (XMCD). Magnetization and blocking temperature are weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in MgO is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic CoPt crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium which have a blocking temperature 3 times smaller than clusters embedded in MgO.
Physical Review B, 2013
We examine the effect of the applied magnetic field amplitude on zero field-cooled/field-cooled (... more We examine the effect of the applied magnetic field amplitude on zero field-cooled/field-cooled (ZFC/FC) curves, through the nonlinear susceptibility of superparamagnetic particles (i.e., at thermondynamic equilibrium, but taking into account the magnetic anisotropy). This nonlinear effect is shown to be the first to manifest itself when going away from the linear response regime (i.e., when the magnetic moment is simply proportional to the applied field), largely before the modification of the macrospin switching energy due to the external field. We demonstrate that it has a significant impact on ZFC/FC curves, especially for the low-temperature behavior of the FC curve, even in usual experimental conditions. We then show how this nonlinearity can be taken into account, in an easy way, to obtain a better modeling of the susceptibility curves and consequently a more reliable determination of the nanoparticles' magnetic properties. The theoretical considerations are confronted in a series of experimental measurements on Co nanoparticles.
Physical Review B, 2010
We report on the magnetic properties of Co clusters embedded in four different matrices ͑Ag, Au, ... more We report on the magnetic properties of Co clusters embedded in four different matrices ͑Ag, Au, Si, and amorphous carbon͒. The recently developed "triple fit" method for treating conventional magnetometry data allows, together with micro-superconducting quantum interference device ͑-SQUID͒ investigations, the detailed study of the influence of the surrounding matrix on the magnetic volume and the magnetic anisotropy of Co nanoparticles. While interdiffusion between matrix and Co atoms cannot be excluded in Si and amorphous C matrices, the structure of clusters embedded in the metallic matrices remains intact. Ag and Au matrices increase significantly the magnetic anisotropy energy of the Co clusters.-SQUID experiments indicate that the magnetic anisotropy of embedded clusters is not affected by a magnetically dead layer and that an anisotropy dispersion must be taken into account for size-selected nanoparticles.
Physical Review B, 2013
An annealing-driven transition from a chemically disordered A1-like structure to a chemically ord... more An annealing-driven transition from a chemically disordered A1-like structure to a chemically ordered L1 0-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios c/a are inferred. This ratio is found to be remarkably different depending on the approach used (c/a > 1 from x-ray absorption at the Co K edge and c/a 0.94 from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2003
In a previous work on 3 nm diameter Co clusters embedded in a Nb matrix, we suggested a core-shel... more In a previous work on 3 nm diameter Co clusters embedded in a Nb matrix, we suggested a core-shell model to account for the cluster structure. In this model, clusters are made up of a pure face-centered-cubic Co core surrounded with a “magnetically dead” CoNb alloyed interface. In the present work, we focus on the structure of this interface using different x-ray based analysis techniques to correlate structure and magnetism in such a nanogranular (Co)Nb system. As a model for the cluster/matrix interface, in situ measurements are also performed on a Co/Nb bilayer grown under an ultrahigh vacuum. No epitaxial growth of Nb on Co is achieved at room temperature. Therefore, we can conclude that magnetoelastic effects are too weak to explain the large magnetic anisotropy previously obtained with the microsuperconducting quantum interference device technique on a single Co cluster embedded in a Nb matrix. On the other hand, interface anisotropy, estimated from the Néel model, is in good ...
The Journal of Physical Chemistry Letters, 2011
The Journal of Chemical Physics, 1998
Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been pro... more Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2–3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination...
Journal of Applied Physics, 1990
The thermal behavior of X/C multilayers (nanometer-thick layers made of tungsten, nickel, or SiWS... more The thermal behavior of X/C multilayers (nanometer-thick layers made of tungsten, nickel, or SiWSi alternating with carbide or pure carbon) was studied. Two types of annealing were performed: the pulsed laser annealing in air and the classical thermal annealing in a vacuum furnace. Depending on the composition and the structure of the layered materials, thermal stability or diffusion mechanisms were observed and further analyzed by small-angle x-ray scattering, transmission electron microscopy, and Auger electron spectroscopy. The results show that the period expansion and the reflectivity evolution, that were observed in some cases after treatment, are caused both by structural changes into the layers and by exchange of matter between layers. These changes always induce a partial graphitization of the amorphous carbon and, in the case of W/C multilayers, the formation of a W2C compound.
Journal of Applied Physics, 1994
Clusters of iron, cobalt, and nickel are produced in a laser vaporization source. The size distri... more Clusters of iron, cobalt, and nickel are produced in a laser vaporization source. The size distributions of the incident clusters are checked by time-of-flight mass spectrometry before deposition at low energy. Studying the near threshold photoionization, Con and Nin clusters exhibit an icosahedral structure while for iron, no clear structure emerges. Neutral clusters were deposited on different substrates at room temperature with thicknesses up to 100 nm in view to determine their structure and magnetic properties. A limited coalescence of the clusters is observed from high-resolution transmission electron microscopy. No icosahedron has been observed but cuboctahedron and interface twins between adjacent particles have been clearly identified in Ni films. Grazing incidence x-ray diffraction experiments reveal a classical phase with grain size around 6 and 4 nm for Fe and Ni films, respectively but an anomalous fcc phase for Co films and a very low grain size of 2 nm. The density of...
Journal of Applied Physics, 1995
Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization rever... more Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization reversal of about 104 μB corresponding to a sensitivity of about 10−16 emu. The detector is a niobium micro-bridge DC superconducting quantum interference device (SQUID), fabricated using electron-beam lithography. It is operational in the temperature range of 0–7 K. Furthermore, we present a method to deposit on the SQUID loop a small number of Co clusters of about 2–5 nm in diameter. The first results obtained on these samples show that there is still a ferromagnetic coupling between the clusters and the magnetization reversal takes place by small avalanches.
Journal of Applied Physics, 2002
We study the temperature dependence of the magnetization switching fields of single nanoparticles... more We study the temperature dependence of the magnetization switching fields of single nanoparticles with an improved micro-superconducting quantum interference device (SQUID) technique, that allows us to perform measurements independently of the critical superconducting temperature of the SQUID. A simplified Néel–Brown model that describes the magnetization reversal by thermal activation over a single anisotropy barrier is compared to measurements on a 3 nm cobalt nanoparticle embedded in a niobium matrix.
Journal of Applied Physics, 1990
Multilayers consisting of alternating thin bilayers of W and Si (period: 1.5<d<9 nm) have b... more Multilayers consisting of alternating thin bilayers of W and Si (period: 1.5<d<9 nm) have been analyzed by x-ray scattering (absolute reflectivity, period, mosaicity, interface roughness, crystallinity, and density) and by cross-sectional transmission electron microscopy observations (periodicity, crystalline phase, and damaged area). Our purpose was to determine the thermal properties of the multilayers with respect to the period value under pulsed laser heating (with a nanosecond Nd-YAG laser at different energy densities up to 1 J/cm2 and at a wavelength λ=0.53 μm) and by furnace annealing (250<T<1000 °C under 10−7 Torr pressure). We propose that two distinct diffusion mechanisms are involved in annealings: first, interdiffusion in the amorphous phase and then crystallization into WSi2, the latter related to a period contraction of about 5–10%. The diffusion coefficients and the crystallization temperature depend drastically on the period value. Simulations of small-a...
Journal of Applied Physics, 2012
Experimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performe... more Experimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performed on samples prepared by physical and chemical pathways. By combining the triple fit method and a careful analysis of ac-experimental curves, we demonstrate an unambiguous and consistent determination method of both the magnetic particle size distribution and anisotropy for diluted granular nanostructures of magnetic clusters. Specifically, we highlight the importance of the size distribution in the determination of the magnetic anisotropy constant as well as the low relevance of the deduced parameters by considering alternative measurements alone.
Journal of Applied Physics, 2011
Thin films (submonolayer) obtained by deposition of size-selected CoPt clusters on graphite surfa... more Thin films (submonolayer) obtained by deposition of size-selected CoPt clusters on graphite surface have been synthesized. The preformed clusters can easily diffuse on the surface and gather to form “islands” or “bunches” of clusters. By changing the cluster size (in the range of 1.8 to 5.8 nm in diameter), very different morphologies can be obtained, going from large ramified islands to bunches of noncontacting clusters having the size of the initially deposited particles.
Journal of Applied Physics, 2011
We have synthesized diluted samples of CoPt clusters and investigated by various techniques the e... more We have synthesized diluted samples of CoPt clusters and investigated by various techniques the effect of a 2 h anneal at 750 K. Transmission electron microscopy observations have put into evidence L10 chemical ordering without any detectable coalescence upon annealing. Magnetic measurements on CoPt clusters embedded in amorphous carbon have been used to provide additional and stronger evidence of the absence of coalescence. We show how the analysis of normalized ZFC curves can be used as a convenient and powerful technique to detect subtle variations in the cluster size distribution.