Showing papers by "Etienne Snoeck published in 2007"

Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach

Abstract: Key limitations of the colloidal semiconductor nanorods that have been reported so far are a significant distribution of lengths and diameters as well as the presence of irregular shapes produced by the current synthetic routes and, finally, the poor ability to fabricate large areas of oriented nanorod arrays. Here, we report a seeded-growth approach to the synthesis of asymmetric core−shell CdSe/CdS nanorods with regular shapes and narrow distributions of rod diameters and lengths, the latter being easily tunable up to 150 nm. These rods are highly fluorescent and show linearly polarized emission, whereby the emission energy depends mainly on the core diameter. We demonstrate their lateral alignment as well as their vertical self-alignment on substrates up to areas of several square micrometers.

Experimental application of sum rules for electron energy loss magnetic chiral dichroism

Abstract: We present a derivation of the orbital and spin sum rules for magnetic circular dichroic spectra measured by electron energy loss spectroscopy in a transmission electron microscope. These sum rules are obtained from the differential cross section calculated for symmetric positions in the diffraction pattern. Orbital and spin magnetic moments are expressed explicitly in terms of experimental spectra and dynamical diffraction coefficients. We estimate the ratio of spin to orbital magnetic moments and discuss first experimental results for the Fe ${L}_{2,3}$ edge.

Stabilization of metastable phases in spatially restricted fields: the case of the Fe2O3 polymorphs.

Abstract: In this work we show how the confinement of particles in a silica matrix with pores, acting as nano-vessels, plays an important role in the formation of pure e-Fe2O3 nanoparticles and their thermal stability. In particular, a HRTEM study of a series of Fe2O3–SiO2 xerogels annealed at different temperatures reveals that, at low temperatures, γ-Fe2O3 nanoparticles are formed and only transform to e-Fe2O3 after subsequent annealing at higher temperatures. These data are complemented by measurements of the SiO2 matrix porosity as well as with calorimetric and structural analysis of the nanoparticles after matrix removal. The gathered data indicate that the SiO2 matrix acts as a barrier, hindering thermal diffusion and particle growth even at the high temperatures where the formation of e-Fe2O3 appears to be favoured, thereby preventing its transformation to α-Fe2O3.

Magnetic easy-axis switching in Co/Pt and Co/Au superlattices induced by nitrogen ion beam irradiation

Abstract: In this paper, the sputtering-grown [Au 6 nm/Co 0.5 nm] × 10 and [Pt 6 nm/Co 0.5 nm] × 15 superlattices are irradiated using N + 150 keV ions with fluences ranging from 10 12 up to 3 × 10 16 N + /cm 2 . The effect of this ion irradiation experiment on perpendicularly magnetized Co/Au and Co/Pt superlattices is presented. Combining structural and magneto-optical measurements, we reveal the differences in the ion induced mixing between miscible Co–Pt and immiscible Co–Au layered systems. For Co/Pt ones, the progressive ballistic intermixing between Co and Pt leads to a decrease of perpendicular anisotropy and to an in-plane reorientation of the easy axis of magnetization. Although, for Co/Au superlattices the mixing occurs but a thermodynamic phase separation is observed. The structural investigations show that for high fluences the Co layers are disrupted in islands with few nanometer size embedded in Au layers. The perpendicular magnetic anisotropy decreases with fluence but no anisotropy reorientation appears.

Influence of interface structure on the tunnelling spin polarization of CoFeB alloys

Abstract: We present measurements of tunnelling spin polarization (TSP) of CoFeB alloys in Al/AlOx/CoFeB tunnel junctions using superconducting tunnelling spectroscopy. We observe that though Co72Fe20B8 (at.%) layers crystallize in the fcc structure, in junctions annealed above 300 °C the measured TSP degrades, indicating that in these junctions the TSP of amorphous CoFeB is higher than its crystalline counterpart.

Multiple-length-scale small-angle X-ray scattering analysis on maghemite nanocomposites

Abstract: Small-angle X-ray scattering (SAXS) analysis has been performed on maghemite–poly(4-vinylpyridine) nanocomposites prepared by in situ precipitation from iron–polymer coordination compounds. According to electron microscopy observations, the nanocomposites contain isolated spherical particles with a narrow size distribution, uniformly distributed throughout the polymer matrix. The scattering intensity of nanocomposites has relevant contributions from both the polymer and the nanocomposites, showing features characteristic of multiscale structured systems, namely two power laws and a Guinier regime. The data have been analysed in terms of Beaucage's unified approach and it is found that the maghemite particle size increases with the iron/polymer weight ratio used in the preparation of the nanocomposites. SAXS curves also feature a bump that was analysed as arising from a second particle population or from interactions. Magnetization and transmission electron microscopy results give arguments favouring the latter interpretation. It is found that the maghemite particle sizes vary linearly with the iron weight ratio used in the preparation of the nanocomposites.

Magnetic Chiral Dichroism Studies using Energy Filtered Images

Abstract: Magnetic thin films of Co80Cr16Ta4 were sputtered onto identical CrMo seed-layers at -200 V bias and 3 different substrate temperatures (150, 200, and 250°C). Energy-filtered transmission electron microscopy (EFTEM) was performed to analyze Cr levels at the grain boundaries as well as inside the grains. These quantitative Cr measurements were used to estimate the local values of magnetocrystalline anisotropy (Ku) and, together with grain size distributions, calculate the product of Ku and the grain volume (KuV), a quantity which is a measure of thermal stability. The results show that the coercivity as well as the percentage of stable grains increased with increasing substrate temperature. The increase in the number of stable grains is produced by the enhancement in the Ku value from Cr depletion of the grain interiors and the magnetic decoupling between the grains from Cr grain boundary segregation.

Determination of strain within Si1-yCy layers grown by CVD on a Si substrate

Abstract: In this work, we performed quantitative measurements of strain in structures consisting of a 30 nm-thick Si1-yCy layer grown by chemical vapour deposition (CVD) on a Si (001) substrate at 550 or 600°C. The total C concentration varies from 0.67 to 1.97% that was measured by SIMS. Geometric phase analysis (GPA) of high resolution transmission electron microscopy (HR TEM) cross-section images and convergent beam electron diffraction (CBED) were used to deduce the strain within these Si1-yCy layers. Finite-element simulations were carried out to estimate the impact of strain relaxation in thin areas of a specimen. These results were compared with the data obtained by high resolution X-ray diffraction and Raman spectroscopy and with the predictions of elasticity theory. Particular interest is paid to the formation of the structural defects within Si1-yCy layers as a function of a C concentration, growth temperature and incorporated strain. Both cross-sectional and plan-view TEM specimen configurations were used to obtain quantitative information on the defect size distribution, their density and structure.

Interlayer Exchange Coupling in Single Crystal Magnetic Tunnel Junctions Studied by Electron Holography

Abstract: Ion irradiation has been performed on epitaxial Fe/MgO/Fe magnetic tunnel junctions exhibiting antiferromagnetic exchange coupling. Electron holography and VSM experiments show that the amplitude and the nature of the coupling are modified by the irradiation. We demonstrate that the roughness induced by the irradiation at the Fe/MgO interfaces destroys the AF coupling. The biquadratic coupling experimentally observed in the irradiated samples is the result of the competition between local antiferromagnetic and ferromagnetic coupling. For the largest irradiation doses, the net coupling becomes ferromagnetic.

Impact of interface crystallization on inelastic tunneling in Al∕AlOx∕CoFeB

Abstract: We report the change in inelastic electron tunneling spectra (IETS) for Al∕AlOx∕CoFeB∕Al junctions when the structure of CoFeB at its interface with AlOx is intentionally changed from quasiamorphous to highly textured fcc. While for the quasiamorphous interface there are signs of the size quantization of magnons, the spectra for the fcc interface show distinct excitations at bias voltages associated with known surface magnon modes in fcc Co. These results demonstrate that IETS can be used as a tool to probe distinct structural changes of the magnetic electrode in tunnel junctions.