O. Boisron

Demixing in cobalt clusters embedded in a carbon matrix evidenced by magnetic measurements

We report on the magnetic properties of cobalt clusters embedded in amorphous carbon using magnetic and structural investigations. From the analysis of the mFC, mZFC, and m(H) curves, we determine the Co magnetic diameter probability density function. An initially magnetically dead interface layer is attributed to a metastable carbide. We found that annealing at 750 K favors the graphitization of the matrix and removes the dead layer without deteriorating the nanoparticle size distribution or changing the magnetic anisotropy constant.

Absence of ferromagnetism in Mn-doped tetragonal zirconia

In a recent letter, it has been predicted within first principle studies that Mn-doped ZrO2 compounds could be good candidates for spintronics application because expected to exhibit ferromagnetism far beyond room temperature. Our purpose is to address this issue experimentally for Mn-doped tetragonal zirconia. We have prepared polycrystalline samples of Y0.15(Zr0.85−yMny)O2 (y = 0, 0.05, 0.10, 0.15, 0.20) by using standard solid state method at equilibrium. The obtained samples were carefully characterized by using x-ray diffraction, scanning electron microscopy, elemental color mapping, x-ray photoemission spectroscopy, and magnetization measurements. From the detailed structural analyses, we have observed that the 5% Mn doped compound crystallized into two symmetries (dominating tetragonal and monoclinic), whereas higher Mn doped compounds are found to be in the tetragonal symmetry only. The spectral splitting of the Mn 3s core-level x-ray photoelectron spectra confirms that Mn ions are in the Mn3+ oxi...

YAG:Ce nanoparticle lightsources

We investigate the luminescence properties of 10 nm yttrium aluminum garnet (YAG) nanoparticles doped with Ce ions at 0.2%, 4% and 13% that are designed as active probes for scanning near-field optical microscopy. They are produced by a physical method without any subsequent treatment, which is imposed by the desired application. The structural analysis reveals the amorphous nature of the particles, which we relate to some compositional defects as indicated by the elemental analysis. The optimum emission is obtained with a doping level of 4%. The emission of the YAG nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder whereas the 13% doped particles hardly exhibit any luminescence. In the latter case, the presence of Ce(4+) ions is confirmed, indicating that the Ce concentration is too high to be incorporated efficiently in YAG nanoparticles in the trivalent state. By a unique procedure combining cathodoluminescence and Rutherford backscattering spectrometry, we demonstrate that the enhancement of the particle luminescence yield is not proportional to the doping concentration, the emission enhancement being larger than the Ce concentration increase. Time-resolved photoluminescence reveals the presence of quenching centres likely related to the crystalline disorder as well as the presence of two distinct Ce ion populations. Eventually, nano-cathodoluminescence indicates that the emission and therefore the distribution of the doping Ce ions and of the defects are homogeneous.We investigate the luminescence properties of 10 nm YAG nanoparticles doped with Ce ions at 0.2%, 4% and 13% that are designed as active probes for Scanning Near field Optical Microscopy. They are produced by a physical method without any subsequent treatment, which is imposed by the desired application. The structural analysis reveals the amorphous nature of the particles, which we relate to some compositional defect as indicated by the elemental analysis. The optimum emission is obtained with a doping level of 4%. The emission of the YAG nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder whereas the 13% doped particles hardly exhibit any luminescence. In the latter case, the presence of Ce4+ ions is confirmed, indicating that the Ce concentration is too high to be incorporated efficiently in YAG nanoparticles in the trivalent state. By a unique procedure combining cathodoluminescence and Rutherford backscattering spectrometry, we demonstrate that the enhancement of the particles luminescence yield is not proportional to the doping concentration, the emission enhancement being larger than the Ce concentration increase. Time-resolved photoluminescence reveals the presence of quenching centres likely related to the crystalline disorder as well as the presence of two distinct Ce ions populations. Eventually, nano-cathodoluminescence indicates that the emission and therefore the distribution of the doping Ce ions and of the defects are homogeneous.

Non-magnetic impurity induced magnetism in rutile TiO2:K compounds

Recent ab initio studies have theoretically predicted room temperature ferromagnetism in several oxide materials of the type AO(2) in which the cation A(4+) is substituted by a non-magnetic element of the 1 A column. Our purpose is to address experimentally the possibility of magnetism in Ti(1-x)K(x)O(2) compounds. The samples have been synthesized via the solid state route method at equilibrium. Our study has shown that Ti(1-x)K(x)O(2) is thermodynamically unstable and leads to a phase separation, in contradiction with the hypothesis of ab initio calculations. In particular, the crystalline TiO(2) grains appear to be surrounded by K-based phase. The oxidization state of the Ti ion is found to be in Ti(4+) as confirmed from the x-ray photoelectron spectra measurement. Nevertheless, K:TiO(2) compounds exhibit weak paramagnetism with the highest magnetic moment of ~0.5 μ(B) K(-1) but no long-range ferromagnetic order. The observed moment in these compounds remains much smaller than the predicted moment of 3 μ(B) by ab initio calculation. The apparent contradictions between our experiments and first-principles studies are discussed.