29 research outputs found
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 candidate 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 & 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+ oxidation state and indicate a local magnetic
moment of about 4.5 {\mu}B/Mn. Magnetic measurements showed that compounds up
to 10% of Mn doping are paramagnetic with antiferromagnetic interactions.
However, higher Mn doped compound exhibits local ferrimagnetic ordering. Thus,
no ferromagnetism has been observed for all Mn-doped tetragonal ZrO2 samples.Comment: 20 pages, 4 figure
YAG nano-light sources with high Ce concentration
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
Bottom-up strategies for the assembling of magnetic systems using nanoclusters
International audienceIn the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context twenty, years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the Low Energy Cluster Beam Deposition (LECBD) technique. In this paper, we focus our attention on the typical case of Co clusters embedded in various matrices to study interface magnetic anisotropy and magnetic interactions as a function of volume concentrations, and on still current and perspectives through two examples of binary metallic 3d-5d TM (namely CoPt and FeAu) clusters assemblies to illustrate size-related and nanoalloy phenomena on magnetic properties in well-defined mass-selected clusters. The structural and magnetic properties of these cluster assemblies were investigated using various experimental techniques that include High Resolution Transmission Electron Microscopy (HRTEM), Superconducting Quantum Interference Device (SQUID) magnetometry, as well as synchrotron techniques such as Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Magnetic Circular Dichroism (XMCD). Depending on the chemical nature of both NPs and matrix, we observe different magnetic responses compared to their bulk counterparts. In particular, we show how finite size effects (size reduction) enhance their magnetic moment and how specific relaxation in nanoalloys can impact their magnetic anisotropy
Correlation between the Extraordinary Hall Effect and Resistivity
We study the contribution of different types of scattering sources to the
extraordinary Hall effect. Scattering by magnetic nano-particles embedded in
normal-metal matrix, insulating impurities in magnetic matrix, surface
scattering and temperature dependent scattering are experimentally tested. Our
new data, as well as previously published results on a variety of materials,
are fairly interpreted by a simple modification of the skew scattering model
Non-magnetic impurity induced magnetism in rutile TiO2:K compounds
International audienceRecent ab initio studies have theoretically predicted room temperature ferromagnetism in several oxide materials of the type AO2 in which the cation A4+ is substituted by a non-magnetic element of the 1 A column. Our purpose is to address experimentally the possibility of magnetism in Ti1−xKxO2 compounds. The samples have been synthesized via the solid state route method at equilibrium. Our study has shown that Ti1−xKxO2 is thermodynamically unstable and leads to a phase separation, in contradiction with the hypothesis of ab initio calculations. In particular, the crystalline TiO2 grains appear to be surrounded by K-based phase. The oxidization state of the Ti ion is found to be in Ti4+ as confirmed from the x-ray photoelectron spectra measurement. Nevertheless, K:TiO2 compounds exhibit weak paramagnetism with the highest magnetic moment of ~0.5 μB K−1 but no long-range ferromagnetic order. The observed moment in these compounds remains much smaller than the predicted moment of 3 μB by ab initio calculation. The apparent contradictions between our experiments and first-principles studies are discussed
ZnO nanostructures for Mid-IR plasmonics
13-17 juin 2016International audienceno abstrac
Tunable mid IR plasmon in GZO nanocrystals
International audienceDegenerate metal oxide nanoparticles are promising systems to expand the significant achievements of plasmonics into the infrared (IR) range. Among the possible candidates, Ga-doped ZnO nanocrystals are particularly suited for mid IR, considering their wide range of possible doping level and thus of plasmon tuning. In the present work, we report on the tunable mid IR plasmon induced in degenerate Ga-doped ZnO nanocrystals. The nanocrystals are produced by a plasma expansion and exhibit unprotected surfaces. Tuning the Ga concentration allows tuning the localized surface plasmon resonance. Moreover, the plasmon resonance is characterized by a large damping. By comparing the plasmon of nanocrystals assemblies to that of nanoparticles dispersed in an alumina matrix, we investigate the possible origins of such damping. We demonstrate that it partially results from the self-organization of the naked particles but also from intrinsic inhomogeneity of dopants