Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Cobalt oxide nanoparticle systems have been prepared by wet chemical processing involving the encapsulation of the nanoparticles by an organic ligand shell ͑oleic acid and oleylamine͒. CoO nanoparticles were easily prepared by this method, while the synthesis of the CoPt/ CoO nanocomposites was achieved using a two step polyol process. CoPt nanoparticles were first obtained by simultaneous reduction of cobalt acetate and platinum acetylacetonate and then used as seeds for the growth upon them of cobalt oxide using a second polyol process. The antiferromagnetic CoO nanoparticles, when field cooled to temperatures below 200 K, show displacement of the magnetization curves ͑along the magnetization axis͒ characteristic of weak ferromagnetism phenomena that are attributed to the uncompensated surface magnetic moments. The transition temperature of the particles is considerably lower than the Néel temperature of CoO and it is followed by an upswing at low temperatures, which is attributed to spins that are loosely coupled to the antiferromagnetic core. In the CoPt/ CoO nanocomposites, magnetic measurements show the appearance of increased coercivity with respect to the as-prepared CoPt particles and unidirectional anisotropy ͑loop shift of H eb = 1125 Oe͒ at temperatures below 20 K, as a result of exchange coupling between CoO and CoPt.

FePt nanoparticles have been encapsulated in insulating and protective MgO shells, using a two step chemical process, in order to prevent sintering during the heat-treatment process required for the L1 0 ordering. The FePt nanoparticles were initially prepared using a standard polyol process and then dispersed in a magnesium oxide solution. As a basis for comparison FePt/ SiO 2 nanocomposites have been also synthesized using a modified aqueous sol-gel route as the second step. The magnetic and microstructural properties of FePt/ MgO and FePt/ SiO 2 nanocomposites are compared with those of FePt nanoparticles. The presence of oxide matrices leads to more homogeneous microstructures and better magnetic properties. While higher coercivity values have been obtained in FePt/ SiO 2 , the MgO matrix is proven to provide better physical and magnetic isolations of the FePt nanoparticles. However, for FePt:MgO molar ratios exceeding 1:20 no L1 0 ordering has been achieved.

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Multiwalled carbon nanotubes (MWCNTs) were used as nanotemplates for the dispersion and stabilization of FePt nanoparticles (NPs). Pre-formed capped FePt NPs were connected to the MWCNTs external surface via covalent binding through organic linkers. Free FePt NPs and MWCNTs-FePt hybrids were annealed in vacuum at 700 °C in order to achieve the L10 ordering of the FePt phase. Both as prepared and annealed samples were characterized and studied using a combination of experimental techniques, such as Raman and Mössbauer spectroscopies, powder X-ray Diffraction (XRD), magnetization and transmittion electron microscopy (TEM) measurements. TEM measurements of the hybrid sample before annealing show that a fine dispersion of NPs along the MWCNTs surface is achieved, while a certain amount of free particles attached to each other in well connected dense assemblies of periodical or non-periodical particle arrangements is also observed. XRD measurements reveal that the FePt phase has the face-centered cubic (fcc) disordered crystal structure in the as prepared samples, which is transformed to the face-centered tetragonal (fct) L10 ordered crystal structure after annealing. An increase in the average particle size is observed after annealing, which is higher for the free NPs sample. Superparamagnetic phenomena due to the small FePt particle size are observed in the Mössbauer spectra of the as prepared samples. Mössbauer and magnetization measurements of the MWCNTs-FePt hybrids sample reveal that the part of the FePt particles attached to the MWCNTs surface shows superparamagnetic phenomena at RT even after the annealing process. The hard magnetic L10 phase characteristics are evident in the magnetization measurements of both samples after annealing, with the coercivity of the hybrid sample over-scaling that of the free NPs sample by a factor of 1.25.

High quality zinc oxide (ZnO) nanowires were grown on n-type Si (100) using vapor-liquid-solid process. We obtained the photoluminescence spectra of ZnO nanowires based on nonlinear optical process using an ultrashort wavelength femtosecond laser as a pumping source. The spectra shows the second harmonic generation phenomenon, as well as the exciton-exciton collision peak at 388 nm and the green emission peak at 515 nm caused by oxygen vacancy. A laser emission peak near 392 nm was observed when pump intensity surpassed 52 mJ/cm2 and a sharp peak about 0.5 nm wide emerged when the energy intensity reached 700 mJ/cm2. We attribute this excitation process to a two-photon absorption process enhanced by Rabi oscillation.