International audienceNi- and Co-substituted ZnO nanoparticles were synthesized using forced hydrolysis of acetate metallic salts in a polyol medium. The X-ray diffraction patterns show a hexagonal wurtzite structure (space group P63mc). The characteristic absorption bands of UV-vis-IR spectra are correlated with the d-d transitions of tetrahedrally coordinated Co2+ and Ni2+ ions in octahedral and tetrahedral sites. The photoluminescence spectra exhibited a typical ZnO UV-exitonic emission band around 380 nm and a broad band between 400 and 500 nm that might be ascribed to the intrinsic defects in the ZnO material. The transmission electron microscopy displays spherical particles with a diameter between 20 and 30 nm. The magnetic measurements reveal that Zn1-xNixO and Zn1-xCoxO nanoparticles show, respectively, ferromagnetic and paramagnetic behavior at 5 K. Homogeneous distributions of Co and Ni ions in the particles observed by filter imaging analysis indicates that there is no evidence of Co or Ni metal throughout the powders
Cobalt nano-rods with the hexagonal close-packed (hcp) structure were prepared by reduction of the long-chain carboxylate Co (II) precursor in polyol. The application of an external magnetic field (µ0H = 1.25 T) during the nucleation and growth steps resulted in a noticeable modification of the mean aspect ratio (length/diameter) of the particles. The particle morphology was also modified as the nano-rods did not exhibit conical heads at their extremities anymore, which are observed for particles prepared without application of an external magnetic field. Besides, the stacking faults density along the c axis of the hcp structure in the cobalt nano-rods has been found to decrease with the increase in the applied magnetic field. The coercive field of randomly oriented nano-rods increased with the aspect ratio, showing the highest value (i.e., 5.8 kOe at 300 K) for the cobalt nano-rods obtained under the highest applied magnetic field. For partially oriented Co nano-rods in toluene solution, the magnetic properties were significantly enhanced with a coercive field of 7.2 kOe at 140 K, while the magnetization saturation reached 92% of the bulk. The MR/MS value was about 0.8, indicating a good orientation of the anisotropic particles relative to each other, making them suitable for the preparation of permanent magnets via a bottom-up approach.
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