The role of synthetic parameters in the magnetic behavior of relative large hcp Ni nanoparticles

The controllable synthesis of relatively large nickel nanoparticles via thermal decomposition of nickel acetate tetrahydrate in oleylamine in the presence of 1-adamantane carboxylic acid (ACA) and trioctylphosphine oxide (TOPO) is reported. High crystalline hcp nanoparticles of different sizes have been prepared at 290 °C, whereas at relative lower temperatures fcc are favored. The particle size was varying between 50 and 150 nm by properly adjusting the proportion of the capping ligands. TOPO-to-ACA ratio was also found to have an influence on the magnetic properties through the potential formation of a NiO shell. Pure hcp Ni nanoparticles over 50 nm in size served as models to illuminate the magnetic behavior of this metastable hexagonal Ni phase. Contrary to the net ferromagnetic characteristics of fcc Ni nanoparticles in the same size range, hexagonal structured particles exhibit superparamagnetic behavior at room temperature and a weak ferromagnetic contribution below 15 K.