Size confined magnetic phase in NiO nanoparticles

The question. of. whether NiO nanoparticles contain metallic ferromagnetic Ni clusters is still. a matter of. debate, and it is very important from. an application point of view. Resolving this problem relies on proper detection probes with high sensitivity and a systematic analysis that would be demonstrated in this study. NiO nanoparticles with mean size ranging from similar to 4 to 80 nm are synthesized by sol-gel method. Synchrotron x-ray diffraction, transmission electron microscopy, Raman and ferromagnetic resonance (FMR) spectroscopy are used to study the size effects on the structures and magnetic properties of nanoparticles. It is found that a minor Ni phase below 1% in NiO nanoparticles is traceable with synchrotron XRD, selective area electron diffraction and static/dynamic magnetic measurements. The Ni phase only exists in NiO nanoparticles with the size ranging from 8 to 20 nm, attributed to the oxygen vacancies in core structure. Our findings provide important information for controlling the magnetic properties of NiO nanoparticles.