Momentum distribution of core and 3d electrons in mechanically strained NiO nanoparticles

Nickel oxide (NiO) nanoparticles was prepared using top-down method in the size range 12–70 nm via ball milling of their bulk. Bulk size NiO was prepared by heating of Ni(NO3)2.6H2O in air. This NiO powder was then milled in planetary ball mill keeping a fixed powder to ball ratio. Strain was introduced in the powder during milling process. The structure of milled samples was studied using x-ray diffraction (XRD) while Vibrating Sample Magnetometer (VSM) measurements were carried out to study magnetization in samples. The peaks in the x-ray diffraction (XRD) pattern correspond to pure NiO phase showing no trace of any other phase. Strained particles showed higher magnetization as compared to relaxed NiO nanoparticles. Positron Annihilation Coincidence Doppler Broadening (PACDB) spectroscopy was used to study the 3d and core electron momentum distribution in the vicinity of Ni atom. It is observed that as milling proceeds and the particles become more and more laminar the positrons get annihilated preferentially at the oxygen core. A relation between PACDB quotient ratio and mechanical strain is established in this study.