  We report studies on magnetization dynamics in NiO nanoparticles of average
size 5 nm. Temperature and time dependence of dc magnetization, wait time
dependence of magnetic relaxation (aging) and memory phenomena in the dc
magnetization are studied with various temperature and field protocols. We
observe that the system shows memory and aging in field cooled and zero field
cooled magnetization measurements. These experiments show that the magnetic
behavior of NiO nanoparticles is similar to spin glasses. We argue that the
spin glass behavior originates from the freezing of spins at the surface of the
individual particles.
