We report the finite size effect and nickel vacancy defects in NiO nanoparticles that result in the formation of magnetic phase separation in uncompensated antiferromagnetic NiO-cores with frustrated and disordered spins at the surface NiO-shell. The inter-particle interaction is probed by analyzing the relaxation dynamics measurements. A significant index for the interaction of n derived from the dynamic magnetization is proposed, which paves the way for the examination of the spontaneous exchange bias mechanism and offers insight into the influence of the particle size and defects.