The magnetic behavior of Ni-ferrite (NiFe2O4) nanoparticles synthesized in a solid state reaction process has been investigated. The cooling field, H-CF, dependence of magnetization in a wide range of temperatures, from 5 to 300 K, has been examined for low and high field regimes. It has been observed that there is a transition region, similar to 3-4 T, between different mechanisms that controls the magnetization. At low fields, 3 T, classical blocking-unblocking of small particles governs the magnetization whereas spin-glass like behavior prevails at high fields, >4 T, starting below a well defined freezing temperature of 50 K. The HCF dependence of magnetic viscosity has shown that there is a significant jump in the relaxation rate of the particles around 4 T which appears as the boundary region for the temperature-dependent magnetization as well. These observations are interpreted as indicating that below the spin freezing temperature there is a boundary field (similar to 4 T) where the strongly pinned surface spins are enabled to be thermally activated while below this field only core spins participate in the magnetic relaxation.