Magnetic nanoparticles of cobalt ferrite have been synthesized by co-precipitation method. XRD and TEM analyses have been performed for structural characterization while for magnetic characterization VSM and PPMS have been used. A detailed study of the magnetic hysteresis loop was performed to elucidate the effects of particle sizes. Temperature dependence of coercivity in these nanoparticles followed a simple model of thermal activation of the particle moments over the anisotropy barriers while the saturation magnetization followed the modified Bloch. The results of size dependent saturation magnetization, coercivity and exchange bias have been explained in the light of the surface spin disorder in ferrite nanoparticles. Magnetic characterization of Ni-CoFe2O4 has been performed and explained with reference to the variation in doping concentration, surface spin disorder and the role of bulk and surface anisotropy. Blocking effects have been studied both in CoFe2O4 and NiFe2O4 systems and it was found that the blocking temperature increases with increasing particle size while it decreases with increasing nickel concentration in CoFe2O4 nanoparticles.