Ac susceptibility of iron-nitride magnetic fluids with various particle number densities was measured. Therelaxation time increases rapidly as the temperature decreases or the inter-particle interaction increases. The analysis of the data suggests that the activation energy is proportional to ${(k_{B}T/J_{typ})}^{alpha}$ with $alpha$~-0.24 in the lower temperature range in which the thermal energy is comparable to the magnetic dipole interaction.