There is no time-dependence of the magnetization when the nanoparticles are either completely blocked () or completely superparamagnetic (). There is, however, a narrow window around  where the measurement time and the relaxation time have comparable magnitude. In this case, a frequency-dependence of the susceptibility can be observed. For a randomly oriented sample, the complex susceptibility is:
where
*  is the frequency of the applied field
*  is the susceptibility in the superparamagnetic state
*  is the susceptibility in the blocked state
*  is the relaxation time of the assembly
From this frequency-dependent susceptibility, the time-dependence of the magnetization for low-fields can be derived: