There is no time-dependence of the magnetization when the nanoparticles are either completely blocked ( T ≪ ≪ T B ) or completely superparamagnetic ( T ≫ ≫ T B ). There is, however, a narrow window around T B 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: