Magnetization and harmonic response for magnetite nanoparticles subject to an applied field of 10 mT, at 50 kHz. The effective relaxation time is calculated for each size using K = 25 kJ/m3 and a surfactant layer thickness of 23 nm. (a) M(t) vs. particle size, (b) Fourier spectrum of M(t) vs. particle size, (c) M(t) for increasingly broad particle size-distributions, where the median diameter is 14 nm, and (d) Fourier transform of M(t) including size-distribution.