Colorize magnetic nanoparticles using a search coil based testing method

•We demonstrated to use the amplitude and phase information of both 3rd and 5th harmonics as magnetic “colors” of magnetic nanoparticles (MNPs).•An easier and simpler way to calibrate amounts of MNPs was developed.•With the same concentration, MNP solution with a larger average particle size could induce higher amplitude, and its amplitude changes greatly with sweeping high frequency.•At lower sweeping frequency, the 5 samples have almost the same phase lag. As the sweeping frequency goes higher, phase lag of large particles drop faster.

Different magnetic nanoparticles (MNPs) possess unique spectral responses to AC magnetic field and we can use this specific magnetic property of MNPs as “colors” in the detection. In this paper, a detection scheme for magnetic nanoparticle size distribution is demonstrated by using an MNPs and search-coils integrated detection system. A low frequency (50 Hz) sinusoidal magnetic field is applied to drive MNPs into saturated region. Then a high frequency sinusoidal field sweeping from 5 kHz to 35 kHz is applied in order to generate mixing frequency signals, which are collected by a pair of balanced search coils. These harmonics are highly specific to the nonlinearity of magnetization curve of the MNPs. Previous work focused on using the amplitude and phase of the 3rd harmonic or the amplitude ratio of the 5th harmonic over 3rd harmonic. Here we demonstrate to use the amplitude and phase information of both 3rd and 5th harmonics as magnetic “colors” of MNPs. It is found that this method effectively reduces the magnetic colorization error.