We developed a detection system for magnetic nanoparticles (MNPs) using the second-harmonic signal. By measuring the second-harmonic signal from MNPs, we can significantly reduce the interference of the excitation field, compared to the case of measuring the fundamental signal. First, we explored the properties of harmonic signals from MNPs when both a DC bias field and an AC field were applied. We obtained the relationship between the DC and AC fields under which the second harmonic reached its peaks. On the basis of these results, we developed a detection system for MNPs using a simple inductive pickup coil gradiometer. The sensitivity of the gradiometer was 7668 V/T and the minimum detectable field was 10 pT at a signal frequency of 38.6 kHz. We could detect 100 μg of MNPs located under the pickup coil at a distance of 30 mm. The position of MNPs was also detected from the contour map of the signal field. This simple detection system will be used in medical applications such as sentinel lymph node biopsy.
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