Detection and susceptibility measurements of a single Dynal bead

In this work we present detection and susceptibility measurement experiments on a single superparamagnetic Dynal bead with a diameter of 1 μm and a magnetic moment of $≈4×108μB$⁠. Accurate bead positioning was achieved via non-invasive AFM nanomanipulation. The detection and magnetic characterization of the bead were performed using ultra-sensitive InSb Hall devices. Single bead detection was demonstrated using a step-wise change of the dc magnetic field; measurements were performed using only the in-phase component of the total ac Hall voltage. Very clear evidence of the bead presence is demonstrated simultaneously with explicit separation of parasitic inductive signals. Additional experiments performed using a sweeping change of the dc field allowed susceptibility measurements of a single Dynal bead. The numerical outcomes of both sweeping and stepping experiments are in a very good agreement. The method presented here opens up new possibilities for the reliable and accurate detection of small magnetic moments, which is of high importance for metrological applications as well as highly sensitive biological, medical, and environmental detectors.