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 mom...