Single Superparamagnetic Bead Detection and Direct Tracing of Bead Position Using Novel Nanocomposite Nano-Hall Sensors

We show that nanocomposite materials consisting of superparamagnetic nanoparticles embedded in a matrix are very good candidates for a novel generation of Hall nanosensors with high spatial and magnetic resolution, capable of detecting superparamagnetic beads suspended at different heights on top of the sensor. Two detection schemes were used: 1) static-the bead was centered on top of the sensor and detected with a combination of ac and dc orthogonal magnetic fields, and 2) dynamic -using nanomanipulation in an electron microscope for moving the bead precisely at different heights above the sensor and using only one magnetic field (ac). The Hall sensors were directly written by focused-electron-beam-induced deposition and the active area was refined by focused ion beam milling. A magnetic field resolution of about 300 μT(Hz)^-1/2 and a spatial resolution of 230 nm(Hz)^-1/2 were measured. Both resolutions could be improved by at least an order of magnitude with the suppression of the parasitic inductive signals.