Single paramagnetic bead detection and direct measurement of the spatial magnetic resolution using novel nanocomposite Hall sensors

We show that nanocomposite materials formed by superparamagnetic nanoparticles embedded in a matrix are very good candidates for a novel generation of Hall nanosensors with high spatial magnetic resolution, capable of detection superparamagnetic beads suspended at different heights on top of the sensor. Two detection schemes were used: the one ex situ, using a combination of AC and DC orthogonal fields, with the bead placed on top of the sensor and the other one in situ, using nanomanipulation. The z spatial resolution can be determined directly from the in situ detection, by suspending the bead at different heights above the sensor, using only one magnetic field (AC) and nanomanipulation. The position of the bead is confirmed by direct electron microscope imaging. The Hall sensors were directly written by an electron beam and the active area was refined by focused ion beam milling below 200 nm, that allowed increasing the spatial magnetic resolution.