Scanning probe measurements on a magnetic bead biosensor

We experimentally demonstrate the sensitivity of an integrated detection scheme for small superparamagnetic beads, intended for medical diagnostic applications. Detection is based on the giant magnetoresistance effect of a 100x3 mu m(2) magnetic multilayer strip. A conductive wire to magnetize the superparamagnetic beads is integrated on the same substrate. By scanning a single bead over the wires and sensor strip using an atomic force microscope, we simultaneously measure topography and sensor resistivity in a three-dimensional volume above the sensor. The observations can be explained well by means of the macroscopically measured sensor resistivity curve and the magnetization of the beads, combined with the Biot-Savart law for the magnetic field of the wire. From these encouraging results, we project that it is possible to detect even a single 300 nm superparamagnetic bead on our sensor.