Detection of magnetic microbeads and ferrofluid with giant magnetoresistance sensors

Giant magnetoresistance sensors based on multilayers [Cu/NiFeCo]_×10/ Ta were fabricated by microfabrication technology. A GMR-bridge was used to detect the magnetic MyOne beads and Ferro fluid. The dependence of the GMR-bridge signals on the surface coverage of MyOne beads was studied. The results show that the GMR sensor is capable of detecting the magnetic beads. The detectable limit of MyOne beads is about 100, and the corresponding signal output is 8 μV. The GMR bridge signal is proportional to the surface coverage of the MyOne beads. The sensitivity of the GMR bridge is inversely proportional to the feature size of the GMR sensor. The GMR bridge integrated with microfludic channel was also used for dynamic detection of ferrofluid (suspension of Fe₃O₄ particles). The results show that the GMR bridge is capable of detecting the flow of ferrofluid, and the sensor signals are proportional to the concentration of the ferrofluid. The detection limit of concentration of the ferrofluid is 0.56 mg/ml, and the corresponding signal is 6.2 μV.