Spin Valve Ring Sensors for Superparamagnetic Bead Detections

We have fabricated spin valve ring shape sensors for the detection of micrometer size magnetic beads. By using a ring shape spin valve sensors, we were able to detect the presence of 2.8 mum size magnetic beads by direct measurement of magnetic dipole fields from the magnetic beads. The abrupt transition position of ring sensor transfer curves was changed from the 5 Oe to -10 Oe due to the presence of the 12 magnetic beads. It was also found that the sensitivities of ring sensors without and with magnetic beads were obtained about 0.11 mV/Oe and 0.08 mV/Oe, respectively. Variations of the abrupt transition position and maximum output signal were explained by the magnetic bead stray field effect. Our results shows that ring shape spin valve magnetic sensors are promising for the detection of biomolecules with magnetic labels.