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We have fabricated a series of highly sensitive spin valve sensors on a micron scale that successfully detected the presence of a single superparamagnetic bead (Dynabeads M-280, 2.8 μm in diameter), and thus showed suitability for identifying biomolecules labeled by such magnetic beads. By polarizing the magnetic microbead on a spin valve sensor with a dc magnetic field and modulating its magnetization with an orthogonal ac magnetic field, we observed a magnetoresistance (MR) signal reduction caused by the magnetic dipole field from the bead that partially cancelled the applied fields to the spin valve. A lock-in technique was used to measure a voltage signal due to the MR reduction. A signal of 1.2 mV rms or 5.2 mΩ of resistance reduction was obtained from a 3 μm wide sensor and a signal of 3.8 mV rms or 11.9 mΩ from a 2.5 μm wide sensor. Micromagnetic simulations were also performed for the spin …