Quantitative digital detection of magnetic beads using pseudo-spin-valve rings for multiplexed bioassays
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(Color online) Simulations and measured MR response of PSV ring. Micromagnetic simulations (using OOMMF, http://math.nist.gov/oommf) show magnetization states of a PSV ring with linewidth as the Py layer is cycled from reverse onion (A) to forward onion state (B) and back, as indicated by the arrows. MR measurements on an actual microfabricated ring confirm that the reverse onion state of the Co layer remains largely undisturbed during the cycling, giving rise to minimum (A) and maximum (B) MR levels. The plateaus correspond to vortex states (C) in the Py layer obtained on the ascending and descending field sweeps.
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(Color online) Results of bead detection. (a) Schematic showing the induced dipole field from a single magnetic bead partially cancelling the externally applied field over the ring. (b) Minor hysteresis loops of the PSV ring taken in both presence (red open circles) and absence (black solid squares) of the bead after saturating the ring at . [(c) and (d)] A single M-450 Dynabead® in solution is isolated and positioned by a bar magnet over the center of the ring. Scale bars are . [(e) and (f)] MR vs time for a critically balanced loop in the absence and presence of the bead, respectively. The null signal obtained for this field range identifies one M-450 Dynabead®.
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