Micromagnetic simulation for detection of a single magnetic microbead or nanobead by spin-valve sensors

Micromagnetic simulation for detecting magnetic beads is performed by using a high-sensitivity spin-valve sensor as the detector. The magnetic beads polarized by a dc magnetic field contribute to a nonuniform dipole field, which can affect the magnetization state of spin-valve sensor, leading to a detectable resistance change. Simulations show that 2 mu m diam microbeads can be detected with similar to 98 mu V voltage change per bead, which is consistent with the experimental results (similar to 118 mu V per bead), indicating that single molecule detection with a micron-sized bead marker is possible. Detection of a single 250 nm nanobead shows a sensitivity of similar to 34 mu V, which is close to the present experimental noise level. This suggests that the detection of a single nanosized marker requires enhanced electronic signal by either reducing sensor dimensions or using high-moment ferromagnetic particles as labels. (C) 2006 American Institute of Physics.