Single molecule detection with magnetic beads - computer simulation

Single molecules e.g. in biological experiments can be detected by labeling with magnetic beads instead of the otherwise used dyes and by analyzing the influence of the beads on a GMR- or TMR-multilayer. The development of suitable sensors has been studied experimentally as well as theoretically by computer simulation in order to optimize the sensor parameters. The local field method with the dipole fields of the beads as additional contributions to the local field H/spl I.oarr//sub ijk/ has been used in the simulations. The bead fields produce vortex like arrangements within the spin ensemble of a magnetic layer. There are 2 important fields: H/sub e/ (in plane) and H/sub 0/ (out of plane). Paramagnetic beads show an induced dipole moment /spl mu/B = m/sub B/H/sub 0/, therefore /spl mu//sub B/ increases with H/sub 0/ and H/sub 0/ = 0 corresponds to the case without beads. Theoretical graphs are presented, which demonstrate some basic dependencies for a GMR-sensor (NiFe/Cu/NiFe).