Magnetic microparticle trapping and mechanical excitation using domain walls in magnetic microstructures

Abstract

We examined the feasibility of using the resonant frequency of magnetic bead-domain wall (DW) couples in a host fluid to measure particle size. Nickel-Iron (Permalloy) rings, made using electron beam lithography, served as the tracks for nucleating and moving DWs, and Invitrogen Dynabeads M-270 magnetic beads were used for the experiment. Tween-20 surfactant in solution and SiO2 capping layers for the structures were used to overcome substrate-bead interaction and maintain bead mobility. The resonant frequency of 40 bead-DW couples was measured and found to lie in a range between 18.3 and 42.7 Hz with a median of 31.1 Hz. In addition, sets of resonance experiments were performed to examine the dependence of the resonant frequency on driving amplitude, DW type, and position on the permalloy (Py) ring. The resonant frequency populations of beads bound to head-head and tail-tail DWs overlapped, but each DW type seemed to be centered around a different frequency. Examining different positions on a ring showed that a large contribution to the spread in resonant frequencies may come from DW pinning due to structural defects or remanent surfacebead interaction. Finally, the resonant frequency is independent of the driving amplitude, a finding which supports the linear spring model for DW-bead interaction. We conclude that resonance measurements made with optical methods reliably distinguish particles of different hydrodynamic radius. This work has also helped identify and address some of the obstacles to improve the reliability of these resonance measurements as indicators of particle size. By demonstrating this detection capability, we can proceed with the development of spin-valve -based resonance devices suitable for clinical applications.