Nanosized corners for trapping and detecting magnetic nanoparticles

Abstract

We present a device concept based on controlled micromagnetic configurations in acorner-shaped permalloy nanostructure terminated with two circular disks, specificallydesigned for the capture and detection of a small number of magnetic beads in suspension.A transverse head-to-head domain wall (TDW) placed at the corner of the structure playsthe role of an attracting pole for magnetic beads. The TDW is annihilated in theterminating disks by applying an appropriate magnetic field, whose value is affected by thepresence of beads chemically bound to the surface. In the case where the beads are notchemically bound to the surface, the annihilation of the TDW causes their release intothe suspension. The variation of the voltage drop across the corner, due to theanisotropic magnetoresistance (AMR) while sweeping the magnetic field, is used todetect the presence of a chemically bound bead. The device response has beencharacterized by using both synthetic antiferromagnetic nanoparticles (disks of 70 nmdiameter and 20 nm height) and magnetic nanobeads, for different thicknesses ofthe protective capping layer. We demonstrate the detection down to a singlenanoparticle, therefore the device holds the potential for the localization and detection ofsmall numbers of molecules immobilized on the particle’s functionalized surface.