The effect of trapping superparamagnetic beads on domain wall motion

Abstract

Domain walls may act as localized field sources to trap and move superparamagnetic beads for manipulating biological cells and DNA. The interaction between beads of various diameters and a wall is investigated using a combination of micromagnetic and analytical models. Domain walls can transport beads under applied magnetic fields, but the mutual attraction between the bead and wall causes drag forces affecting the bead to couple into the wall motion. Therefore, the interaction with the bead causes a fundamental change in the domain wall dynamics, reducing the wall mobility by five orders of magnitude.

Transport dynamics of superparamagnetic microbeads trapped by mobile magnetic domain walls

“…Fig. 4(d) plots the VV curves of two 2.8 μm diameter beads driven around CoFe ( =1910 kA m -1 ) 41 and NiFe ( =800 kA m -1 ) 20 rings of the same dimensions. The bead on the CoFe ring exhibits a (785 μm/s) higher than that of the bead on the NiFe ring, proportional to the ratio of between the two materials.…”

“…Once the bead is trapped in the potential well of the DW, the DW can be used to manipulate individual beads. Indeed, bead transport has been realized by either stepping a bead from one DW trap site to the next 19,[21][22][23][24]27 or moving it continuously with a propagating DW 20,22,25,26 . Continuous transport is limited, however, by the maximum interaction force, or binding force , between the bead and DW, which must overcome the hydrodynamic drag force on the bead as it is pulled through the host fluid 20 .…”

“…Following the model of Bryan et al 20 , maximum coupled transport speeds, , were estimated by equating with viscous drag, assuming Stokes form 6 where /2, with a viscosity, , of water (10 -3 Pa s) and a near surface correction factor = 2.2 for a bead dragged parallel to a surface 35 . As seen in Fig enhance the moment of the DW and thus increase bead-DW magnetostatic interaction and binding forces.…”

“…Although several estimates of the binding strength between a wall and trapped bead have been reported 19,20,23,24,27 , these calculations have generally been limited to model parameters which do not accurately represent the size and magnetic state of the bead-DW system. In this work, we use a combination of micromagnetic modeling and numerical calculation to predict bead-DW interaction forces for experimentally relevant geometries.…”

Nanowire spintronics for storage class memories and logic

“…Magnetic field sensors [66] are in the later stages of development for automotive applications. The magnetic field from the head-to-head DWs found in soft magnetic nanowires is being developed to control secondary systems such as nerve cells [67] and paramagnetic beads [68,69] in biology, and laser-cooled (less than 100 mK) paramagnetic atoms in atomic physics. This could lead to novel developments in tissue engineering, proteomics and quantum computation.…”