Domain structure and force analysis of micro-magnetic patterns for bio-functionalized bead actuation

Abstract

One of the important items for the determination of the macroscopic behavior of the thin magnetic film is the arrangement of magnetic domains as a function of an applied external field. These domain structures of the magnetic thin films are observed by the Magnetic transmission x-ray microscopy. The magnetic transmission x-ray microscopy is a novel technique to image element specifically magnetic domain structures. As the element-specific magnetic contrast which is due to x-ray magnetic circular dichroism scales with the projection of the magnetization onto the photon propagation direction both out-of-plane and in-plane magnetic domain structures can be studied. While observing the domain structures, enhance magnetic contrast and eliminates non-magnetic background so that, each image was normalized to a reference image taken at saturation state. Using magnetic transmission x-ray microscopy, we characterized the domain structure of the magnetic patterns including half disk and full disk and observed the vortex state of the magnetic pattern in demagnetized state. The change in vortex state for the magnetic patterns by changing the applied filed is also measured by x-ray microscopy. Furthermore, the demagnetized states for the full disk micro-magnet were determined by OOMMF simulation which is good agreement with the magnetic domain structure measured by X-ray microscopy. In addition, the non-linear dynamics of superparamagnetic beads moving around the periphery of patterned magnetic disks in the presence of an in-plane rotating magnetic field was studied here. Two different dynamical regimes are observed in experiments, including (1) phase-locked motion at maximum driving frequencies, (2) phase-slipping motion above a first critical frequency fc1. The force calculations, Phase-locked and Phase-slipping angles are calculated by the superparamagnetic bead around a disk micro-magnet under an applied in-plane rotating field in clockwise direction. Due to the magnetic field gradient produced by the micro-magnets, the beads can be trapped at the location of high induced field. Under the rotating field, there could be a phase lag of bead from the field direction, and the schematics for governing forces on a moving bead are discussed. ⓒ 2016 DGIST