Ratchet effects for paramagnetic beads above striped ferrite-garnet films

TL;DR

This paper models the motion of paramagnetic beads above striped ferrite-garnet films to explore ratchet effects, enabling controlled directional transport and separation of beads by size through external magnetic field modulation.

Contribution

It introduces a comprehensive model for bead motion that demonstrates various ratchet phenomena, including height-dependent separation and tunable thresholds, advancing magnetic bead manipulation techniques.

Findings

Beads exhibit net directional motion under periodic external field modulation.

Height-dependent ratchet allows separation of beads by size.

The separation threshold can be tuned by adjusting the field amplitude.

Abstract

We calculate the motion of a small paramagnetic bead which is manipulated by the stripe domain pattern of a ferrite-garnet film. A model for the bead's motion in a liquid above the film is developed and used to look for ratchet solutions, where the bead acquires net coherent motion in one direction when the external field is modulated periodically. We consider three cases. First, the ratchet, where the beads all go in the same direction. Second, the height dependent ratchet, where beads at different heights go in opposite direction. This case can be used to separate beads of different sizes, as considered in J. Phys. Chem. B 112, 3833 (2008). Third, we describe how the separation threshold can be tuned by changing the amplitude of the applied field. Finally, we describe a pseudo ratchet, where the external modulation is not periodic and the ratchet changes direction periodically.