Counter-rotation of magnetic beads in spinning fields

Abstract: A magnetized bead in a magnetic field seeks to minimize its magnetic free energy by aligning its magnetic moment with the field direction and by moving towards the maximum of the field's intensity. However, when the bead is coupled to a substrate it is forced to roll. The two otherwise independent degrees of freedom, translation and rotation, become tightly coupled giving rise to subtle and often counterintuitive effects. Here we investigate one such, easily reproducible, yet stunning effect : A neodymium bead placed on top of a laboratory magnetic stirrer. When the stirrer's magnet spins at slow rates the bead naturally follows the field. However, surprisingly, at high spinning rates the bead suddenly inverts its direction and runs, to the surprise of the observer, in the opposite direction, against the driving field direction.This effect, experimentally investigated in [J.Magn.Magn.Matter, 476, 376-381, (2019)], is here comprehensively studied, with numerical simulations and a theoretical approach complementing experimental observations.