Nonlinear phenomena in systems of magnetic holes

Experimental studies are made of the response of bound pairs of magnetic holes (nonmagnetic microspheres in ferrofluid) subjected to a rotating magnetic field. For increasing driving frequency, the motion of the system goes through a transition from a state where the pair axis follows the magnetic field with a constant phase delay to a state where the phase delay increases in a series of kinks. The equation of motion for the rotating pair is found analytically and numerical solutions show good agreement with experiments.