Breakup of dipolar rings under a perpendicular magnetic field.

An experimental and theoretical study of the breakup process of rings, formed by magnetic microspheres, under the application of an external magnetic field perpendicular to the plane of the ring is presented. We found experimentally that when the value of the external magnetic field falls below a lower critical field the dipoles rotate in the ring without any distortion of the ring structure. However, exceeding the upper critical field causes sudden breakup of the ring into short chains aligned with the field. Between the lower and upper critical fields the system is in a metastable state, and hence, it is very sensitive to external perturbations. The spiral opening was found experimentally to be the lowest energy transition from the ring to the chain conformation. We worked out an analytic approach and we performed computer simulations, the results of which are in good agreement with experiments.