Break-up of dipolar rings under an external magnetic field

An experimental and theoretical study of the deformation and break-up proce ss of rings, formed by magnetic microspheres, under the application of an e xternal magnetic field is reported in this Letter. When the external magnet ic field is applied parallel to the plane of the rings, we found that the b reak-up process has three different outcomes depending on the way of applic ation and time history of the external held: (a) deformation into a compact set of dipoles with a triangular lattice structure, (b) opening into a sin gle chain, and (c) break-up into two chains with various relative sizes. A thorough theoretical investigation of the breakup process has been carried out based on computer simulations, taking into account solely the dipole-di pole and dipole-external field interactions, without thermal noise. The exp erimental results and the simulations are in good agreement. (C) 2000 Elsev ier Science B.V. All rights reserved.