We use quaternion molecular dynamics to determine the equilibrium structure, the stability, and the dynamics of fragmentation of magnetic particle aggregates as a function of applied magnetic field and temperature. In a colloidal suspension the particles form complex one-dimensional structures. At low fields and beyond a critical size, rings are more stable than chains. Rings orient perpendicularly to an applied field and can be activated to fragment into chains. This activation barrier vanishes and rings fragment spontaneously at a higher critical field.

• Received 31 October 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3049