Aggregation kinetics and stability of structures formed by magnetic microspheres

Stable rings formed by magnetization-controllable microspheres at zero fiel d are observed and reported in this Rapid Communication. The magnetic micro spheres were made by plating glass beads with a critical thickness of Ni fi lm. We found that the ring leading to magnetic flux closure is the most sta ble configuration. At high concentrations, all individual rings, chains, an d clusters join together to form a netlike structure. A computer simulation based on the dipole-dipole interaction, without thermal noise, has been ca rried out, and the results are in good agreement with the experimental obse rvations. Based on an analytic approach we give a simple explanation of the formation and stability of rings. [S1063-651X(99)50105-0].