Evidence of multiple states of ordered structures and a phase transition in magnetic fluid films under perpendicular magnetic fields

When a magnetic fluid thin film is subjected to a perpendicular magnetic fi eld H, the particles in the film agglomerate and form particle columns. Wit h the increasing H, the columns evolve from a disordered column phase to th e first-level hexagonal structural pattern and finally reach the second-lev el hexagonal structural pattern through a phase transition. During the tran sition phase, each column split into two columns. The split of a column may be attributed to the further alignment of the magnetic particles in each c olumn under higher H's. Hence the dipolar repulsive force in each column be comes dominant and causes the split of a column. The formation of the secon d-level hexagonal structure reveals that the minimum energy of the equilibr ium system of magnetic fluid thin films under perpendicular magnetic fields has multiple local minima when H varies. (C) 1999 American Institute of Ph ysics. [S0003-6951(99)01241-3].