The pattern formation of magnetic fluid films containing magnetic microdrops under applied perpendicular magnetic fields was investigated. At a zero field, these drops of size several μm were initially near-circular. As the field strength increased, these drops changed to dumbbells or branched structure. When the field strength increased again, a labyrinthine structure appeared. At even higher field strength, the connectivity of the drops broke up; and finally an equilibrium hexagonal structure formed inside the film. A new phase transition exists between the labyrinthine structure and the hexagonal structure, which has not been seen before. Also, the reverse process of pattern formation is irreversible.
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