A hexagonal ordered structure of magnetic columns, which results from an agglomeration of magnetic particles, is obtained in a magnetic fluid film when a magnetic field is applied perpendicularly to the film surface. The evolution of the initially ordered structure in the magnetic fluid film during the heating and cooling process is investigated under a given magnetic field. For the heating process, the columns remain unchanged until the temperature exceeds a critical temperature. As the temperature is further increased, column particles start to disperse into the liquid carrier. As a result, portions of columns disappear. As the temperature continue to rise, the ordered structure changes to a disordered column state, or even a monodispersed state. On the other hand, when the temperature is lowered, the magnetic particles in the carrier condense out of solution and finally an ordered structure of columns is achieved. However, this structural evolution during a thermal cycle is irreversible.
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