Magnetochromatics of the magnetic fluid film under a dynamic magnetic field

The structure and the corresponding magnetochromatics of homogeneous magnetic fluid films under dynamic magnetic fields perpendicular to the film surfaces are investigated in this letter. During the application of a magnetic field from 0 up to 200 Oe, the structure in the magnetic fluid film evolves from a monodispersed state to a disordered-column state, and then to an ordered hexagonal structure. The column spacing for the instant hexagonal structure during the dynamic process was measured and found to vary from 2.41 to 1.83 mum when the field strength was raised from 60 to 200 Oe. Within this range, the hexagonal structure diffracts the visible light like an optical grating. Due to the dynamic variation of the column spacing with the increasing field strength, the color of the diffracted light changed unceasingly from red to blue when observed from a fixed point above the film. The results strongly suggest the possibility that optical devices may be developed by utilizing the magnetochromatics of magnetic fluid films under dynamic magnetic fields. (C) 2001 American Institute of Physics.