We investigate the phase transitions induced by external fields in magnetic colloidal suspensions. We report video microscopy experiments showing the onset of unidimensional phase-segregated patterns composed of self-similar periodic structures induced by a pulsed magnetic field. Similar periodic patterns are shown to occur in the presence of combined shear flow and dc magnetic fields. The wavevector of these patterns can be tuned by the shear viscosity or the frequency of the pulsed field.