Here we demonstrate a novel magnetic functional material composed of montmorillonite (Mt) with superparamagnetic coating Fe3O4 dispersed in the aqueous solution. Colloidal stability can be obtained through keeping 2 < pH < 3. After applying an external magnetic field, Mt@Fe3O4 exhibits long-range order and liquid crystalline phase. The transmittance of the liquid crystal is dependent on the solid content of Mt@Fe3O4, magnetic field strength and magnetic field direction. We also observe reflective phenomena when we modulate the magnetic field. The photonic bandgap can be tuned by changing the angle between incident light and magnetic field. The varying observation points also make a difference to the reflection. This facile approach for fabricating Mt@Fe3O4 with ordered periodic structures and optically tunable property is of interest for a variety of advanced optics applications with low cost and environment-friendly.