Novel convective instabilities in a magnetic fluid

A field induced novel instability was observed in a horizontal magnetic fluid layer placed in a uniform vertical magnetic field with a focused laser beam parallel to the field passing through the layer. The unstable diffraction patterns include triangles and other polygons. We found that the critical field for the onset of this instability increases with increasing the thickness of the layer. These results are consistent with our linear stability analysis. A new bifurcation was also observed when an additional horizontal held was applied.