  When applying a red-detuned retro-reflected laser beam to a large cloud of
cold atoms, we observe the spontaneous formation of 2D structures in the
transverse plane corresponding to high contrast spatial modulations of both
light field and atomic spins. By applying a weak magnetic field, we explore the
rich resulting phase space and identify specific phases associated with both
dipolar and quadrupolar terms of the atomic magnetic moment. In particular we
demonstrate spontaneous structures in optically induced ground state coherences
representing magnetic quadrupoles.
