  Magnetic x-ray diffraction combined with x-ray focusing optics is used to
image individual antiferromagnetic spin density wave domains in a chromium
single crystal at the micron scale. The cross section for non-resonant magnetic
x-ray scattering depends on the antiferromagnetic modulation vector and spin
polarization direction and allows these quantities to be extracted
independently. The technique is used to show that the broadening of the
nominally first order "spin-flip" transition at 123 K, at which the spins
rotate by 90 deg., originates at the walls between domains with orthogonal
modulation vectors. During cooling the transition begins at these walls and
progresses inwards. The modulation-vector domains are themselves unchanged.
