  Neutron diffraction measurements of a high quality single crystal of CaFe2As2
are reported. A sharp transition was observed between the high temperature
tetragonal and low temperature orthorhombic structures at TS = 172.5K (on
cooling) and 173.5K (on warming). Coincident with the structural transition we
observe a rapid, but continuous, ordering of the Fe moments, in a commensurate
antiferromagnetic structure is observed, with a saturated moment of
0.80(5)muB/Fe directed along the orthorhombic a-axis. The hysteresis of the
structural transition is 1K between cooling and warming and is consistent with
previous thermodynamic, transport and single crystal x-ray studies. The
temperature onset of magnetic ordering shifts rigidly with the structural
transition providing the clearest evidence to date of the coupling between the
structural and magnetic transitions in this material and the broader class of
iron arsenides.
