Cerium-Iron Magnetic Coupling in Single Crystal CeFeAsO at Low Temperatures*

Neutron and synchrotron resonant X-ray scattering techniques have been used to determine the intricate magnetic structure of single crystal CeFeAsO at low temperatures, way below the spin-density wave (SDW) transition around 130 K associated with Fe moments ordering. Our synchrotron X-ray scattering results at the Ce $L_{II-}$edge clearly show a magnetic transition that is specific to the Ce ordering at $T_{Ce}$ = 4 K, whereas neutron diffraction data indicate a transition at $T*$ = 12 K with unusual order parameter. Detailed order parameter measurements on the (100) {\&} (101) magnetic reflections by neutrons show an anomaly at $T$ = 4 K which we associate with the Ce ordering. The successive transitions at $T_{Ce}$ and $T$* can also be clearly identified by two anomalies in heat capacity measurements. We argue that the higher transition temperature observed in neutron measurements reflects Fe-Ce combined rearrangement prior to the complete ordering of the Ce. The effect of the weak Ce-Fe coupling on the rearrangement of Fe ordering is yet another example of the vulnerability of the Fe-SDW as influenced by minute doping or by relatively low applied pressures.