Robust magnetic order of Ce 4f-electrons coexisting with superconductivity in CeFeAsO_1-xF_x

The electrical resistance of CeFeAsO_1-xF_x (x = 0.06 and 0.08) has been measured in magnetic fields up to 40 T. At zero field, the sample with x = 0.06 shows a structural phase transition around T _S ≈ 100 K, followed by a spin-density-wave (SDW) transition around T _SDW ≈ 30 K. For x = 0.08, the structural phase transition is suppressed down to T _S ≈ 60 K without a clear anomaly associated with the Fe-SDW transition, and superconductivity shows up at T _C ≈ 25 K. At lower temperatures, both samples show a clear resistive peak around T _N ≈ 4 K, which is associated with the antiferromagnetic (AFM) transition of Ce 4f-electrons. Strikingly, zero resistance is recovered upon a further lowering of the temperature to below T _N for x = 0.08. Moreover, we found that the AFM transition of Ce 4f-electrons at 4 K hardly change when a magnetic field of up to 40 T was applied. This was true even in the case of x = 0.08, for which the superconductivity had been partially suppressed at such a large field.