Specific heat investigation in high magnetic field of the magnetic ordering of the rare-earth lattice in RFeAsO: The case of Sm

We have investigated the evolution of the low-temperature specific heat anomaly (TN=5.4 K in zero field) in polycrystalline SmFeAsO samples with magnetic fields up to 35 T. The anomaly remains very sharp up to 16 T and becomes rounded with little shift in temperature at higher fields. Doped superconducting SmFeAsO0.85F0.15 sample shows a similar behavior up to 16 T. The initial slope of the critical field dBc /dT is 160 T/K for undoped SmFeAsO and 70 T/K for doped SmFeAsO0.85F0.15 with Bc(T) defined at the peak of the specific heat anomaly. The insensitivity to the application of an external magnetic field is unique to Sm and is not observed in CeFeAsO whose anomaly shifts with initial slope dBc /dT=5.7 T/K. We argue that SmFeAsO (F) presents an unprecedented case of spin reorientation at the antiferromagnetic transition.