Magnetic ordering of the RE lattice in REFeAsO: the odd case of Sm. A specific heat investigation in high magnetic field

Abstract

We have investigated the evolution of the low temperature specific heat anomaly (TN=5.4K in zero field) in polycrystalline SmFeAsO samples with magnetic fields up to 35T. The anomaly remains very sharp up to 16T and becomes rounded with little shift in temperature at higher fields. Doped (superconducting) SmFeAsO0.85F0.15 sample shows a similar behavior up to 16T. The initial slope of the critical field dBc/dT is 160T/K for undoped SmFeAsO and 70T/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=5.7T/K. We argue that SmFeAsO(F) presents an unprecedented case of spin reorientation at the antiferromagnetic transition.