  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.
