Comment on ``Universal Spin-Flip Transition in Itinerant   Antiferromagnets"
  In a recent paper [Phys. Rev. Lett. 91, 117201 (2003)], it is argued that an itinerant antiferromagnet in an external magnetic field undergoes a spin-flip transition, in marked contrast with the behavior of a localized antiferromagnet: for a weak magnetic field, the magnetization is parallel to the field, and flips to the perpendicular configuration at a critical value of the field. In this Comment we show that these conclusions are incorrect. The canted state (not considered in the Letter) is the antiferromagnetic ground state of the system up to a critical value of the field where the normal state is restored, in qualitative agreement with the behavior of the magnetization in a localized antiferromagnet. 
N. Dupuis