Universal Spin-Flip Transition in Itinerant Antiferromagnets

We report a universal spin flip (SF) transition as a function of temperature in spin-density-wave (SDW) systems. At low temperatures the antiferromagnetic (AFM) polarization is parallel to the applied field and above a critical temperature the AFM polarization {\it flips} perpendicular to the field. This transition occurs in {\it any} SDW system and may be considered as a qualitative probe of the itinerant character of AFM in a given material. Our SF transition resolves the longstanding puzzle of the SF transition observed in cromium and may be at the origin of the equally puzzling SDW-I to SDW-II transition in Bechgaard salts for which we make experimental predictions.