Phase boundaries of a uniaxial system in an applied field of arbitrary direction

A procedure is put forward for the specification of a suitable phase diagra m for a uniaxial system with higher-order anisotropy which undergoes a spin -reorientation transition via coherent rotation of magnetization in an appl ied field of arbitrary strength and direction. The approach generalizes rec ent studies for a field applied in one of the principal directions. The ana lysis is valid for bulk and thin-film systems and is relevant to all experi mental techniques that involve the implementation of an external field. The representation is especially promising in the context of thickness-driven transitions in ultrathin systems. It is generally valid, beyond the uniaxia l restriction, for any remagnetization process which takes place within a g iven plane.