Bulk and surface spin-flop transitions in an antiferromagnetic XYZ chain

Spin-flop transitions are studied within a spin-1/2 antiferromagnetic XYZ c hain immersed in a uniform bias magnetic field. For a special choice of ani sotropy the bulk spin-flop transition occurs at a critical field Hb charact erized by the onset of "accidental" degeneracy in the energy spectrum that may indicate a hidden symmetry. On an open chain the bulk transition is pre ceded by a surface spin-flop transition induced by a surface magnon that tu rns soft at a new critical field H-s<H-b, while an antiferromagnetic domain wall is realized in the ground state as the bias field approaches Hb. Thes e T=0 phase transitions could be observed through electron-spin resonance o n magnetic chains doped with nonmagnetic ions. [S0163-1829(99)01137-6].