Magnetic-field-induced reorientation in thin antiferromagnetic films: Spin-flop transition and surface-induced twist effects

Phenomenological equations for antiferromagnetic structures in a thin antiferromagnetic layer are derived and investigated in a broad range of magnetic fields and layer thicknesses. Corresponding phase diagrams magnetic field vs layer thickness are presented. Depending on thickness and values of the intrinsic and surface-induced interactions, the spin-flop transition occurs either as first-order transition (similar as in bulk materials) or continuously via twisted structures that are inhomogeneous across the layer thickness. The antiferromagnetic twisted phases have the same physical nature and display similar properties as spatially modulated phases observed in confined liquid crystals (Freedericksz effect) or in thin ferromagnetic films and multilayers.