Magnetic states and reorientation transitions in antiferromagnetic superlattices

Equilibrium spin configurations and their stability limits have been calculated for models of magnetic superlattices with a finite number of thin ferromagnetic layers coupled antiferromagnetically through spacers. Depending on values of applied magnetic field and uniaxial anisotropy, the system assumes collinear (antiferromagnetic, ferromagnetic, various ``ferrimagnetic'') phases, or spatially inhomogeneous (symmetric spin-flop phase and asymmetric, canted and twisted, phases) via series of field induced continuous and discontinuous transitions. Contrary to semi-infinite systems a surface phase transition, so-called ``surface spin flop,'' does not occur in the models with a finite number of layers. It is shown that ``discrete jumps'' observed in some Fe/Cr superlattices and interpreted as ``surface spin flop'' transition are first-order ``volume'' transitions between different canted phases. Depending on the system these collinear and canted phases can co-exist as metastable states in broad ranges of the magnetic fields, which may cause severe hysteresis. The results explain magnetization processes in recent experiments on antiferromagnetic Fe/Cr and Co/Ru superlattices.