Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers

In antiferromagnetically coupled superlattices grown on (001) faces of cubic substrates, e.g., based on material combinations as Co/Cu, Fe/Si, Co/Cr, or Fe/Cr, the magnetic states evolve under competing influence of bilinear and biquadratic exchange interactions, surface-enhanced fourfold in-plane anisotropy, and specific finite-size effects. Using phenomenological (micromagnetic) theory, a comprehensive survey of the magnetic states and reorientation transitions has been carried out for multilayer systems with even number of ferromagnetic sublayers and magnetizations in the plane. In two-layer systems $(N=2)$ the phase diagrams in dependence on components of the applied field in the plane include ``swallow-tail-type'' regions of (metastable) multistate coexistence and a number of continuous and discontinuous reorientation transitions induced by radial and transversal components of the applied field. In multilayers $(N>~4)$ noncollinear states are spatially inhomogeneous with magnetization varying across the multilayer stack. For weak fourfold anisotropy the magnetic states under influence of an applied field evolve by a complex continuous reorientation into the saturated state. At higher anisotropy they transform into various inhomogeneous and asymmetric structures. The discontinuous transitions between the magnetic states in these two layers and multilayers are characterized by broad ranges of multiphase coexistence of the (metastable) states and give rise to specific transitional domain structures.