Spin-Reorientation Transitions in Multiferroics with Cycloidal Spin Ordering

Phase transitions in multiferroics with spatially modulated structures as ground states are studied in this work. The key physical mechanisms needed to implement modulated states of various kinds (cycloidal and helicoidal magnetic phases) and the conditions of phase transitions within modulated states are discussed. The domain of physical parameters where the ground state of the spin cycloid becomes unstable is determined for single crystals and films of bismuth ferrite. It is demonstrated that the domains of existence and stability of spin cycloids differ. Analytical estimates of their boundaries are obtained. The behavior of the spin system in a (110) BiFeO3 film in the magnetic field is studied, and diagrams of the ground magnetic states are plotted.