Purely Magnetic Nature of Spin-Reorientation Transitions in Orthoferrites.

Magnetic and structural characteristics of ErFeO$_3$, TmFeO$_3$ and YbFeO$_3$ single crystals were studied in the vicinity of the spin-reorientation transitions. In all crystals the behavior of the absolute value of the magnetization $M(T)$ and the rotation angle $\theta(T)$ are incompatible with the conventional Landau mean field theory but can be described with no fitting parameters by the proposed modified mean field theory. X-ray measurements found no symmetry-lowering lattice distortions and suggest a purely magnetic nature of the transition. Successful fitting of experimental data in several materials with a wide range of magnetic parameters demonstrates the generality of the proposed description of $\Gamma_4(G_x, F_z) \to \Gamma_{24}(G_{xz}, F_{xz}) \to \Gamma_2(G_z, F_x)$ orientation phase transitions in orthoferrites.