Symmetry and spin-flip transitions in antiferromagnetic crystals

A symmetry analysis of spin-flip transitions (collapse of magnetic moments of sublattices in an external magnetic field) in antiferromagnetic crystals is performed. The directions of the magnetic field in which a spin-flip transition (SFT) can be observed are found for all crystallographic classes. It appears that depending on the specific crystallomagnetic structure of the antiferromagnet, three groups can be separated. In the first group, SFT can occur with any orientation of the external magnetic field H, in the second the SFT must be observed only for certain orientations of H, and in the third the SFT is absent for any orientation of H. The relativistic interactions that destroy the spin-flip phase are indicated for each specific case. It is also shown that in a certain group of antiferromagnets belonging to the trigonal, hexagonal, and cubic syngonies SFT occurs as a first-order phase transition. In antiferromagnetic crystals belonging to the crystalline classes Cn, Dn (n = 3, 4, 6), T, and O, the phase diagram contains a line of phase transitions that separates the spin-flip phase and the spatially nonuniform, noncollinear phase.

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