Properties of phase transitions in easy-axis tetragonal antiferromagnets

Properties of phase transitions in a magnetic field H parallel to the easy axis have been investigated, and it has been shown, in particular, that the nature of the transition of the magnetic subsystem from the antiferromagnetic phase to the angular phase depends on the "sign" of the Dzyaloshinskii interaction. The conditions for orientation of the antiferromagnetism vector l in the basis plane in fields larger than the threshold field have been determined. It is shown that the transition from the angular phase to the state where the resulting magnetic moment m is parallel to the easy axis takes place in the field corresponding to a spin-flip transition. From an analysis of the configuration of the magnetic subsystem for arbitrary orientation of the external magnetic field, it follows that the values of the critical angle (psi(cr)) for which a first-order phase transition takes place satisfy the condition psi(cr)much less than 1 in the case in which the anisotropy constant f in the basis plane is of the order of the first anisotropy constant b. Usually parallel to f parallel to much less than parallel to b parallel to so that the tricritical point in the phase diagram H-y,H-z satisfies the condition H(y)similar to H-z. (C) 1999 American Institute of Physics. [S1063- 7834(99)02711-2].