KINKS IN ONE-DIMENSIONAL TWO-SUBLATTICE ANTIFERROMAGNET WITH BIQUADRATIC EXCHANGE INTERACTION

Nonlinear wave phenomena of an antiferromagnet have been actively investigated in the last years (see [1-3]). Three possibilities are known to analyse nonlinear waves in such magnetics: (i) on the basis of the system of the Landau—Lifshitz equations [4, 5]; (ii) on the basis of the Heisenberg Hamiltonian [6]; (iii) on the basis of analysis of an order parameter of antiferromagnet [7]. In the simplest case these models lead to the sine-Gordon equation. The solutions of sine-Gordon equation describe such experimental results as a domain wall movement in weak ferromagnet i.e. ferromagnet with Dzyaloshinsky interaction [3] or soliton excitations in quasi-one-dimensional chains like TMMC [6]. However, the analysis of soliton dynamics with velocities v about the minimum phase velocity of spin wave c is impossible because soliton width tends to zero when v —l c. Therefore, it is necessary to take into account high order derivatives on space coordinates to finite soliton width in this region of parameter v. In other words, it is necessary to include in our consideration the biquadratic exchange interaction in an antiferromagnet Hamiltonian. The problem of simplification of the system of the Landau—Lifshitz equations for antiferromagnet if high order derivatives are taken into account was considered in [8]. On the other hand, a biquadratic exchange interaction is significant for high spin systems [9, 10]. The role of biquadratic interaction for nonlinear dynamics of ferromagnets was investigated in [11-14].