Bar’yakhtar’s exchange evolution for ultrafast spin dynamics of antiferromagnets

The concept of Bar’yakhtar’s exchange relaxation is applied to the description of spin dynamics of antiferromagnets for the pure exchange interactions of spins with exact accounting for effects of longitudinal evolution of sublattice spins. Linear regime of spin dynamics, including magnon propagation, is shown to be significantly different from that for both types of non-compensated magnets, ferromagnets or ferrimagnets. Some nontrivial features of exchange spin evolution of highly-nonlinear states, including dynamically-induced nonequivalence of sublattice spins and long-living uniform in space spin oscillations with exchange frequencies, are analyzed. These oscillations persist during the time of relativistic relaxation, which is much longer than the exchange relaxation time.