  Chaotic regimes of the microwave energy absorption are experimentally
observed and analyzed for two-dimensional metallorganic antiferromagnet
(NH_3)_2(CH_2)_4MnCl_4 at low temperatures under the conditions of nonlinear
antiferromagnetic resonance. Relaxation oscillations of energy absorption are
investigated in detail. Their frequency spectra, frequency-amplitude
characteristics, and dependences of absorbed power on driving power and static
magnetic field are studied. It is shown that the dynamics of relaxation
oscillations undergoes a transition to chaos by "irregular periods".
Peculiarities of the transition are described consistently. Among other things,
the conditions for the emergence of energy absorption regimes with a spike-like
and a saw-tooth signal structure are determined, and the characteristics of
chaotic oscillations such as the dimensions of strange attractors are
calculated. The chaotic dynamics is found to be high-dimensional with a large
contribution from noise which is of deterministic origin in the antiferromagnet
under investigation.
