Chaotic regimes of the microwave energy absorption are experimentally observed and analyzed for two-dimensional metallorganic antiferromagnet (NH 3 ) 2 (CH 2 ) 4 MnCl 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 in-1 vestigation.
The resonance properties of a new Cu 2 OSeO 3 ferrimagnet have been investigated in a wide range of frequencies (17-142 GHz) at liquid helium temperature. The resonance data were used to plot the frequencyfield dependence of the ferrimagnetic spectrum described within the model of an anisotropic two-sublattice ferrimagnet. The effective magnetic anisotropy corresponding to the gap in the spin wave spectrum has been estimated (3 GHz). It is found that the spectrum has a multicomponent structure which is due to the diversity of the types of magnetization precession. As the amplitude of the high-frequency magnetic field increased, an additional absorption was observed in the external magnetic field lower than the field of the main resonance. The detected additional absorption corresponds to the nonuniform nonlinear parametric resonance, connected with nonuniformity of magnetic structure in the ferrimagnetic crystal Cu 2 OSeO 3 .
The electron paramagnetic resonance (EPR) spectrum of KTb(WO4)2 is investigated in the frequency range 14–120 GHz and magnetic field range 0–70 kOe at helium temperature. The observed triplet structure of the spectrum is interpreted as a manifestation of resonance in three-site clusters. On this basis a value of the g factor (g≈13.3) and an estimate of the gap (δ≈1 K) are obtained for the quasi-doublet ion Tb3+ in the crystalline field of KTb(WO4)2, and the parameters of the dipole (Id≈1.6 K) and exchange (Iex≈0.9 K) AFM interactions of the nearest neighbors in the chains are determined for the corresponding singlet magnet model. A first-order structural phase transition, induced by an external magnetic field lying in the basal plane of the crystal, is observed.
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