Modulational instability of nonlinear spin waves in easy-axis antiferromagnetic chains. II. Influence of sample shape on intrinsic localized modes and dynamic spin defects

An indirect method of producing intrinsic localized modes (ILM’s) is to drive an unstable uniform mode to large amplitude. In this investigation the intimate connection between the shape-dependent demagnetization factor, the modulation instability, and ILM behavior is established. The stability of the antiferromagnetic resonance (AFMR) against breakup into such ILM’s is shown to depend strongly on the frequency difference between the linear AFMR and the long-wavelength limit of the spin-wave dispersion curve. This difference, which stems from the long-range dipole-dipole interactions, depends on the demagnetization factor and hence the sample shape. Here it is demonstrated initially with linear perturbation analysis and later with molecular-dynamics simulations that the instability characteristics and ILM production properties depend strongly on the sign and magnitude of the frequency difference. The simulations show that when the AFMR frequency is coincident with the spin-wave band, a spatially coherent train of ILM’s appears, but this coherence is lost at long times. When the AFMR frequency is inside the spin-wave band, the Suhl instability populates the degenerate spin waves, whose subsequent modulational instability leads to traveling and decelerating ILM’s. When the AFMR frequency is below the spin-wave band, a large-amplitude threshold for the instability appears. Above this threshold ILM’s and localized dynamic spin flops occur, confined to specific lattice regions.