When excited to sufficiently large amplitude, the uniform spin-wave mode of an antiferromagnet can undergo various pattern-forming instabilities. In the antiferromagnets experimentally investigated, these patterns become increasingly localized in space giving rise to so-called intrinsic localized modes. When the dipolar fields are included in the spin Hamiltonian, it can be shown that the type of instability and the resultant spin pattern depend sensitively on the shape of the crystal. Molecular dynamics simulations are used to extend the perturbation analysis valid at short times and to reveal the fully nonlinear structure of the spin patterns at long times. Connections to results from microwave pump-probe measurements will be presented.
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