Discrete Breathers in Anisotropic Ferromagnetic Spin Chains

We prove the existence of discrete breathers (time-periodic, spatially localized solutions) in weakly coupled ferromagnetic spin chains with easy-axis anisotropy. Using numerical methods we then investigate the continuation of discrete breather solutions as the intersite coupling is increased. We find a band of frequencies for which the 1-site breather continues all the way to the soliton solution in the continuum. There is a second band, which abuts the first, in which the 1-site breather does not continue to the soliton solution, but a certain multi-site breather does. This banded structure continues, so that in each band there is a particular multi-site breather which continues to the soliton solution. A detailed analysis is presented, including an exposition of how the bifurcation pattern changes as a band is crossed. The linear stability of breathers is analyzed. It is proved that 1-site breathers are stable at small coupling, provided a non-resonance condition holds, and an extensive numerical stability analysis of 1-site and multisite breathers is performed. The results show alternating bands of stability and instability as the coupling increases.