Solitons on a discrete ferromagnetic spin chain

Discrete ferromagnetic spin chains with planar single-ion and exchange anisotropy and with an externaldc fieldB applied in the easy plane are considered in the classical limit. We find two different static in-plane kink structures, which are symmetric with respect to a given spin or to a given bond of the chain, respectively. The first structure is unstable against in-plane fluctuations for all applied fields. The second one, on the other hand, exists (independent of the anisotropy parameters) only up to a critical fieldB_e and is stable against in-plane fluctuations in its whole domain of existence. Depending on the anisotropy parameters, this latter configuration may, however, become unstable against out-of-plane fluctuations at a critical valueB_c<B_e of the external field. Both types of kink structures can also move along the chain, by developing an out-of-plane spin component. With increasing velocity, the kink motion leads to a rapid reduction of the critical field marking the existence limit of these spin structures. The discreteness is found to have substantial influence on the spectrum of the solitons.Work supported by the Bundesministerium für Forschung und TechnologieWork supported by Schweizerischer Nationalfonds