Solitons on a discrete ferromagnetic spin chain

Abstract

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.