NONTOPOLOGICAL THERMAL SOLITONS IN ISOTROPIC FERROMAGNETIC LATTICES

The paper deals with the properties of thermally excited solitons of t he isotropic spin-g ferromagnetic chain with nearest-neighbor logarith mic interactions. The exact statistical mechanics of the interacting s oliton gas is developed for the general case (arbitrary S, temperature , and magnetic field). At low temperatures the model's thermodynamics coincides with that of the Heisenberg model. We present analytical app roximations of the leading-order asymptotic behavior of the energy in three limiting cases: (a) zero field, low temperature, classical limit ; (b) zero held, T --> 0, S finite (quantum limit); (c) zero field, hi gh temperature, classical limit. Cases (a) and (c) are examples of a d ense gas of [nontopological] solitons; results are in agreement with t hose obtained by the transfer integral method. Case (b) illustrates th e behavior of a dilute, yet strongly interacting soliton gas; results for the thermodynamics are very close to (but not identical with) spin -wave and/or Bethe ansatz predictions.