New analytic results are presented for the low-
T thermodynamics of the Ising-Heisenberg linear ferromagnetic in a magnetic field
H0. For small
H0 the thermodynamic functions show unexpected and interesting structure as a function of
H0 and the anisotropy $\ensuremath{\Delta}$. The thermal and magnetic energy gaps have singularities, not necessarily at the same $\ensuremath{\Delta}\ensuremath{-}{H}_{0}$ location, as changes occur in the type of excitation dominating the low-
T behavior. The results may relate to quantum solitons in the linear ferromagnet.