Excitation Spectrum and Thermodynamic Properties of the Ising-Heisenberg Linear Ferromagnet

New analytic results are presented for the low-$T$ thermodynamics of the Ising-Heisenberg linear ferromagnetic in a magnetic field $H_0$. For small $H_0$ the thermodynamic functions show unexpected and interesting structure as a function of $H_0$ and the anisotropy $\Delta$. The thermal and magnetic energy gaps have singularities, not necessarily at the same $\Delta -H_0$ 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.