Date of Original Version
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 Δ. The thermal and magnetic energy gaps have singularities, not necessarily at the same Δ−H0 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.
J. D. Johnson and Jill C. Bonner. "Excitation spectrum and thermodynamic properties of the Ising-Heisenberg linear ferromagnet." Phys. Rev. Lett. 44, 616 (1979).
Available at: http://dx.doi.org/10.1103/PhysRevLett.44.616