Spin-wave theory applied to the low-temperature properties of the spin-1/2 ferromagnetic chain with the y-direction exchange anisotropy

Abstract In this paper, we apply the spin-wave theory within the simple self-consistent method, and provide a simple way to investigate the spin- 1 / 2 ferromagnetic chain with the y -direction exchange anisotropy. It is found that the anisotropy has an influence on the ground-state and low-temperature properties of the system. Anisotropy dependences of both the susceptibility maximum and specific heat maximum fit well to the exponential laws. The linear decrease is shown for the position of the susceptibility maximum. The position of the specific heat maximum displays three kinds of the anisotropy dependence, i.e., the power law, constant law and exponential law. The specific heat coefficient is calculated to be a monotonically increasing function of the anisotropy in the zero-temperature limit. At some low temperature, the Heisenberg behavior is observed to cross over the XY behavior, which agrees with the experimental result of the quasi-one-dimensional compound (C 6 H 11 NH 3 )CuBr 3 . In the isotropic and anisotropic cases, our results are in good accord with the exact Bethe-ansatz results and quantum Monte Carlo estimates, and have some advantages over the findings of Green-function method and the renormalization group method at low temperatures.