Dyson-Maleev mean-field theory of ferrimagnetic spin chain

Based on the method of Dyson-Maleev mean-field theory, we study the Heisenberg ferrimagnetic spin chain at zero temperature. The energy spectrum exhibits the coexistence of a nearly gapless branch and a gapful branch. Compared with numerical results of other methods, the energy gap, Δh=1.32, of Dyson-Maleev mean-field theory is reasonable and only a little bit lower. At finite temperature, the properties of energy gap, magnetization, uniform susceptibility, internal energy, specific heat are calculated respectively. The trends of these thermodynamic properties in this method are consistent with the existed results of other methods.