Considering the attractive interaction between two magnons with opposite wave vectors in a Heisenberg ferromagnet, we propose the model of magnon-pairs, which is suitable for low-temperature environment. A dressed magnon is an energy quantum of the magnon-pairs whose energy is a monotonically increasing function of absolute temperature. Based on the model, we re-investigate the excitation mechanism and thermodynamic properties of the Heisenberg ferromagnet. The correction factor e(0) plays an important role in studying the low-temperature properties of a ferromagnet.
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