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##### Collective and intrinsic degrees of freedom in the Heisenberg ferromagnet

**Authors**: J. da ProvidĂȘncia, C. Fiolhais and M. Brajczewska

**Ref.**: Journal of Physics A: Mathematical and General **22**, 703-715 (1989)

**Abstract**: A separation between collective and intrinsic degrees of freedom in the Heisenberg model of a ferromagnet at a finite temperature is achieved by reformulation the thermal boson expansion previously obtained by the present aultors. In the new approach, all the collective modes (spin waves) have wavenumbers lower than a certain value. The intrinsic energy turns out to be the usual mean-field energy minus a correlation energy due to the excitations of spin waves. The influence of the cutoff in momentum space on the critical temperature is studied and the temperature dependence of the spin-wave renormalisation factor and the magnetisation are calculated.