Abstract
We consider an idealized model, represented by an isotropic Heisenberg Ferro-magnet, which interacts with a free electron gas via ans-d-exchange interaction. By decoupling the equation of motion of double-time Green's functions, we derive the system's magnon dispersion law and find an expansion in powers of thes-d exchange coupling constant. The zero-order term is equal to the well-known Tjablikow result of an undisturbed Heisenberg Ferromagnet, the linear term produces a temperature dependent energy gap in the magnon spectrum, while the quadratic term proves to be responsible for kohn-like anomalies. With these results we can discuss the efficiency of the considered model, but calculation of the magnon life-times clearly shows the limits of the applied decoupling method.
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