Magnon-Paramagnon Effective Theory of Itinerant Ferromagnets

The present work is devoted to the derivation of an effective magnon-paramagnon theory starting from a microscopic lattice model of ferromagnetic metals. For some values of the microscopic parameters it reproduces the Heisenberg theory of localized spins. For small magnetization the effective model describes the physics of weak ferromagnets in accordance with the experimental results. It is written in a way which keeps O(3) symmetry manifest,and describes both the order and disordered phases of the system. Analytical expression for the Curie temperature,which takes the magnon fluctuations into account exactly, is obtained. For weak ferromagnets $T_c$ is well below the Stoner's critical temperature and the critical temperature obtained within Moriya's theory.