LOCAL-BAND THEORY OF ITINERANT FERROMAGNETISM .5. STATISTICAL-MECHANICS OF SPIN-WAVES

The functional-integral method of the previous paper is generalized to allow a discussion of the dynamical spin fluctuations in itinerant-electron magnets. The magnitude fluctuations and nonstatic spin fluctuations are treated as a perturbation on the nonlinearly interacting static spin fluctuations. The result is a free energy which is a sum of a Stoner contribution together with correlation corrections, a classical Heisenberg free energy as found in the previous paper, and a quantum spin-wave free energy minus its zero-frequency part. © 1979 The American Physical Society.