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.

• Received 14 August 1978

DOI:https://doi.org/10.1103/PhysRevB.19.4698