Abstract
In microscopic theories of phase transitions occurring in itinerant-electron systems, physical phenomena are generally considered in the random-phase (RPA) or mean-particle-field approximation. We describe here a many-body theoretical method of calculating the appropriate order-parameter susceptibility function which goes beyond the RPA. A diagrammatic analysis of the equation of motion for a quantity related to is made, and it is shown how one can systematically and self-consistently include the effect of order-parameter fluctuations on . The method is applied here to the paramagnetic phase of an itinerant-electron ferromagnet. A mean-fluctuation-field approximation (MFFA) which includes the contribution of one internal spin fluctuation to is discussed in detail. Its temperature-dependent contribution to goes roughly as . A self-consistent solution of the MFFA equation for leads to a Curie-Weiss-like behavior for it. We make an explicit comparison of our results with experimental values for Ni, and find good agreement in the range . In the Stoner or RPA theory the Curie-Weiss law is ascribed to the part of the particle-field term . This is smaller than the MFF term by a factor , and for Ni, is only 5% of the latter. The Curie-Weiss-like law observed in metallic paramagnets is therefore due to the mean spin-fluctuation field, as also realized by Murata and Doniach, and by Moriya and Kawabata. Going beyond the MFFA, we calculate the contribution of the simplest spin-fluctuation correlation diagram. The contribution of this diverges logarithmically as . When this term becomes comparable to the MFFA, we are well in the critical regime which cannot be conveniently discussed by this method. This criterion is used to provide a first-principles estimate of the static and dynamic critical regimes in Ni. The former obtains for and the latter for . We show how spin fluctuations suppress ferromagnetism in a two-dimensional system and plot vs for a Ni-like film in the MFFA. The method developed here can be applied to discuss fluctuation effects in the ferromagnetic phase, in superconductivity, and in itinerant-electron antiferromagnetism.
- Received 14 February 1974
DOI:https://doi.org/10.1103/PhysRevB.10.4014
©1974 American Physical Society