A Spin Fluctuation Theory of Itinerant-Electron Systems — The Single-Site Approximation

Abstract

It is well known that the band theory based on the Hartree-Fock approximation has succeeded in describing the ground-state properties such as the magnetic moment, the Fermi surface and the cohesive energy of transition metals. Despite its great success, however, the band theory cannot account for physical phenomena such as the Curie-Weiss susceptibility, the persistence of local moments above the Curie temperature and the large magnetic entropy. In recent years much progress has been made in the theoretical understanding of the finite-temperature properties of transition metals [1–4]. It has been recognized that the effect of spin fluctuations, which is completely neglected in the Hartree-Fock approximation, plays predominant roles in discussing the thermodynamical properties of magnetic metals. Recently, the present author has developed a spin fluctuation theory in which the effect of local spin fluctuations is taken into account by means of the functional-integral method combined with the alloy-analogy approximation [5–8]. We will discuss in this note the finite-temperature properties of ferromagnetic metals (Sec.2), of antiferromagnetic metals (Sec.3) and of concentrated ferromagnetic alloys (Sec.4), calculating the transition temperature, the average magnetic moment, the amplitude of local moments, the spin susceptibility and the specific heat. Supplementary discussions on the validity of the single-site approximation and its extention to a pair theory are given also (Sec.5).KeywordsLocal MomentSpin FluctuationSpin SusceptibilityCurie ConstantAverage Magnetic MomentThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.