Quantum and thermal spin fluctuations in itinerant-electron magnets

Itinerant-electron magnetism is investigated using the Hubbard model. Local spin fluctuations are taken into account through mapping to the Anderson model. Intersite spin fluctuations are taken into account by the lid-expansion method, with d being the spatial dimensionality. The free energy as a function of magnetization, which is similar to Landau's phenomenological one, is derived at the microscopic level. There are two mechanisms of Curie-Weiss behavior of the susceptibility. One is due to specific structures of the dispersion relation of quasiparticles and is of leading in lid or O[(l/d(0)], and the other is due to the mode-mode coupling between intersite thermal spin fluctuations and is of higher order in l/d. The formation of light quasiparticles can occur in the vicinity of the Mott transition, caused by momentum-dependent self-energy corrections due to intersite quantum spin fluctuations. It can explain the so called spin-bag or spin-gap behavior of high-T-c cuprate oxides.