Abstract
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 $1/d$-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 $1/d$ or $O[{(1/d)}^{0}]$, and the other is due to the mode-mode coupling between intersite thermal spin fluctuations and is of higher order in $1/d$. 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.
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