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Ferromagnetism in the t-t' Hubbard model is investigated on a square lattice.Correlation effects in the form of self-energy and vertex corrections aresystematically incorporated within a spin-rotationally-symmetric scheme whichexplicitly preserves the Goldstone mode and is therefore in accord with theMermin-Wagner theorem. Interplay of band dispersion and correlation effects onferromagnetic-state stability are highlighted with respect to both long- andshort-wavelength fluctuations, which are shown to have substantially differentbehaviour. Our approach provides a novel understanding of the enhancement offerromagnetism near van Hove filling for t'~0.5 in terms of strongly suppressedsaddle-point contribution to the destabilizing exchange part of spin stiffness.Finite-temperature electron spin dynamics is investigated directly in terms ofspectral-weight transfer across the Fermi energy due to electron-magnoncoupling. Relevant in the context of recent magnetization measurements onultrathin films, the role of strong thermal spin fluctuations in low dimensionsis highlighted, in the anisotropy-stabilized ordered state, by determining thethermal decay of magnetization and T_c within a renormalized spin-fluctuationtheory.

physical review. b, condensed matter and materials physics

Quantum and thermal fluctuations in a two-dimensional correlated band  ferromagnet -- Goldstone-mode preserving investigation with self-energy and  vertex corrections