Spin fluctuations in itinerant electron antiferromagnetism and anomalous properties of Y(Sc)Mn₂

An analysis of spin-fluctuation (SF) effects in itinerant electron magnets with antiferromagnetic instabilities is presented in two well-distinguished regimes of SF's. One, related to soft-mode (SM) fluctuations, is dominated by strongly coupled low-frequency SF's giving rise to the increase of the unsaturated local magnetic moments with temperature. The other, the localized moments (LM) regime, is characterized by dispersionless SF's, local in the real space, and by saturated thermally induced moments giving rise to the Curie-Weiss susceptibility. It is shown that both thermal and zero-point SF's play an important role in SM as well as in LM regimes. The presented description of the SF behavior generalizes the conventional mode-mode coupling theory based on a weak-coupling constraint and establishes a link between SM and LM regimes of SF's. The results are shown to give a quantitative description of the SF behavior and effects of frustration in the Y(Sc)Mn₂ system.