Soft-mode Spin Fluctuations in Itinerant Electron Magnets

Abstract

This paper critically overviews recent developments of the theory of itinerant magnets with soft-mode spin fluctuations (SF) which are slowed down near some points of the Brillouin zone. With respect to the role of spatial dispersion and quantum effects a novel classification scheme for different regimes of SF with anisotropic spectrum is presented including generalized Fermi liquid (FL), soft-mode (SM), and localized moments (LM) ones. By introducing a dimensionless spin anharmonicity parameter it is shown that conventional SF theories are related to the weak coupling limit and cannot be applied to real magnets where mode-mode coupling of SF is strong. A variational SM theory is presented accounting for effects of strong spin anharmonicity induced by zero-point SF. Basing on the SM theory temperature dependencies of the spin anharmonicity parameter, local magnetic moments, magnetic susceptibility, thermal expansion, and specific heat are analyzed. It is shown that low temperature specific heat anomalies arising near critical points in the quantum SM regime are strongly affected by zero-point SF and caused by them strong anharmonicity, which may lead to a heavy fermion and non-Fermi liquid behavior. In the high temperature LM regimes the Curie-Weiss law for the susceptibility is shown to be accompanied by a saturation of local moments which may be affected by quantum dynamical effects reducing their values. This may lead to changes in the Rhodes-Wohlfarth plots and to a breakdown of the conventional criterion for itinerant magnetism. A novel criterion separating magnets with LM and soft-mode SF caused by itinerant electrons is suggested.