Functional Integral Approach to the Low-Temperature Properties of Itinerant Ferromagnets

Abstract

Although there have been a lot of investigations of the itinerant electron magnetism based on the functional integral formalism [1], almost all the treatments employ static or classical approximation. From the sense of this approximation, applicability of these theories ought to be restricted to the high-temperature properties. Therefore in order to give more adequate descriptions, in particular, when we discuss the ordered phase, we must be faced with the problem of dealing with the dynamical effects. Even in the paramagnetic state in the local moment limit, we have to take into account the dynamical effects in order to obtain the Curie constant proportional to S(S+1) rather than S² obtained in the classical approximation. This has been regarded as one of the main difficulties of the existing treatments. Extension of the theory to take into account dynamical effects is therefore highly desirable.