Theory of Disordered Itinerant Ferromagnets I: Metallic Phase

TL;DR

This paper develops a comprehensive disordered Fermi liquid theory for itinerant ferromagnets, incorporating quenched disorder and soft-mode effects, explaining experimental observations and providing a basis for understanding phase transitions.

Contribution

It introduces a saddle-point solution for a disordered ferromagnet within the Q-field theory, extending previous models by including quenched disorder and analyzing soft-mode effects.

Findings

Spin waves cause a square-root frequency dependence of conductivity in 3D.

The effect persists at nonzero temperatures, unlike weak localization.

In 2D, spin waves do not produce a logarithmic frequency dependence.

Abstract

A comprehensive theory for electronic transport in itinerant ferromagnets is developed. We first show that the Q-field theory used previously to describe a disordered Fermi liquid also has a saddle-point solution that describes a ferromagnet in a disordered Stoner approximation. We calculate transport coefficients and thermodynamic susceptibilities by expanding about the saddle point to Gaussian order. At this level, the theory generalizes previous RPA-type theories by including quenched disorder. We then study soft-mode effects in the ferromagnetic state in a one-loop approximation. In three-dimensions, we find that the spin waves induce a square-root frequency dependence of the conductivity, but not of the density of states, that is qualitatively the same as the usual weak-localization effect induced by the diffusive soft modes. In contrast to the weak-localization anomaly, this effect persists also at nonzero temperatures. In two-dimensions, however, the spin waves do not lead to a logarithmic frequency dependence. This explains experimental observations in thin ferromagnetic films, and it provides a basis for the construction of a simple effective field theory for the transition from a ferromagnetic metal to a ferromagnetic insulator.