Theory of disordered itinerant ferromagnets. I. Metallic phase

A comprehensive theory for electronic transport in itinerant ferromagnets i s developed. We first show that the Q-field theory used previously to descr ibe a disordered Fermi liquid also has a saddle-point solution that describ es a ferromagnet in a disordered Stoner approximation. We calculate transpo rt coefficients and thermodynamic susceptibilities by expanding about the s addle point to Gaussian order. At this level, the theory generalizes previo us random-phase-approximation-type theories by including quenched disorder. We then study soft-mode effects in the ferromagnetic state in a one-loop a pproximation. In three dimensions, we find that the spin waves induce a squ are-root frequency dependence of the conductivity, but not of the density o f states, that is, qualitatively the same as the usual weak-localization ef fect induced by the diffusive soft modes. In contrast to the weak-localizat ion anomaly. this effect persists also at nonzero temperatures. In two dime nsions, however, the spin waves do not lead to a logarithmic frequency depe ndence. 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 ins ulator.