Spin-Density-Wave Mechanisms of Antiferromagnetism

Abstract

The salient properties of spin-density-wave (SDW) states of an electron gas are reviewed. Antiferromagnetism in metals can arise from SDW's whether or not the metal contains localized magnetic moments (with spin degrees of freedom). If there are no local moments, a SDW ground state of the conduction electron gas is required, containing one or more collective-electron magnetization waves. Large amplitude SDW's are observed in Cr and, very likely, small amplitude SDW's will eventually be observed in a number of other metals, including some of the nontransition elements. Localized d or f electrons can be antiferromagnetically ordered by SDW's even though the conduction electron gas, in which they are imbedded, has of itself a nonmagnetic ground state. Exchange interactions between the localized-electron spins and the spin polarization density of excited SDW's are responsible for excitation of the latter, which in turn orient and order the former. Indirect exchange interactions (Ruderman-Kittel) between localized spins are shown to be equivalent to, and derivable from a multiple SDW excitation of the electron gas. However, the indirect-interaction model is inappropriate for a discussion of long range order, since the SDW excitations are then concentrated in a few discrete waves. Observed effects such as truncation of the Fermi surface by repopulation of k space, the energetic importance of commensurateness between the SDW wavelength and lattice constant, and the occurrence of multiply periodic states of long range order are natural and immediate consequences of SDW theory.