Non-nesting spin density wave antiferromagnetism in FeAs: first principles calculations

Abstract

The antiferromagnetic state of FeAs is unusual in several aspects, and seems to be rather complicated. The Fe spins form an incommensurate magnetic spiral pattern with a relatively short wavelength, about 15 A. To model this, we have performed first principles calculations and found that the nearest-neighbor AFM ordering is energetically favorable, with the lowest-energy pattern reproducing the experimentally found nearest neighbor correlations. Other antiferromagnetic orderings are also very stable, even though higher in energy; the Fermi-surface geometry thus plays little role. We calculate the bare Lindhard susceptibility in the AFM state and find that the observed spin-density wave ordering vector textbf{Q} $simeq(0,0,0.4)$ is textit{not} that given by this calculation. Thus the observed pattern must be due to a subtler mechanism, such as frustration created by the competing ground states.