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.
