Non-nesting spin-density-wave antiferromagnetism in FeAs from first principles

The antiferromagnetic (AFM) state of FeAs is very different from that of the FeAs-based superconductor parent compounds, and it is rather complicated, with the Fe spins forming an incommensurate magnetic spiral pattern with a wavelength of about 15 A. To model this, we perform first-principles calculations and find the nearest-neighbor AFM ordering to be energetically favorable, with the lowest-energy pattern reproducing the experimentally found nearest-neighbor correlations. Other AFM orderings are also very stable, although higher in energy. Unlike in the superconductor parent compounds, the Fermi-surface geometry thus plays a small role. We calculate the bare Lindhard susceptibility in the AFM state and find that the observed spin-density-wave ordering vector Q similar or equal to (0,0,0.4) is not that given by this calculation. This is again unlike the superconductor parent compounds, which generally show a magnetic pattern matching the Lindhard susceptibility maximum. Thus in FeAs, the observed pattern must be due to a subtler mechanism.