We performed first principles calculations for magnetic domain wall structures in free-standing Fe monolayers with lattice constants matching with those of bcc Fe(1 1 0) and W(1 1 0) substrates by using the full-potential linearized augmented plane-wave method that incorporates intra-atomic noncollinear magnetism. The self-consistent calculations predict atomically narrow domain walls with widths of about 8 and 14 angstrom for the monolayers with the lattice constants of the Fe and W, respectively. These results are qualitatively consistent with and support domain walls having a 6 angstrom width in the Fe monolayers on a W(1 1 0) substrate recently observed in spin-polarized scanning tunneling microscopy experiments. (c) 2004 Elsevier B.V. All rights reserved.