  A few theoretical models predict Fe to display both ferromagnetic and
antiferromagnetic phases, depending on the volume of the unit cell. A proper
choice of the substrate allows the growth of thin Fe overlayers with structures
different from the bcc bulk one. The Cu3Au(001) substrate is a suitable
candidate for testing these magnetic properties since it has a lattice
parameter (3.75 Angstroms) which lies closer to the ferromagnetic fcc Fe phase
(3.66 Angstroms) than to the ferromagnetic bcc phase (4.07 Angstroms). We have
investigated Fe films up to 40 Angstroms thickness by means of Grazing
Incidence X-Ray Diffraction (GIXRD) and Photo/Auger--Electron Diffraction (ED).
The combination of GIXRD and ED allows one to obtain quantitative information
on the in--plane spacing "a" from the former technique, and the ratio between
the vertical spacing "c" and "a", from the latter one. At low coverage the film
grows pseudomorphic to the fcc substrate up to a limit thickness of 8
Angstroms. Above this limit the film is characterized by the coexistence of the
pseudomorphic phase with another tetragonally strained phase. The latter phase
is shown to fall on the epitaxial curve of tetragonally distorted fcc Fe phase.
Finally, above 17 Angstroms the development of a body centered phase "alpha",
whose unit cell is rotated of 45 deg. with respect to the substrate one, has
been clearly observed. "alpha" is the dominating phase for film thickness
higher than 25 Angstroms and its lattice constant evolves towards the
orthomorphic phase in strict quantitative agreement with epitaxial curves
calculated for the tetragonally distorted bcc iron phase.
