Surface x-ray structure analysis of periodic misfit dislocations in Fe/W(110)

We have carried out a surface x-ray diffraction analysis of the geometric structure of a 13-monolayer-thick Fe film on W(110). The Fe adlayer is characterized by a well-ordered two-dimensional array of misfit dislocations that have a periodicity of 35.84 and 50.76 Angstrom along the [001] and [(1) over bar 10] directions, respectively. In the (110) plane the average Fe structure is isotropically strained by +1.2% with respect to (bulk) bcc Fe, corresponding to lattice constants of 2.901 Angstrom along [001] and 4.103 Angstrom along [(1) over bar 10]. In the surface normal direction ([110]) we find that the Fe film is also strained by +0.22% (c=4.062 Angstrom). This corresponds to a slightly laterally distorted bct phase with a=b=2.887 Angstrom, c=2.901 Angstrom, and gamma=89.4degrees. The intensity distribution along 14 satellite rods was analyzed quantitatively using a sine-wave modulation ansatz for the lateral and vertical displacements of the Fe atoms out of their average positions. We find maximum amplitudes in the order of about 0.7 Angstrom for the normal and lateral modulations, respectively. The modulation amplitudes continuously decrease with distance from the Fe/W(110) interface. The adlayer/substrate registry is characterized by a 37/34 coincidence between the Fe- the W atoms, where the vertical Fe corrugation is directly related to the Fe-adsorption site. The implications of the unexpected strain state of the film for its mechanical stress and magnetic anisotropy are discussed.