Observation of the fcc-to-bcc Bain transformation in epitaxial Fe ultrathin films on Cu sub 3 Au(001)

A continuous fcc-to-bcc crystallographic transition via a homogeneous tetragonal lattice deformation (Bain transformation) with increasing Fe coverage was observed in molecular-beam grown epitaxial Fe ultrathin films on Cu sub 3 Au(001), contrary to the usual case of a discontinuous martensitic transformation of Fe. With increasing Fe film thickness, a continuous compression of the interlayer distance perpendicular to the film plane and a simultaneous continuous expansion of the in-plane atomic distance was observed. We did not find evidence for the coexistence of fcc and bcc phases. In the approx =1-12 ML (monolayer) thickness range, the films do exhibit some atomic disorder and do not grow pseudomorphous, but form twisted crystallographic domains that are rotated in the film plane about the film normal direction. The atomic volume of tetragonal states was found to follow closely face-centered tetragonal (fct) or body-centered tetragonal (bct) 'epitaxial lines' according to strain-energy calculations [Surf. Rev. Lett. 1 (1994) 15], including a crossover from ferromagnetic high-moment high-volume fct to bct Fe. The results were obtained by in situ X-ray photoelectron diffraction, high-energy electron diffraction, and exp 37 Fe conversion-electron Mossbauer spectroscopy. Correlated with the Bain transformation is a Fe spin reorientation from preferentially perpendicular (for fct) to in-plane (for bct) spin direction at 25 K.