Thickness-dependent fcc to bcc structural change in iron films:Use of a 2-ML Ni/W(110) substrate

The fcc to bcc structural change in the growth of ultrathin iron films is studied for films grown on a (111) fcc surface, in an attempt to mimic the geometry of the martensitic transition in bulk iron, where the interface is formed from the close-packed planes of the two structures. The use of a 2-ML Ni/W(110) substrate allows good wetting and lattice matching of the iron, without introducing either significant interdiffusion at the interface or a large amount of magnetic material. Low-energy electron diffraction and angle-resolved Auger electron spectroscopy show that the iron films grow as a slightly distorted (111) fcc surface for 3 ML, after which a surface cell intermediate to fcc and bcc appears in a specific Kurjumov-Sachs (KS) orientation. Thicker films show a simultaneous relaxation of the surface unit cell to a bcc structure and the movement of the layer stacking from the fcc position to the KS bcc position. The approximate layer-by-layer evolution of the structure is in marked contast to the complicated growth transition seen for iron films grown on (001) fcc surfaces. This promises to be a useful system to isolate the relation of the structural transition and magnetism, without important contributions from other growth processes.