Nucleation of bcc iron in ultrathin fcc films

Needle-shaped bcc nucleation centers in fee films of Fe on Cu(100) are observed by scanning tunneling microscopy. They form virtually without mass transfer and nearly under conservation of volume, which causes a large strain within the nascent bce grain. The corresponding strain energy almost equals the gain in structural energy, rendering the bce nucleation very sensitive to any effect influencing this subtle balance. We suggest that modifying the film by straining, alloying, or surface adsorption may inhibit the bcc nucleation and lead to thick metastable fee films.