Evidence for martensitic fcc-bcc transition of thin Fe films on Cu(100)

We report on the martensitic fcc-bcc phase transformation for Fe films grown at room temperature on Cu(100) in a thickness range from 10 to 20 monolayers. bcc(110) grains are highly elongated along 〈011〉fcc{∥}[11{}1]bcc and exhibit characteristic island growth with steps along [001]bcc. The martensitic (i.e., sudden, collective, nondiffusive) transformation in the bulk of the film is demonstrated by STM topography, including the behavior of steps at grain boundaries and the presence of tilted surfaces arising from fcc-bcc interfaces.