Structural Instability of Ferromagnetic fcc Fe Films on Cu(100).

Ultrathin fcc Fe films on Cu(100) up to 5 monolayers (ML) show a peculiar structural arrangement that is intimately linked with the ferromagnetic coupling within the film. Our structural analysis of the 5 \ifmmode\times\else\texttimes\fi{} 1 phase finds an increased atomic volume in combination with a complex reconstruction pattern with sinusoidal shifts and vertical buckling. The atomic configuration differs considerably from the bulklike structure of the interior of antiferromagnetic Fe films above 5 ML. This points towards a structural instability of the ferromagnetic fcc phase.