Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of fcc-to-bcc transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic fcc phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained bcc-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7-8 ML films reconstructs by forming non-close-packed structures with bcc-like bond angles. The formation of these bcc-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering.
Corresponding author: M. Schmid (schmid).
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