An STM-study of the Invar surface

Full text: In 1896 C. Guillaume discovered the vanishing thermal expansion coefficient of Invar (Fe65Ni35) due to which it quickly found its way into industrial applications (e.g. shadow masks for TV-tubes, semiconductor laser packages and more). In the meantime there are other systems known (mostly based on iron plus 3d-transition metals) that show the same or similar material characteristics. Many attempts have been made to explain this so-called Invar effect (e.g. R. J. Weiss, E. F. Wassermann) but still it is not fully understood. New ideas of the crystallographic structure of ultra thin Fe films that include a nanomartensitic phase prior to the actual martensitic transformation (fcc to bcc) motivate a reinvestigation of the Invar structure. It seems that the nanostructure plays a more important role in the origin of the physical properties of Invar than previously thought. STM images at room temperature and at the temperature of liquid nitrogen should reveal this and perhaps permit to track the Invar effect down to the atomic scale. Refs. 4 (author)