Heteroeptiaxial growth of Co1-xFex alloy monolayers on W(1Â 1Â 0)

We studied the initial growth of Co1-xFex films on W(1 1 0) using scanning tunneling microscopy and spectroscopy (STM and STS), in combination with low-energy electron diffraction (LEED). For Co-rich alloys the deposition at room temperature followed by high-temperature annealing is not equivalent to the deposition at high substrate temperatures, in contrast to the case of Fe-rich alloys. While room temperature deposition results in a structure consisting of small islands even after annealing, deposition at high substrate temperatures leads to large coalesced areas. Sequential deposition of pure Co and pure Fe results in phase separated areas of Co- and Fe-rich areas. Only, in the case of co-deposition of the two elements a homogeneous alloy monolayer can be grown. The Co1-xFex submonolayer starts to grow pseudomorphic with the substrate for all compositions up to a coverage of 0.7 pseudomorphic monolayers. A Co-rich monolayer with a Co concentration of more than 60% and exceeding a coverage of 0.7 monolayers is transformed partly into a close-packed structure. The occurrence of close-packed areas show up as a 8×1 superstructure in the LEED pattern.