The growth and structure of ultra thin films of terbium deposited on C
o(1010BAR) have been studied by X-ray photoemission, Auger spectroscop
y and low energy electron diffraction at 600 K. Two stages are observe
d in the growth mode: the growth of about two Tb layers is followed by
an interdiffusion process, leading to the formation of an alloy. We a
lso show that two-dimensional ordering of the film occurs at 600 K in
the monolayer range.