Size effect on magnetism of Fe thin films in Fe/Ir superlattices

In ferromagnetic thin films, the Curie temperature variation with the thickness is always considered as continuous when the thickness is varied from n to n + 1 atomic planes. We show that it is not the case for Fe in Fe/Ir superlattices. For an integer number of atomic planes, a unique magnetic transition is observed by susceptibility measurements, whereas two magnetic transitions are observed for fractional numbers of planes. This behavior is attributed to successive transitions of areas with n and n + 1 atomic planes, for which the T-c's are not the same. Indeed, the magnetic correlation length is presumably shorter than the average size of the terraces. Monte Carlo simulations are performed to support this explanation.