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

In ferromagnetic thin films, the Curie temperature variation with the thick ness 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 su perlattices. For an integer number of atomic planes, a unique magnetic tran sition is observed by susceptibility measurements, whereas two magnetic tra nsitions are observed for fractional numbers of planes. This behavior is at tributed 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 len gth is presumably shorter than the average size of the terraces. Monte Carl o simulations are performed to support this explanation.