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.