A FePt single layer, a FePt/Cr bilayer, and a FePt/Pt/Cr trilayer were fabricated. Transmission electron microscope images revealed that the grain size of a FePt single layer was around 50 nm, and decreased to 20 nm with a Cr underlayer. When a Pt layer was inserted between the FePt magnetic layer and the Cr underlayer, L10 FePt(001) thin films with perpendicular magnetic properties can be obtained with a grain size of around 20 nm determined primarily by the Cr underlayer. The intergranular interactions determined from the Kelly-Henkel plot were the exchange coupling between magnetic grains in the FePt single layer. In contrast, negative values of the δM curve were obtained for the FePt/Cr bilayer and the FePt/Pt/Cr trilayer, implying the presence of dipole interactions in both films. Smaller grains and the negative δM of FePt/Cr were derived from the diffusion of Cr into the FePt layer.