  We have investigated the relationship between the domain structure and the
magnetic interactions in a series of FePt ferromagnetic thin films of varying
thickness. As-made films grow in the magnetically soft and chemically
disordered A1 phase that may have two distinct domain structures. Above a
critical thickness $d_{cr}\sim 30$ nm the presence of an out of plane
anisotropy induces the formation of stripes, while for $d<d_{cr}$ planar
domains occur.
  Magnetic interactions have been characterized using the well known DCD-IRM
remanence protocols, $\delta M$ plots, and magnetic viscosity measurements. We
have observed a strong correlation between the domain configuration and the
sign of the magnetic interactions. Planar domains are associated with positive
exchange-like interactions, while stripe domains have a strong negative
dipolar-like contribution. In this last case we have found a close correlation
between the interaction parameter and the surface dipolar energy of the stripe
domain structure. Using time dependent magnetic viscosity measurements, we have
also estimated an average activation volume for magnetic reversal, $\langle
V_{ac}\rangle \sim 1.37\times 10^{4}$ nm$^{3},$ which is approximately
independent of the film thickness or the stripe period.
