  The interface between FePt--L1$_0$ and MgO(001) alloys has been studied using
density functional calculations. Because the stacking of the face-centered
tetragonal L1$_0$ phase is formed by alternating Fe and Pt planes, both the Fe-
and Pt-terminated contact layers were studied. Furthermore, due to the large
mismatch between the in-plane lattice constants of both systems, we have chosen
some common $a$ values for both alloys in order to explore in detail the
adsorption energy, the electronic structure and the interface magnetism. The
adsorption energy has been calculated by subtracting the energy of clean FePt
and MgO alloys from the total energy. The preferred adsorbed geometric sites
for Fe/Pt atoms are when they lie on $top$ of the O species, having a smaller
adsorption energy for the remaining positions. We found that expanding the MgO
lattice enhances the magnetic moment of the Fe species but the Pt moments
remain almost constant.
