We compare structural and magnetic properties of CoPt films sputtered at 900 K on MgO(110) (with a Pt(110) buffer layer) and MgO(001) (with a Pt(001) buffer layer) substrates. We obtain a layer with the L1(0) tetragonal structure. The equiatomic L1(0) phase is a "natural" multilayer, which consists in a stacking along the [001] direction of pure Co and pure Pt monolayers. At this temperature, the growth of CoPt yields nearly single orientation epitaxial films: CoPt(110)[001]//MgO(110)[001] and CoPt(001)[110]//MgO(001)[110] as shown by transmission electron microscopy. On MgO(110) Substrates, the long-range chemical ordering is incomplete in contrast with the case of MgO(001) substrates, where long-range order is nearly perfect. Despite incomplete chemical ordering, a large in-plane magnetic anisotropy is present for the films grown on the MgO(110) substrate. This is interesting for the magnetic recording writing with a classical recording head and reading with a magnetoresistance head. The structural study of the CoPt films grown on MgO(110) has pointed out that three variants of the L1(0) phase coexist. The proportion of [100] and [010] variants, oriented at 45degrees with the ordering growth direction, is much higher than the proportion of the [001] variant. In fact. the simulation of magnetization loops has shown that the easy magnetization axis is within the plane along the [1,-1,0] direction. This anisotropy is favored for the [100] and [010] variants. On MgO(001), the CoPt films grow as a single variant with the concentration modulation and the magnetic anisotropy along the growth direction. (C) 2003 Elsevier B.V. All rights reserved.