Magnetic anisotropy and microstructure in sputtered CoPt(1 1 0) films

We compare structural and magnetic properties of CoPt films sputtered at 900 K on MgO(1 1 0) (with a Pt(1 1 0) buffer layer) and MgO(0 0 1) (with a Pt(0 0 1) buffer layer) substrates. We obtain a layer with the L10 tetragonal structure. The equiatomic L10 phase is a “natural” multilayer, which consists in a stacking along the [0 0 1] direction of pure Co and pure Pt monolayers. At this temperature, the growth of CoPt yields nearly single orientation epitaxial films: CoPt(1 1 0)[0 0 1]//MgO(1 1 0)[0 0 1] and CoPt(0 0 1)[1 1 0]//MgO(0 0 1)[1 1 0] as shown by transmission electron microscopy. On MgO(1 1 0) substrates, the long-range chemical ordering is incomplete in contrast with the case of MgO(0 0 1) 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(1 1 0) 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(1 1 0) has pointed out that three variants of the L10 phase coexist. The proportion of [1 0 0] and [0 1 0] variants, oriented at 45° with the ordering growth direction, is much higher than the proportion of the [0 0 1] 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 [1 0 0] and [0 1 0] variants. On MgO(0 0 1), the CoPt films grow as a single variant with the concentration modulation and the magnetic anisotropy along the growth direction.