Characterization of metastable crystal structure for Co-Pt alloy thin film by x-ray diffraction

Co50Pt50 and Co75Pt25 (at. %) alloy epitaxial films with the close-packed plane parallel to the substrate surface are prepared on MgO(111) substrates with and without Ti(0001) underlayer at 300 °C. The structural properties are investigated by using a combination of out-of-plane, in-plane, and pole-figure x-ray diffractions. The crystal structure is determined by taking into account the order degree and the atomic stacking sequence of close-packed plane. Formation of a metastable ordered fcc-based L11-CoPt phase is recognized for the Co50Pt50 film deposited on MgO(111) substrate, whereas a metastable ordered hcp-based Bh-CoPt phase is involved in the Co50Pt50 film deposited on Ti(0001) underlayer in addition to an L11-CoPt phase. The Co75Pt25 film deposited on MgO(111) consists of a mixture of ordered hcp-based phases of Bh-CoPt and D019-Co3Pt. An accurate order degree is calculated by comparing the out-of-plane superlattice and fundamental reflection data. A simplified method for estimating the order degree, where there are only two kinds of parameter: intensity ratio of superlattice to fundamental reflection and out-of-plane lattice spacing, is also proposed. The order degree estimated by the simplified method agrees in a small error less than a few percentages with the accurate value.