Effect of Composition on the Ordered Phase Formation in Co-Pt Thin Film Deposited on MgO(111) Single-Crystal Substrate

Co_xPt_100-x (at. %) alloy epitaxial films with the close-packed plane parallel to the substrate surface are prepared on MgO(111) substrates heated at 300 °C by varying the Co content of x from 0 to 100. The stacking sequence of close-packed plane (the volume ratios of fcc and hcp crystals, V_fcc and V_hcp) and the order degree are quantitatively determined. The films with Pt-rich and nearly equiatomic compositions primarily consist of fcc crystals (V_fcc >0.9). With increasing the x value beyond 56, the V_hcp value drastically increases. The films with Co-rich compositions consist of mixtures of hcp (V_hcp ≈0.8) and fcc (V_fcc ≈ 0.2) crystals. The films with 31 ≤x≤ 86 involve metastable ordered phases. The order degree of S_L11,Bh is kept constant around 0.25 in a compositional range of 31 × 75. From the viewpoints of volume ratios of fcc and hcp crystals and order degree, the crystal structure is determined to vary A1 A1 + SFs (SFs: stacking faults) L1₁+ SFs ⇒L1₁+ Bh (+D0₁₉) ⇒A3 + SFs with increasing the x value. The films including ordered phases show strong perpendicular magnetic anisotropies. It is shown that the uniaxial magnetocrystalline anisotropy energy of Co_xPt_100-x film can be kept >10⁷ erg/cm³ while controlling the saturation magnetization from 600 to 1150 emu/cm³.