Compositional modulated atomic layer stacking and uniaxial magnetocrystalline anisotropy of CoPt alloy sputtered films with close-packed plane orientation

An atomic layer stacking structure in hexagonal close packed (hcp) Co_100−xPt_x alloy films with c-plane sheet texture was directly observed by a high-angle annular dark-field imaging scanning transmission electron microscopy. The analysis of sequential and/or compositional atomic layer stacking structure and uniaxial magnetocrystalline anisotropy (K_u = K_u1 + K_u2) revealed that (1) integrated intensity of the superlattice diffraction takes the maximum at x = 20 at. % and shows broadening feature against x for the film fabricated under the substrate temperature (T_sub) of 400 °C. (2) Compositional separation structure in atomic layers is formed for the films fabricated under T_sub = 400 °C. A sequential alternative stacking of atomic layers with different compositions is hardly formed in the film with x = 50 at. %, whereas easily formed in the film with x = 20 at. %. This peculiar atomic layer stacking structure consists of in-plane-disordered Pt-rich and Pt-poor layers, which is completely different from the so-called atomic site ordered structure. (3) A face centered cubic atomic layer stacking as faults appeared in the host hcp atomic layer stacking exists in accompanies with irregularities for the periodicity of the compositional modulation atomic layers. (4) K_u1 takes the maximum of 1.4 × 10⁷erg/cm³ at around x = 20 at. %, whereas K_u2 takes the maximum of 0.7 × 10⁷erg/cm³ at around x = 40 at. %, which results in the maximum of 1.8 × 10⁷erg/cm³ of K_u at x = 30 at. % and a shoulder in compositional dependence of K_u in the range of x = 30–60 at. %. Not only compositional separation of atomic layers but also sequential alternative stacking of different compositional layers is quite important to improve essential uniaxial magnetocrystalline anisotropy