An atomic layer stacking structure in hexagonal close packed (hcp) Co100-xPtx 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 (Ku = Ku1 + Ku2) 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 (Tsub) of 400 °C. (2) Compositional separation structure in atomic layers is formed for the films fabricated under Tsub = 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) Ku1 takes the maximum of 1.4 × 107erg/cm3 at around x = 20 at. %, whereas Ku2 takes the maximum of 0.7 × 107erg/cm3 at around x = 40 at. %, which results in the maximum of 1.8 × 107erg/cm3 of Ku at x = 30 at. % and a shoulder in compositional dependence of Ku 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.
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