Influence of composition, many-body effects, spin-orbit coupling, and disorder on magnetism of Co-Pt solid-state systems

Systematic trends in the magnetism of Co-Pt solid-state systems are explored by studying the effects of composition, electron correlations, spin-orbit coupling (SOC), and long-range order. Ab initio fully relativistic calculations were performed for bulk Co, a substitutional Pt impurity in Co, Co3Pt, CoPt, CoPt3, and a substitutional Co impurity in Pt. Many-body effects beyond the local spin-density approximation (LSDA) were included via the dynamical mean-field theory (DMFT); a comparison with results based on the Brooks orbital-polarization (OP) scheme was also made. The disorder was treated within the coherent-potential approximation. We found that while the spin magnetic moments mu(spin) at the Co atoms monotonously increase with increasing Pt concentration, the orbital magnetic moments mu(orb) do not follow the trend of mu(spin). Magnetic moments mu(spin) and mu(orb) at Pt atoms do not depend on Pt concentration monotonously. Most of these trends can be understood in terms of hybridization and site-dependent SOC. The OP scheme of Brooks corrects the deficiencies of a pure LSDA only if the Pt concentration is low. The LSDA+DMFT scheme provides mu(orb)/mu(spin) ratios for Co atoms which agree with experiment even for high Pt content. Introducing disorder leads to an enhancement of mu(spin) at Co atoms and to a suppression of mu(spin) at Pt atoms. The mu(orb) at Co atoms does not exhibit any systematic dependence on the degree of order, while mu(orb) at Pt atoms decreases with increasing disorder for all Pt concentrations.