Competition between ordering, twinning, and segregation in binary magnetic 3d-5d nanoparticles: A supercomputing perspective
The benefit of massively parallel supercomputers for technologically relevant applications in the field of materials science is demonstrated at the example of first-principles total energy calculations of magnetic binary transition metal nanoparticles containing up to 1415 3d and 5d transition metal atoms. The simulations, which take into account structural optimizations without symmetry constraints, reveal the size-dependent evolution of the energetic order of single crystalline and multiply twinned Fe-Pt nanoparticles up to 4 nm in diameter, which are discussed as promising building blocks for future ultra-high density data recording media. Although at small diameters, multiply twinned morphologies are preferred, we can show that an energetic crossover to a single crystalline, ordered arrangement can be expected at diameters around four nanometers. The comparison with Co-Pt indicates that the contributions of the interfaces in multiply twinned structures are of similar importance as the surface and cannot be neglected especially for small particle sizes. The results imply that for Co-Pt particles segregation of Pt to the surface and the formation of a Pt-depleted subsurface layer is also dominant for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, whereas for Fe-Pt multiple twinning is the most important equilibrium mechanism for small particle sizes. Hybrid combinations of the most favorable ordering motifs, that is, L10-type ordering in the particle core in combination with segregation in the outer shells, may thus lead to highly stable morphologies, which could dominate the growth process.
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