Int. J. Quant. Chem. 112 (2012) 277 Competition between ordering, twinning and segregation in binary magnetic 3d-5d nanoparticles: A supercomputing perspective Markus E. Gruner and Peter Entel * Faculty of Physics and Center for Nanointegration, CeNIDE, University of Duisburg-Essen, 47048 Duisburg, Germany (Dated: November 28, 2013) Abstract 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 calcula- tions 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 symme- try constraints, reveal the sized dependent evolution of the energetic order of single crystalline and multiply twinned Fe-Pt nanoparticles up to 4nm in diameter, which are discussed as promis- ing building blocks for future ultra-high density data recording media. While 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 com- parison 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 CoPt particles segregation of Pt to the surface and the formation of a Pt depleted subsurface layer is dominant also for nanometer-sized single crystalline particles and may help to stabilize particles with partial L1 0 order, while for FePt multiple twinning is the most important equilibrium mechanism for small particle sizes. Hybrid combinations of the most favor- able ordering motifs, i. e., L1 0 -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. * Electronic address: Markus.Gruner@uni-due.de,Peter.Entel@uni-due.de 1