**On the conditions for ordered hexagonal mm2 Co3Pt**

X Lu and DE Laughlin and JG Zhu, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 491, UNSP 165570 (2019).

DOI: 10.1016/j.jmmm.2019.165570

A lattice-based Monte Carlo model is used to investigate the ordering conditions necessary to produce the ordered Co3Pt phase (mm2) In this phase, Co and Co50Pt50 atomic layers alternate along the c-axis of the parent hexagonal structure. It is found that perpendicular ordering where the Co and Co(50)Pt(50 )layers alternates, is strongly correlated with in-plane ordering of the Co50Pt50 layers. In these layers the Co atoms and Pt atoms form alternating rows in parallel. Such atomic ordering within the plane is only facilitated by a deformation of the hexagonal lattice such that the ratio between the atomic spacing transverse to the row and along the row, b/a, becomes significantly smaller and the structure becomes orthorhombic. At adequate lattice spacings and b/a ratio, both in-plane ordering and perpendicular ordering occur simultaneously. In addition, the Co50Pt50 layers are also staggered alternately, forming a structure with the Imm2 space group. The ordered Co3Pt with Imm2 symmetry is also found to be sensitive to changes in the Pt composition.

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