Structure determination of CoPt nanoparticles: Chemical ordering and its effect on magnetic properties

Due to the huge magnetocrystalline anisotropy of bulk CoPt crystallized in the L1$_{0}$ phase, CoPt nanoparticles have been widely studied during the last decade. In order to determine the intrinsic magnetic properties of CoPt clusters, we synthesize benchmark samples: 3 nm diameter CoPt clusters, pre-formed in the gas phase, are embedded in an amorphous carbon matrix under UHV conditions. The transition from the chemically disordered A1 to the ordered L1$_{0}$ phase is then obtained by annealing. Chemical ordering has clearly been evidenced by different techniques (HRTEM, GIXRD). In the case of nanoparticles, this phase transition goes with a magnetic anisotropy increase much lower than for the bulk. Besides, XMCD measurements have revealed a $\mu $L/$\mu $S increase for Co and Pt atoms and a strong $\mu $S enhancement for Co upon L1$_{0}$ ordering. F. Tournus et al. Phys. Rev. B 77, 144411 (2008) Thanks are due to the CLYM (Centre Lyonnais de Microscope) for the access to the tranmission electron microscope