Intermetallic compounds such as FePt with an ordered face-centred tetragonal (L10) phase are very promising candidates for next-generation magnetic data storage media and permanent magnet applications. As-synthesized, FePt thin films and nanoparticles have a chemically disordered face-centred cubic (fcc) structure (A1 phase). When annealed at high temperatures, they undergo a transition from an A1 phase to an L10 phase or to a chemically ordered fcc (L12) phase, depending on the chemical composition. Owing to the chemical ordering and strong spin–orbit coupling, L10 FePt exhibits extremely large magnetocrystalline anisotropy energy (MAE). However, despite extensive studies of this material system, a fundamental understanding of 3D chemical order/disorder, crystal defects and the resulting magnetic properties at the individual atomic level remains elusive.
To tackle this challenging problem, we have recently determined the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an FePt nanoparticle to correlate 3D atomic arrangements and chemical order/disorder with material properties at the single-atom level. We identified rich structural variety and chemical order/disorder, including 3D atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We showed, for the first time, that experimentally measured 3D atomic coordinates and chemical species with 22 pm precision can be used as direct input for quantum mechanics calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy (1). We believe that this work will not only revolutionize 3D atomic structure characterization in the physical sciences, but also transform our understanding of material properties and functionality at the individual atomic level (2).
1. Y. Yang, C.-C. Chen, M. C. Scott, C. Ophus, R. Xu, A. Pryor Jr, L. Wu, F. Sun, W. Theis, J. Zhou, M. Eisenbach, P. R. C. Kent, R. F. Sabirianov, H. Zeng, P. Ercius and J. Miao, "Deciphering chemical order/disorder and material properties at the single-atom level", Nature 542, 75-79 (2017).
2. J. Miao, P. Ercius and S. J. L. Billinge, "Atomic electron tomography: 3D structures without crystals", Science 353, aaf2157 (2016).